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IntroductionOperational amplifier (op amp for short) is basically a voltage amplifying device designed to be used with components like capacitors and resistors, between its in/out terminals, or is simply a linear Integrated Circuit (IC) having multiple-terminals. In electronics, the open-loop voltage gain of the actual operational amplifier is very large, which can be seen a differential amplifier with infinite open loop gain, infinite input resistance and zero output resistance. In addition, it has positive and negative inputs which allow circuits that use feedback to achieve a wide range of functions. And meanwhile, it can be further simplified into an ideal op amp model, referred to as an ideal op amp (also called ideal OPAMP).CatalogIntroductionⅠ Ideal Op Amp Characteristics1.1 Infinite Input Resistance1.2 Zero Output Impedance1.3 Infinite Open-loop Gain1.4 Infinite Common-mode Rejection Ratio1.5 Infinite BandwidthⅡ Assumptions of Ideal Op AmpⅢ Working Characteristics of Ideal Operational Amplifiers3.1 Work in Linear Region3.2 Work in Nonlinear RegionⅣ Analysis of Ideal Operational Amplifier CharacteristicsⅤ Balanced Resistance Presets5.1 The Role of Balanced Resistance5.2 Input Balancing Resistor ExplanationⅥ Ideal Op Amp EquationsⅦ Several Common Op Amp CircuitsⅧ Difference Between Ideal Op-amp and Practical Op-ampⅠ Ideal Op Amp CharacteristicsWhen analyzing various application circuits of operational amplifiers, the integrated operational amplifier is often regarded as an ideal operational amplifier. The so-called ideal op amp is to idealize various technical indicators of op amps, and it must have the following characteristics.Characteristics of An Ideal Op-Amp1.1 Infinite Input ResistanceThe input terminal of an ideal operational amplifier does not have any current to flow in. In electronics, op amps are voltage gain devices. They amplify a voltage fed into the op amp and give out the same signal as output with a much larger gain. In order for an op amp to receive the voltage signal as its input, the voltage signal must be dropped across the op amp. If you know the concept of a voltage divider, voltage drops primarily across components with high impedances, proportionally according to ohm’s law by the formula V=IR. So the greater the resistance (or impedance) of a device, the greater the voltage drop across that device is. To make sure that the voltage signal drops fully on the op amp, it must have a very high input impedance, so that the voltage drops fully across it. If it had a low input impedance, the voltage may not drop across it and it would not receive the signal. This is why op amps must have high-input impedances.It’s also easy to make the input impedance lower (put a resistor in parallel) or the source impedance higher (put a resistor in series).Figure 1. Ideal Op Amp Symbol and Transfer Characteristic Curve 1.2 Zero Output ImpedanceThe output of an ideal op amp is a perfect voltage source, no matter how the current flowing to the amplifier load changes, the output voltage of the amplifier is always a certain value, that is, the output impedance is zero. In practice, zero output impedance is actually a distinct property from infinite input impedance, but for a very long time infinite input impedance was approached only with compromises in offset voltage and noise. 1.3 Infinite Open-loop GainIn an open-loop state, the differential signal at the input has an infinite voltage gain. This feature makes the operational amplifier very suitable for practical applications with upper negative feedback configuration. 1.4 Infinite Common-mode Rejection RatioAn ideal operational amplifier can only respond to the difference between the voltages at both ends of V+ and V-. In addition, the same part of the two input signals (ie common mode signal) will be completely ignored. What’s more, a high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI). An example is audio transmission over balanced line in sound reinforcement or recording. 1.5 Infinite BandwidthThe ideal operational amplifier will amplify the input signal of any frequency with the same differential gain, which will not change with the change of signal frequency.Ⅱ Assumptions of Ideal Op AmpThe op amp can be considered a voltage controlled current source, or it is an integrated circuit that can amplify weak electric signals. Based on it, for an ideal OPAMP, what is the relationship between it and these electrical signals?First, assume that the current flowing into the input of the op amp is zero. This assumption is almost completely correct for FET op amps, because the input current for FET op amps is below 1pA. But for dual high-speed op amps, this assumption is not always correct, because the input current of it can sometimes reach tens of microamperes.Second, assume that the gain of the op amp is infinite, so the op amp can swing the output voltage to any value to meet the input requirements. It means that the output voltage of the op amp can reach any value. In fact, when the output voltage is close to the power supply voltage, the op amp will saturate. Maybe this hypothesis does exit, but needs a limit in practical. For example, at higher frequencies, the internal junction capacitors of transistor come into play, thus reducing the output and therefore the gain of amplifier. The capacitor reactance decreases with increase in frequency bypassing the majority of output. The opamp is in saturation state.Figure 2. Op Amp SaturationFor example, as per datasheet of LM741, large signal voltage gain is 200V/mv. It means an open loop gain of 200,000. If you operate an op-amp in open-loop condition(i.e. without negative feedback) ,even microvolts of input voltage (input offset voltage of LM741 is 3mv) will drive the output to saturation.In most of the amplifier circuits op-amp is configured to use negative feedback which greatly reduces the voltage gain (i.e. closed loop gain). In oscillators and schmit triggers, Op-amp is configured to use positive feedback. Comparator circuit is an example of the circuit which utilizes open-loop gain of op-amp. Its output will be always at saturation either positive or negative saturation. In an integrator circuit, the DC gain should be limited by adding a feed back resistor in parallel with capacitor ;else the output will get saturated .Even in amplifier circuits, the amplitude of the input signal and the voltage gain of the circuit should be balanced so that the output voltage does not exceed power supply voltage . For example for a non-inverting amplifier with a voltage gain of 100, the maximum permissible input voltage will be 150 mv if the VCC is 15 Volts. If you apply a signal of 200 mv ,the op-amp output will goto saturation as the required output will be 20 volts which exceeds the VCC of 15 Volts.Third, the assumption of infinite gain also means that the input signal must be zero. The gain of the op amp will drive the output voltage until the voltage (error voltage) between the two input terminals is zero. The voltage between the two input terminals is zero. The zero voltage between two input terminals means that if one input terminal is connected to a hard voltage source like ground, the other input terminal will also be at the same potential. In addition, since the current flowing into the input terminal is zero, the input impedance of the op amp is infinite.Fourth, of course, the output resistance of an ideal op amp is zero. An ideal op amp can drive any load without any voltage drop due to its output impedance. At low currents, the output impedance of most op amps is in the range of a few tenths an ohm, so this assumption is true in most cases. Ⅲ Working Characteristics of Ideal Operational Amplifiers3.1 Work in Linear RegionWhen the ideal op amp works in the linear region, the output and the input voltage show a linear relationship. Where u0 is the output voltage of the integrated op amp; u+ and u- are the voltages at the non-inverting input terminal and the inverting input terminal, respectively. Auo is the open loop differential voltage magnification. According to the characteristics of the ideal op amp, two important characteristics of the ideal op amp in the linear region.1) Zero differential input voltageSince the open-loop differential voltage magnification of an ideal op amp is equal to infinity, and the output voltage is a certain value, the voltage values at the non-inverting input terminal and the inverting input terminal are approximately equal. Just like short circuit between input and output, but it is fake. Because it is an equivalent short circuit, not a real short circuit, so this phenomenon is called "virtual short".2) Zero input currentSince the open-loop input resistance of an ideal op amp is infinite, no current flows into the op amp at either input. At this time, the current at the non-inverting input terminal and the inverting input terminal are both equal to zero. Like an disconnection, but an equivalent disconnection, so this phenomenon is called "virtual break". Virtual short and virtual break are two important concepts for analyzing the ideal op amp working in the linear region.In fact, the ideal operational amplifier has the characteristics of "virtual short" and "virtual break". These two characteristics are very useful for analyzing linear amplifier circuits. The necessary condition for virtual short is negative feedback. When negative feedback is introduced, at this time, if the forward terminal voltage is slightly higher than the reverse terminal voltage, the output terminal will output a high voltage equivalent to the power supply voltage after the amplification of the op amp. In fact, the op amp has a respond time changing from the original output state to the high-level state (the golden rule of analyzing analog circuits: the change of the signal is a continuous change process). Due to the feedback resistance of the reverse end change will inevitably affect its voltage, when the reverse end voltage infinitely close to the forward end voltage, the circuit reaches a balanced state. The output voltage does not change anymore, that is, the voltage at the forward end and the reverse end is always close. (Note: The analysis method is the same when the voltage decreases.) 3.2 Work in Nonlinear RegionWhen the op-amp operates in the nonlinear region, the output voltage no longer increases linearly with the input voltage, but saturates. The ideal op amp also has two important characteristics when operating in the nonlinear region.1) When u+ ≠ u-, the output voltage of the ideal op amp reaches the saturation value.When u+ > u-, the op-amp operates works in positive saturation region with a positive output voltage.When u+ < u-, the op-amp operates works in negative saturation region with a negative output voltage.Ideal op amp operates in the nonlinear region, u+ ≠ u-, there is no “virtual short”.2) The input current is equal to zero.Although the input voltage u+ ≠ u- above, the input current is considered to be zero. Ⅳ Analysis of Ideal Operational Amplifier CharacteristicsAs for Op-amp, there's probably a description like this: three-terminal element (circuit structure with double-ended input, single-ended output), ideal transistor, high-gain DC amplifier.(1) High input resistanceUnder this situation, the current flowing into the input terminal is close to 0, almost no signal source current is used, which is close to the voltage control characteristic. And virtual break is derived from this.(2) Lower output resistanceIt has the characteristics of adapting to any load. And the impedance of the subsequent load circuit will not affect the output voltage.(3) Infinite voltage amplification(4) Under a certain supply voltage condition, the amplifier can only work in closed-loop (negative feedback) mode, and the actual amplification is limited. Because op-amps themselves don't have a 0V connection but their design assumes the typical signals will be more towards the center of their positive and negative supplies. Thus, if your input voltage is right at one extreme or forces the output toward one supply, chances are it won't work properly. Working in open-loop mode is the like a comparator, and the output is high level orlow level.In the closed-loop (limited amplification) state, the amplifier is randomly compare the potentials of the two input terminals. The output stage makes immediate adjustments when they are not equal. So the final purpose of amplification is to make the potentials of the two input terminals equal. And virtual short is derived from this. Ⅴ Balanced Resistance Presets5.1 The Role of Balanced Resistance1) A suitable resistance is generally required to ensure that the input impedance is matched.2) In order to reduce the input current imbalance, the in-phase resistor should be equal to the parallel value of the two resistors at the reverse end in theory. In practice, as a result of the closed loop, especially in deep negative feedback conditions, the misalignment is not obvious at the output. And there is no need of in-phase grounding resistor when the misalignment is not the main problem. Because a balanced resistor is the starting point for an ideal op amp. In-phase grounding resistance is useful for bipolar op amps, and has no meanings for MOS-type op amps.3) Ground input termination resistance: it is necessary for impedance matching and high frequency setting.4) Bias current and offset current.For operational amplifiers with bias current greater than offset current, input resistance matching can be reduced, and precision circuits can compensate bias current to a minimum. If the bias current and offset current are similar, the matching resistance will increase the error.5) Set for the bias current at the input, the purpose of which is to equalize the impedance of the invertingand non-inverting inputs, so that two inputs with equal bias currents are assumed to have equal voltage drops, thereby counteraction can be made. 5.2 Input Balancing Resistor ExplanationA op-amp is connected to an inverting amplifier:Set the input resistance for R1, feedback resistance for Rfi,Assume that the non-inverting end is not connected to a balanced resistor, but grounded directly.Set the input bias current for the op-amp IB (same voltage in inverting and non-inverting end).The current flows through R1 and Rf are represented by I1 and If.Inverting voltage is V-, The op-amp gain is A.Use KCL in the inverting end (set the input signal to 0).Where (0-V-)/R1- (A+1)V- /Rf=IBFrom the above equation, it follows that V-=-(IB×R1×Rf/(Rf+(A+1)R1))At this time, the output voltage of the op-amp is Vo=A×(IB×R1×Rf/(Rf+(A+1)R1))The above formula can be approximated as Vo=IB×((A×R1)/Rf)If the in-phase terminal passes through a resistor R2 to ground and R2=R1/Rf, then the voltage at the in-phase terminal is V+=-IB×R2KCL is applied to the inverted terminal, where (0-V-)/R1+(A×(V+-V-)-V-)/Rf=IBAt this time the output voltage of the op-amp is Vo=0. Ⅵ Ideal Op Amp EquationsUnderstanding the basic conditions of an ideal op amp, and combining it with the Kirchhoff's current law (KCL) node voltage method and the superposition theorem of the node, is an effective method to analyze the ideal op amp circuit.As shown below, find the output voltage uoFigure 3. OPAMP Circuit1) Equation based on KCLFrom the concept of virtual break, i+=i-=0, then i1=i2, i3=i4, so (a)Based on virtual break, u+=u-, then (b)2) Node voltage methodList the node voltage equations for node 1 and node 2, and get (c)Note: Because the output current of the op amp is unknown at 1) and 2), it is not possible to list the KCL equation or node voltage equation at the output of the op amp. In addition, the op amp output uo in 2) should be treated as an independent voltage source. 3) Superposition theoremWhen there are multiple signal inputs, choosing the superposition theorem to solve can simplify the analysis and calculation process. The size of the output signal uo can be regarded as the superposition of the output signal obtained by the independent action of u1 and u2. When u1 acts alone, the u2 terminal is grounded, and the op amp output is: (d)Therefore, the final output of the operational amplifier is:   (e) Ⅶ Several Common Op Amp CircuitsNon-inverting Amplifier CircuitA non-inverting amplifier is an op-amp circuit configuration which produces an amplified output signal. It provides a high input impedance along with all the advantages gained from using an operational amplifier. Inverting Amplifier CircuitAn inverting amplifier (also known as an inverting operational amplifier or an inverting op-amp) is a type of operational amplifier circuit which produces an output which is out of phase with respect to its input by 180 degrees out of phase with respect to input signal. In the following figure, two external resistors to create feedback circuit and make a closed loop circuit across the amplifier. Op-amp as AdderAn adder circuit can be made by connecting more inputs to the inverting op amp. The circuit diagram of a summing amplifier is as shown in the following figure. Differential AmplifierDifferential amplifier is an analog circuit with two inputs and and one output in which the output is ideally proportional to the difference between the two voltages. It is a very useful op-amp circuit and by adding more resistors in parallel with the input resistors as shown in the following. Composite AmplifierThe composite amplifier is termed as a combination of multiple operational amplifiers that are cascaded together with a negative-feedback loop around the entire network. The resistance in the circuit is generally selected at the K ohm level, the ratio of the resistance affects the gain and bias, in addition, the supply current, frequency response and capacitive load driving capability of the op amp determine their specific values in circuits. If it is used in a high-frequency circuit, the resistance needs to be reduced to obtain a better high-frequency response, but it will increase the input bias current, thereby increasing the current of the power supply. Ⅷ Difference Between Ideal Op-amp and Practical Op-ampIdeal op amps use no power, have infinite input impedance, unlimited gain-bandwidth and slew rate, no input bias current, and no input offset. They have unlimited voltage compliance.Practical op amps consume some power, have very high input impedance have limited gain-bandwidth and limited slew rate, have some input bias current and input offset voltage. Voltage compliance is limited by the power supply rail, or frequently even less.Still practical op amps are very useful because most of the limitations listed above are way better than what your circuit needs.For an ideal amplifier, it does not draw any current at all from its input. Assuming a two input amplifier the signal current in both input probes is zero. In other words the input impedance must be infinite. The output, should operate as the output of an ideal voltage source. This means that the potential between the output and the ground must be A(v2−v1), no matter how much current would a load connected to the output would draw. In other words the output impedance must be zero.For a real amplifier, the input impedance must be as large as possible while the output impedance must be as low as possible.In fact, An op-amp in real life, however, cannot operate with zero current flow. Frequently Asked Questions about Ideal Op Amp1. What is characteristic of ideal opamp?Ideal op amps will have infinite voltage gain, infinitely high impedance, zero output impedance, its gain is independent of input frequency, it has zero voltage offset, its output can swing positive or negative to the same voltages as the supply rails, and its output swings instantly to the correct value. 2. How does an ideal op amp work?An operational amplifier, or op amp, generally comprises a differential-input stage with high input impedance, an intermediate-gain stage, and a push-pull output stage with a low output impedance (no greater than 100 Ω). ... Open-loop voltage gain runs very high, on the order of 1 million. 3. Why are op amps not ideal?Op-amps with FET inputs have an Ibias that is so small that this method becomes less practical. Instead of measuring the voltage drop across a resistor, one can monitor the change in voltage across a capacitor as it is charged by the bias current. 4. How are real op amps different from ideal op amps?In real op amps, the amplified signal will not fully reach the DC supply rails. They will fall short of it. In an ideal op amp, the output will swing instantly to the amplified voltage value. There will be no time delay between the time the voltage is input into the op amp till the time it is output. 5. What are the four main ideal characteristics of an open-loop op amp?An ideal op amp is usually considered to have the following characteristics:Infinite open-loop gain G = vout / vInfinite input impedance Rin, and so zero input currentZero input offset voltageInfinite output voltage rangeInfinite bandwidth with zero phase shift and infinite slew rateZero output impedance R

IntroductionA band pass filter is an electronic device or circuit that allows signals between two specific frequencies to pass. That is, allowing signals in a specific frequency band to pass while shielding other frequency bands. In other words, a band-pass filter attenuates frequency components in other ranges to an extremely low level, as opposed to the concept of a band-stop filter. For example, the RLC tank is an analog band-pass filter, it is a resistor - inductor - capacitor circuit (RLC circuit). These filtering circuit can also be made by connecting low-pass filters and high-pass filters.How to Design Band Pass Filter CircuitCatalogIntroductionⅠ Band Pass Filter Circuit CharacteristicsⅡ Band Pass Filter Parameters2.1 Center Frequency2.2 Cut-off Frequency2.3 Bandwidth2.4 Quality FactorⅢ Types of Band Pass Filter3.1 Active Band Pass Filter3.2 Passive Band Pass FilterⅣ Band Pass Filter Equation4.1 Cutoff Frequency of Band Pass Filters4.2 General Form of Second-order BPF Transfer Function4.3 Second-order Band Pass Filters4.4 High-Q second-order Band Pass Filters4.5 Dual-operational Amplifier BPF (High-Q)4.6 Second-order Band Pass Filters (Voltage-controlled Type)Ⅴ Band Pass Filter ApplicationsⅠ Band Pass Filter Circuit CharacteristicsAn ideal band pass filter should have a completely flat pass band, no amplification or attenuation. And all frequencies outside the pass band will be completely attenuated. In addition, the conversion outside the pass band is completed in an extremely small frequency range. But in fact, there is no ideal band-pass filter. Because the filter cannot completely attenuate all frequencies outside the desired frequency range, especially there is an attenuated but not isolated frequency range outside the desired pass band. This is usually called the filter roll-off phenomenon, and it is expressed in dB per decade of attenuation amplitude. Generally, the filter design should ensure that the roll-off range is as narrow as possible, so that the performance of the filter is closer to the design requirement. However, as the roll-off range gets smaller and smaller, the pass band becomes no longer flat and causes ripples.The high-pass filter has a low cut-off frequency, and the low-pass filter has a high cut-off frequency. When the high cut-off frequency is lower than the low cut-off frequency, combining the two circuits, and it is possible to design a band pass filter. The gain of the band pass filter is adjusted by the feedback resistor and the current limiting resistor.Figure 1. Band Pass Filter Circuit PartsA band pass filter with a high quality factor refers to a filter with a narrow pass band. In other words, a high-Q factor means that fewer unwanted frequency signals will pass. A low-Q factor means that the pass band is very wide, to allow a wider range of frequencies to pass through.  Ⅱ Band Pass Filter Parameters2.1 Center FrequencyIt usually defined as the midpoint between the two 3dB points of a band pass filter (or a band stop filter), generally expressed by the arithmetic average of the two 3dB points. It is a frequency when the impedance of the entire circuit is a real number.2.2 Cut-off FrequencyIt refers to the frequency point on the right of the low-pass filter and the frequency point on the left of the high pass filter in the pass band. That is, the boundary frequency. It is usually defined as a standard by 1dB or 3dB relative loss point. The band pass filter has two cutoff frequencies, the low cutoff frequency fp1 and the high cutoff frequency fp2. 2.3 BandwidthThe difference between two cut-off frequencies. The bandwidth is defined as B=fp2－fp1.2.4 Quality FactorThe reciprocal of the damping coefficient is called the quality factor, which is an important indicator of the frequency selection characteristics of band pass and band stop filters. In short, it is the ratio of the center frequency to the bandwidth. What’s more, it can be used to describe the shape of the transfer function graph. Ⅲ Types of Band Pass Filter3.1 Active Band Pass FilterFigure 2. Active Band Pass Filter CircuitThe active band pass filter is a cascade of high-pass and low-pass filters and amplifier components. The circuit diagram of the active band pass filter consists of three parts. The first part is the high-pass filter. Then, use the op amp for amplification. The last part of the circuit is the low-pass filter.3.2 Passive Band Pass FilterFigure 3. Passive Band Pass Filter CircuitPassive band pass filters are a combination of passive high-pass and low-pass filters. Passive filters use only passive components, such as resistors, capacitors, and inductors. Therefore, passive band pass filters are also used as passive components and do not use op amps for amplification. Ⅳ Band Pass Filter Equation4.1 Cutoff Frequency of Band Pass FiltersThe characteristic of the band pass filter is that the output signal amplitude in the pass band is independent from the frequency. When f<fp1 or f>fp2, the output signals attenuate quickly. The amplitude-frequency characteristics are shown in the figure:Figure 4. BPF Bandwidth(The broken line is the ideal BPF frequency characteristic, and the solid line is the actual BPF frequency characteristic)The resonance frequency is between fp1 and fp2, where the gain of the filter is the largest, and the bandwidth of the filter is the difference between fp2 and fp1.It can be seen from the frequency characteristics of BPF that it can be composed of LPF and HPF in series, as long as the fpL of LPF (ie, fp2 of BPF) is greater than fpH of HPF (ie, fp1 of BPF).4.2 General Form of Second-order BPF Transfer FunctionFrequency CharacteristicsWhere Aup is the pass-band magnification, center frequency , Q factor Normalized frequency characteristicsNormalized amplitude - frequency characteristicsFigure 5. Amplitude - frequency CharacteristicsFigure 6. Frequency CharacteristicsIt can be seen that the frequency characteristic of the band pass filter is completely determined by the center frequency ωo and the quality factor Q.When f>fo, as the frequency f increases, the amplitude increases. According to the definition of cutoff frequency, the denominator of amplitude-frequency characteristic , that is (since f>fo, take a positive value)1) Upper cutoff frequency（take a positive value）When f<fo, as the frequency f decreases, the output signal amplitude will decrease. According to the definition of cutoff frequency, the denominator of amplitude-frequency characteristic , that is (since f<fo, take a negative value)2) Lower cutoff frequency（take a negative), get the bandwidth When the center frequency fo and bandwidth B (or Q) are known, the upper and lower cutoff frequencies fp1 and fp2 can be calculated. On the contrary, when the upper and lower cutoff frequencies fp1 and fp2 are known, the center frequency fo and bandwidth B (or Q) can be calculated. (where ),  4.3 Second-order Band Pass FiltersA simple second-order band pass filter circuit is shown in the figure below, where R1 and C1 constitute a low-pass filter circuit, and C2 and R3 constitute a high-pass filter circuit.Figure 7. Second-order Band Pass Filter Circuit(1) Transfer FunctionIn order to reduce the amount of parameters matching, generally take C1=C2=CTake , , that is The transfer function can be obtained by using the node current method.(2) Frequency Characteristicswhere band-pass amplification (The negative sign means that the input and output are inverted. Because the filter circuit is an inverting filter.)Center frequency (C1=C2=C), Q factor When Aup, Q, and ωo are known, the resistance of each resistor is (R3 can be calculated with ωo/Q), (Aup<2Q2)When the pass band amplification factor Aup is small, Q should not be too large (that is, the simple second-order BPF has poor selectivity), otherwise R2 will become very small (R2 is generally greater than 1K), which will attenuate the input signal seriously. In order to make the system stable, Aup is generally between 1 and 10, and Q can be between 1 and 20.(3) Design StepsExample: It is known that Aup=5, center frequency fo=450Hz, bandwidth B=200Hz (). Try to calculate the parameters of the band-pass filter and verify.First, according to the center frequency fo, check the parameter table and determine C1, C2, and operational amplifier parameters according to the nominal value.fo=450Hz, take C1=C2=0.01uF(103 capacitor). Since the center frequency is not high, the requirement can be met by using LM358 operational amplifier.Second, calculate the resistance of each resistor. Among them, the range of R1 and R3 should be between 10K ~510K, and R2 should be between 1K ~100K, otherwise the capacitor C needs to be reselected.Substituting the relevant parameters into the above formula, the result is R1=15.9K, R2=15.5K, R3=159K.Third, use simulation software to verify on the computer, and try to take the nominal value of each relevant resistance. The simulation schematic diagram and simulation results are shown in the figure below. The result values obtained from the AC small signal analysis and transmission characteristic analysis basically meet the requirements.Figure 8. Filtering Circuit with LM358Figure 9. Simulation Schematic DiagramFigure 10. Voltage - Time Simulation (ui)Figure 11. Voltage - Time Simulation (ui, uo)The circuit requires a small number of components, and it can work with dual power supplies or with a single power supply (the non-inverting termination is connected to a 1/2Vcc bias potential). In fact, it is widely used in single power supply systems. Because the quality factor Q cannot be too high. Almost all band pass filter circuits with a larger bandwidth B (with a smaller Q value) adopt this circuit form. 4.4 High-Q second-order Band Pass FiltersThe high-Q second-order band pass filter circuit is shown in the following figure. This circuit can work with dual power supplies or single power supplies, which is convenient to use in a single power supply system. Since the Q value can be made larger, it is particularly suitable as a band pass filter.Figure 12. Bandpass Filter Circuit(1) Transfer FunctionIn order to reduce the amount of parameters matching, generally take C1=C2=CWhere , , that is , , getting The transfer function can be obtained by using the node current method.(2) Frequency Characteristics, where band-pass amplification Center frequency , Q factor In order to make the system stable, Aup and Q must be greater than 0, that is, 2RfR4-RFR3>0, which must be guaranteed . Adjusting can control Aup and Q. is more closer to 2, the greater the Aup and Q values. Adjust the capacitor C to select the center frequency ωo. When the values of Aup, Q and ωo are known, and the ratio between and is determined, the resistance of each resistor is , , (3) Design stepsExample: It is known that Aup=5, center frequency fo=1kHz, bandwidth B=50Hz (). Try to calculate the parameters of the band-pass filter and verify.First, according to the center frequency fo, check the parameter table and determine C1, C2, and operational amplifier parameters according to the nominal value. (Aup: 1 ~10)fo=1kHz, take C1=C2=0.01uF. Since the center frequency is not high, the requirement can be met by using LM358 operational amplifier.Second, according to the value of Aup and Q, initially determine the value of and .Since Aup and Q are large, is 1.8, is 0.5, and is 3.6.Third, calculate the resistance of each resistor. The range of R1 and R3 should be between 10K and 510K, and R2 should be between 1K and 100K. Otherwise, the ratio of and needs to be reselected.Substituting the relevant parameters into the above formula, the result is:R1=229K, R3=63.7K, R2=4.18K, R4=127.4K: RF takes 36K, Rf takes 10K.Fourth, use simulation software to verify on the computer, and try to take the nominal value of each relevant resistance. The simulation schematic diagram and simulation results are shown in the figure below. The result values obtained from the AC small signal analysis and transmission characteristic analysis basically meet the requirements.Figure 13. High-Q BPF CircuitFigure 14. Voltage - Frequency SimulationFigure 15. Voltage - Time Simulation4.5 Dual-operational Amplifier BPF (High-Q)The BPF circuit with high-Q value formed by dual operational amplifiers is shown in the figure. With fewer components, a very high-Q value can be obtained when the pass band amplification factor Aup is fixed equal to 2, so it is also a commonly used BPF circuit.Figure 16. Dual-amp BPF(1) Transfer functionAccording to the rule of futility, where, , that is , where , so (2) Frequency CharacteristicsCompared with the standard form of the second-order BPF transfer function, the following parameters can be obtained:pass-band magnification , , center frequency When R4=R5,R2=R3=R,C1=C2=C, Aup=2, , （）It can be seen that when Aup=2 (that is, when R4=R5), the value of Q can be very large.(3) Design stepsAccording to the center frequency fo, check the parameter table to determine C. When C is determined, the resistance R is calculated from the center frequency. Meanwhile, etermine R1 based on the Q value. 4.6 Second-order Band Pass Filters (Voltage-controlled Type)The second-order voltage-controlled BPF is shown in the figure. Among them, R1 and C1 constitute a low-pass filter, R2 and C2 constitute a high-pass filter. (The voltage positive feedback is introduced through the voltage R3 to form a voltage-controlled band-pass filter.)Figure 17. Second-order Bandpass Filter (voltage controlled)Rf/RF cannot be 3 to avoid self-excitation.(1) Transfer FunctionWhere ，that is , , so The transfer function can be obtained by using the node current method.In order to reduce the amount of parameters matching, generally take R1=R3=R,R2=2R,C1=C2=CWhere In order to make the system stable, the coefficient of the first term in the denominator must be larger than 0, that is, 3−Auf>0, in other words, Auf<3.(2) Amplitude - frequency Characteristicswhere band-pass amplification , center frequency , Q factor It can be seen that the closer Auf is to 3, the larger the Q value. The narrower the pass band B, and the better the selectivity.(3) Design StepsAccording to the center frequency, look up the table and initially determine C1=C2=Ccalculate resistance , that is , Calculate bandwidth based on upper and lower cutoff frequencies , Calculate the quality factor Calculate by Q and determine the resistances Rf and RF.As a special case, the center frequency fo=1KHz is known, so C1=C2=C=0.01uF，R2=2R=31.8K, getting Auf=2.95, that is . If Rf=10K, calculate RF=19.5K.For high pass and band pass filters, the output of the op amp is not required to be 0 at static state. And the single power supply operating mode can be selected. In the low-pass or band-stop filter circuit, it is a DC-to-AC DC amplifier circuit, which generally requires the circuit to work in a dual power supply state. Ⅴ Band Pass Filter ApplicationsThe filtering circuit has a wide range of uses.According to different frequency amplitude characteristics, filter circuits can be divided into low pass filter circuit (LPF), high-pass filter circuit (HPF), band pass filter circuit (BPF), band stop filter circuit (BEF) and all-pass filter circuit (APF) . The BPF is mainly used to highlight signals in useful frequency bands and weaken signals or interference and noise in other frequency bands to improve the signal-to-noise ratio. Therefore, band pass filters are often used in wireless receivers and transmitters to receive useful signals while preventing unwanted frequencies from passing through.In addition to the fields of electronics and signal processing, an specific application of band pass filters is in the field of atmospheric sciences. A very common example is to use it to filter the weather data in the last 3 to 10 days, only the cyclone as a disturbance remains in the domain. Frequently Asked Questions about Band Pass Filter1. What is a bandpass filter used for?A band pass filter is an electronic circuit or device which allows only signals between specific frequencies to pass through and attenuates/rejects frequencies outside the range. Band pass filters are largely used in wireless receivers and transmitters, but are also widely used in many areas of electronics. 2. What is the bandwidth of a bandpass filter?The bandwidth of a bandpass filter is usually defined as the 3 dB bandwidth. Similarly, the 1 dB bandwidth is the point at which the signal amplitude decreases by 1 dB from its maximum value (above and below the center frequency). 3. How does bandpass filter work?A bandpass filter works to screen out frequencies that are too low or too high, giving easy passage only to frequencies within a certain range. Band-pass filters can be made by stacking a low-pass filter on the end of a high-pass filter, or vice versa. Attenuate means to reduce or diminish in amplitude. 4. How is bandpass filter calculated?So all frequencies between the low cutoff frequecny and the high cutoff frequency are the passband of the bandpass filter. The gain of the circuit is determined by the formula, gain (AV)= -R2/R1. Thus, for example, to have a gain of 10, R2 must be 10 times the value of R1. 5. What is the use of band pass filter?Bandpass filters are widely used in wireless transmitters and receivers. The main function of such a filter in a transmitter is to limit the bandwidth of the output signal to the band allocated for the transmission. This prevents the transmitter from interfering with other stations.

IntroductionLED lights cannot directly use the conventional mains grid voltage, because of the characteristics of LED lighting. In order to meet the special voltage and current requirements of LEDs, a specially designed voltage conversion device must be used to make LEDs work normally. This device is an LED driver. LED drivers are usually switching mode devices that convert the input voltage (Typically 120-220 VAC or 12 VDC) into a voltage at which the current drawn by the LED's is equal to its drive current. The drive current is regulated for optimum brightness, led service life, and battery life. A drive current lower than the maximum drive current of an LED can greatly prolong service life. As a key part of LED lighting, the quality of LED drivers directly affects the performance of LED lighting.Choose the Correct LED Drivers For LED LightsNo matter how good the quality of the LED driver is, failure and maintenance are inevitable. This article will analyze the 10 failures in LED lighting design and its application based on the relevant technology and practical experience of the LED driver.CatalogIntroductionⅠ LED Driver Failure Analysis1.1 Forward Voltage Drop (Vf) Range1.2 Power Margin and Derating Requirements1.3 LED Working Characteristics1.4 Test Session1.5 Different Load with Different Test Results1.6 LED Driver Circuit Problem1.7 Wrong Phase Wring1.8 Grid Fluctuation1.9 Frequent Line Trips1.10 Drive CoolingⅡ LED Driver Maintenance2.1 Multimeter to Detect LED Driver2.2 Identify LED Power SupplyⅢ LED Driver Circuit Modulation3.1 Pulse Width Modulation (PWM)3.2 Pulse Frequency Modulation (PFM)3.3 Sliding-Mode ModulationⅣ One Question Related to LED DriverⅠ LED Driver Failure AnalysisThe LED driver is measuring current passing through LEDs using sense resistor and then increase or decrease the voltage to maintain constant current continuously. LEDs are kind of diode so they need DC voltage to operate so most of the LED drivers are boost and can vary output supply in wide range (example, 16V to 38V). They also have dimming control by PWM signal from microcontroller OR by having a manual potentiometer to change sense resistor. According to them, LED driver failures are complex, but we can follow the steps below to analyse.1.1 Forward Voltage Drop (Vf) RangeLED lamp load end is generally composed of a number of LEDs connected in series, and its working voltage Vo=Vf×Ns, where Ns represents the number of LEDs. And the Vf of an LED varies with temperature. Generally, at a constant current, Vf becomes lower at high temperatures and becomes higher at low temperatures. The LED lamp load working voltage is VoL at high temperature, and VoH represents a value at low temperature. When selecting an LED driver, consider that the driver output voltage range is greater than VoL～VoH.If the maximum output voltage of the LED driver is lower than VoH, the maximum power of the lamp may not reach the actual power required at low temperature. If the minimum voltage of the selected LED driver is higher than VoL, the output of the driver may exceed the working range at high temperature. And LED driver will work unstable, making the lights flicker.Considering the overall cost and lamp efficiency, don’t blindly pursue the ultra-wide output voltage range of the LED driver. Because the driver voltage is only in a certain range, its efficiency is the highest. When the range is exceeded, the efficiency and power factor (PF) will deteriorate. In addition, if the design of the driver output voltage range is too wide, high costs and unoptimized efficiency will be made. 1.2 Power Margin and Derating RequirementsIn general, the nominal power of the LED driver refers to the data measured under the rated environment and rated voltage. Taking into account different applications, most LED driver suppliers will provide power derating curves in their product specifications (common load vs. ambient temperature derating curves and load vs. input voltage derating curves).As shown in Figure 1, the red curve represents the power derating curve when the input is 120Vac, and its load varies with the ambient temperature. When the ambient temperature is lower than 50℃, the LED driver is allowed to be 100% full load. When the ambient temperature is as high as 70℃, the LED driver can only be derated to 60% of the load. When the ambient temperature changes between 50℃~70℃, the driver load varies with the temperature linearly.Figure 1. Power Derating Curve (Load vs Ambient Temperature)The blue curve represents the power derating curve when the input is 230Vac or 277Vac, and its load varies with the ambient temperature. The principle is similar to the above mentioning.As shown in Figure 2, the blue curve represents the derating curve of the LED driver when the ambient temperature is 55°C, its output power varies with the input voltage. When the input voltage is 140Vac, the load of the driver is allowed to be 100%, and the input voltage will be adjusted downward. If the output power remains the same, the input current will rise, resulting in input terminal loss and lower efficiency. When the device temperature rises, exceeding the rated temperature, which may cause the device to fail.Figure 2. Power Derating Curve (Load vs Input Voltage)Therefore, when the input voltage is less than 140Vac, the output load of the LED driver is required to linearly decrease as the input voltage decreases. According to the above derating curve and corresponding requirements, when choosing a LED driver, actual applying needs are important, as well as the derating margin. 1.3 LED Working CharacteristicsWhen the required input power is a fixed value, such as a fixed error of 5%, the output current can only be adjusted to the specified power for each lamp. Due to different working ambient temperature and different lighting time, the power of each lamp will vary greatly.Although there are considerations for marketing and business factors. However, the volt-ampere characteristic of the LED lamp determines that the LED driver is a constant current source, and its output voltage varies with the series voltage Vo of the LED load. When the efficiency of the driver is basically unchanged, its input power changes with Vo. And meanwhile, the overall efficiency of the LED driver will increase after thermal equilibrium. Under the same output power, the input power will decrease compared to the boot time.Therefore, when formulating requirements, LED driver users should first understand the operating characteristics of LEDs. Avoid suggesting indicators that do not meet the principles of operating characteristics, and indicators that far exceed actual requirements, resulting in excess quality and cost waste. 1.4 Test SessionSample test problems, for example, multi-brand LED driver samples all failed during the test. The reason is that a self-dual voltage regulator is used to directly power the LED driver for testing. After power on, the voltage regulator is gradually adjusted from 0Vac to the rated operating voltage of the LED driver. This kind of test operation can easily make the LED driver start and work with the load when the input voltage is very small, but this situation will cause the input current to be far greater than the rated value. And the internal input terminal related components, such as fuses, rectifier bridges, thermistors, etc. will fail due to excessive current or overheating, damaging the LED driver.The correct test method is to adjust the voltage regulator to the rated operating voltage range of the LED driver, and then connect the driver to power-on test. Of course, technically improving the design can also avoid the failure caused by this kind of test misoperation. That is, a starting voltage limit circuit and an input undervoltage protection circuit are set at the input of the driver. When the input does not reach the start-up voltage set by the driver, the driver does not work. When the input voltage drops to the input undervoltage protection point, the driver enters the protection state. Although the driver has a self-protection function and will not fail, you must carefully understand whether the purchased LED driver product has this protection before testing (considering the actual application environment of the LED driver, most LED drivers currently do not have this set). 1.5 Different Load with Different Test ResultsOn the one hand, when the LED driver is tested with LED lights, the result is normal; on the other hand, when driver tested with an electronic load, the result may be abnormal. Usually this phenomenon has the following reasons:1) The output instantaneous voltage or power of the driver exceeds the working range of the electronic load instrument. (Especially in CV mode, the maximum test power should not exceed 70% of the maximum power of the load. Otherwise, the load may instantaneously have overpower protection when loading, causing the driver to fail to work.)2) The characteristics of the electronic load instrument used are not suitable for measuring constant-current device. And the load voltage gear jumping, result in the drive to fail to work.3) Because there is a large capacitor inside the input of the electronic load meter. The test is equivalent to connecting a large capacitor in parallel with the driver output, which may cause the driver's current sampling work to be unstable. As we all known, the LED driver is designed to meet the working characteristics of LED lamps. The most practical test method is to use the LED lamp as a load, and connects an ammeter and a voltmeter in series to test. 1.6 LED Driver Circuit ProblemThe following conditions often cause damage to the LED driver:Connect AC to the DC output terminal of the driver, causing the driver to fail.Connect AC to the DC/DC output or input of the driver, causing the driver to fail.Connect the output terminal of the constant current to the dimming light, causing the driver to fail.Connect the phase wire to the ground wire causing the driver has no output and the housing is charged. 1.7 Wrong Phase WringTake an international example: the rated working voltage between each phase line and the neutral line is 220V, and the voltage between the phase line and the phase line is 380V. If the driver is connected to two phase wires, after power on, the LED driver input voltage exceeds the rated range, which cause the product to fail.As shown in Figure 3, V1 represents the first phase voltage, V2 represents the second phase voltage, and R1 and R2 respectively represent the drivers normally installed on the line. When the neutral line (N) on the circuit is disconnected, the drivers R1 and R2 on the two branches are connected to the 380V voltage after being connected in series. Because of the difference in input internal resistance, when one of the drivers is charged to start, the internal resistance becomes smaller. Most of the voltage may be applied to another driver, causing the overvoltage damage. Therefore, it is recommended that switches or short-circuiters on the same distribution branch should be disconnected together, not just cut off the neutral line. What’s more, do not put the power distribution fuse on the neutral line to avoid bad effect of the neutral line on the circuit.Figure 3. Neutral Line Open Circuit Diagram 1.8 Grid FluctuationWhen wires of a transformer grid branch is too long and there is large power equipment on the branch, the grid voltage will fluctuate sharply when the large equipment starts and stops. It even causes the grid to be unstable. When the grid voltage exceeds 310V, the drive may be damaged (even if there is an LED lightning protection device, it is useless. Because the lightning protection device is to deal with pulse spikes of tens of uS level, and the fluctuation of the grid may reach tens of mS, or even hundreds of mS) . Therefore, special attention should be paid when there is large electric machinery on the street lighting branch power grid. It is best to monitor the fluctuation range of the power grid or to supply power to the grid transformer separately. 1.9 Frequent Line TripsToo many lights are connected on the same branch, which leads to overload on a certain phase and uneven power distribution among the phases, resulting in frequent line trips. 1.10 Drive CoolingAlthough the LED has high luminous efficiency, only a small part of the energy flowing through the LED is radiated in the form of visible light. And most of the remaining energy is consumed in the LED in the form of heat, so the LED generates more serious heat. When the driver is installed in a non-ventilated environment, the driver housing should be in contact with the lamp housing as much as possible. If possible, apply thermal glue or a thermal pad on the contact surface between the housing and the lamp housing to improve the heat dissipation of the LED driver and ensure the reliability of the driver.Ⅱ LED Driver Maintenance2.1 Multimeter to Detect LED DriverMeasuring the output voltage of the no-load LED driver with a multimeter, if the output voltage is not detected, does it mean that the driver is broken? Look at the following steps:1) The voltage of the non-isolated LED power supply in the no-load state is about 300V tested with a multimeter, and it is about 220V with a PFC.2) Isolating the LED power supply, the voltage in the no-load state, tested with a multimeter, is about 3-5V more than the total voltage of the rated LED series. However, although the output voltage can be tested under no load, it does not mean that it can be normal under load. At this time, it is necessary to connect the corresponding LED light board to see the performance of the LED lighting. If there is no flicker, the output voltage is also equal to the total voltage of LED lights in series connection. This situation can be considered normal, otherwise it fails. If there is no output voltage at no load, the power supply must be broken. 2.2 Identify LED Power SupplyThe LED power supply is widely used in many applications. So how to distinguish the quality of LED power supply is particularly important. A few methods are briefly introduced below. LED Driver ICThe power of IC drive, the quality of IC directly affects the whole power supply. The lighting manufacturer should understand the IC design solution and calculate the cost of the driver, so as to purchase power products at a reasonable price. TransformersThe control chip can be regarded as the brain center of the power supply, while the transformers determine the power and temperature resistance. The transformer is responsible for the transfer of "AC to DC". However, the energy overload will damage the device. The core of the transformer is the magnetic core and the wire package. Electrolytic Capacitors and Ceramic CapacitorsThe quality and life requirements of input electrolytic capacitors are important. However, people tend to ignore the quality requirements of the output capacitor. In fact, the life of the output capacitor also has a great impact on the life of the power supply. The output end has a switching frequency of up to 60,000 times per second, which causes the parasitic resistance of the capacitor to heat up and produce substances similar to scale. Finally, the electrolyte heats up and bursts. Ceramic capacitors: The materials are divided into X7R, X5R and Y5V, and the actual capacitance value of Y5V can only reach 1/10 of the actual value. In addition, the nominal capacitance value only refers to when a capacitor works at 0V. Therefore, this tiny chip with poor options will also lead to a price difference in cost and greatly shorten the life of the power supply. Circuit Design and Welding ProcessJudgment of the pros and cons of the design: Aside from the professional point of view, it can be distinguished by some intuitive methods, such as the neat layout of the components, and soldering points. As for flying leads and manually adding components, it is a serious lack of techniques and efficiency. As we all know, the quality of mechanized production of wave soldering process is definitely better than manual welding. Because the machining process is more neat and uniform. Identification method: whether there is red glue on the back board.The flashing phenomenon of the lamp after a period of use is basically caused by the power supply or the weak welding of the lamp beads. However, it is extremely difficult to detect the virtual welding of products through aging, so AOI must be used to detect the quality of the power supply. Batch Inspection of Aging Racks and High Temperature Aging RoomsNo matter how good the power products are controlled by materials and production processes, they still need to be tested for aging. Because the incoming inspection of electronic components and transformers is difficult to control. Only through the aging of the entire batch of power supplies and the high temperature sampling inspection of the high temperature room. This is a wide-ranging screening to determine whether the materials have safety hazards. Ⅲ LED Driver Circuit ModulationThe LED driver circuit is divided into constant-voltage type and constant-current type according to the power supply to the LED. Constant-current switch type LED driver circuit samples the current flowing through the LED lamp, and gives the output control signal to control the on and off of the switching power tube, which aims to adjust the output current as the set value. The dimming control circuit mainly includes SCR dimming circuit, pulse width modulation (PWM), pulse frequency modulation (PFM), sliding mode control, PWM_PFM, PSM, etc. Let's take pulse width modulation (PWM), pulse frequency modulation (PFM), and sliding mode modulation to introduce in detail below. 3.1 Pulse Width Modulation (PWM)Pulse width modulation, shown in the figure below, refers to the stability of the output voltage by changing the on-time of the switching power tube in each cycle at a specific frequency. That is adjusting the duty cycle to obtain stable output voltage. When the output voltage changes due to the working environment, noise and other factors, the error amplifier samples the voltage change and sends the signal to the control circuit. The control circuit adjusts the duty cycle of the switching power tube signal to maintain the stability of the output voltage.Figure 4. PWM Modulation Based on BUCK StructureFigure 4 (a). Voltage ModeFigure 4 (b). Peak Current ModeFigure 4 (c). Average Current Mode3.1.1 Advantages of PWM(1) The PWM modulation method has high efficiency under heavy load, and has a good dynamic response to load changes.(2) The output ripple voltage is small and the linearity is high.(3) The frequency is stable, the duty cycle adjustment is not restricted, the control is simple, and both the current control mode and the voltage control mode are applicable.3.1.2 Disadvantages of PWM(1) The efficiency of PWM modulation method decreases at light load.(2) The transient response is slow during constant-voltage driving, and a more complicated compensation circuit is required.(3) Accurate current detection circuit is required for constant-current driving. 3.2 Pulse Frequency Modulation (PFM)The pulse frequency modulation is shown in the figure below. Under the condition of a certain on-time of the switching power tube, the output voltage can be controlled by adjusting the off time. When the output voltage changes, the error amplifier samples the feedback signal and sends the output signal compared with the reference signal to the control circuit. The control circuit analyzes the error signal and generates a square wave signal with constant pulse width and varying frequency to control the switch power tube to maintain the stability of the output voltage.Figure 5. Pulse Frequency Modulation Based on BUCK Structure3.2.1 Advantages of PFM(1) The PFM modulation has very high efficiency, better frequency characteristics and higher voltage regulation rate at light load.(2) The PFM modulation has a relatively high transmission signal-to-noise ratio and a good anti-interference ability.(3) The output voltage has a large adjustable range and low power consumption.3.2.2 Disadvantages of PFM(1) The efficiency of the PFM modulation will decrease under heavy load.(2) The frequency spectrum of the output ripple is scattered and irregular.(3) The load adjustment range is very small, resulting in high filtering costs. 3.3 Sliding-Mode ModulationSliding-mode modulation mode, the full name is sliding mode variable structure control,  is a discontinuous control. As shown in Figure 6, the sliding mode makes the system structure change purposefully according to its current state, which force the system to make small amplitude and high frequency up and down movements along the designed trajectory under response conditions. That is, sliding mode movement. Reduce the system's sensitivity to disturbances and load jumps.Figure 6. Sliding Mode Control Based on BUCK Structure3.3.1 Advantages and Disadvantage of Sliding ModeIt has the advantages of fast dynamic response, strong robustness and wide stability range, but it also has a problem that the operating frequency is not fixed. Ⅳ One Question Related to LED Driver4.1 QuestionHow long do LED drivers last?4.2 AnswerWhile the light function of an LED may last for years, drivers can give out much sooner. This is why we recommend name brand LED bulbs for the home, especially those with 25,000 hour rated lives. In general, high power white LEDs use much more current, and need of more complicated drivers. Frequently Asked Questions about LED Drivers Failure Analysis and Maintenance1. What is a LED driver IC?They are configured as either inductorless (charge pump) or switching regulator-based LED drivers that support driving white LEDs in series, parallel or combination. ... Topologies include boost regulator, buck regulator, buck/boost, SEPIC topology LED drivers, and more. 2. What is a LED driver used for?LED drivers are electrical devices that prevent damage to LEDs by regulating the forward voltage (VF) of the LED that changes with temperature, avoiding thermal runaway while delivering a constant current to the LED. LED drivers also aid efforts to meet new energy requirements (e.g., Energy Star). 3. How do I choose an LED driver?Use an LED driver with at least the same value as your LED(s). The driver must have a higher output power than your LEDs require for extra safety. If the output is equivalent to the LED power requirements, it is running at full power. Running at full power may cause the driver to have a shorter life span. 4. Why do LED drivers fail?LED FailureThe LEDs usually fail, because they have been connected to a constant LED driver in parallel. If the LEDs have failed you may want to also replace the LED driver. We usually recommend using a model with an adjustable output, and trimming down the output voltage slightly, to avoid over powering the LEDs. 5. How long do LED drivers last?Namely, the life of the driving circuit expires prior to when the LED stops emitting light or has its brightness dropped. The typical nominal lifetime of these elements is often times less than 25,000 hours, while the lifetime of LED itself could be as long as 50,000-100,000 hours. 6. Why do my LED drivers get hot?If the LED driver is trying to draw DC (not a balanced load circuit) then that can also cause the transformer to overheat.7. What is the difference between a transformer and an LED driver?LED drivers and electronic transformers for retrofit LED lighting are not interchangeable. They differ in output and load compatibility i.e. which LED lights they will work with. The fundamental difference between the two is that LED drivers output DC while electronic halogen transformers output 12VAC.

IntroductionWhat is the difference between a three-phase AC motor and a single-phase AC motor? If you pay attention to it, you will find that single-phase AC motors have more equipment than three-phase AC motors, which is the start capacitor (starting capacitor). The most common one is in various household appliances. Almost all household appliances with motors are equipped with start capacitors. This article will start with the explanation of the principle of the motor start capacitor, and then describe in detail the failure phenomena, causes and test methods of the start capacitors in the two common home appliances, air conditioners and fans. In addition, the article will also explain some basic questions about start/run capacitors. If you want to learn the related knowledge of motor start capacitors, this article must be worthy of your reading.How to test a RUN or START CAPACITOR the CORRECT wayCatalogIntroductionCatalogI The Principle and Causes of Damage of Start Capacitor 1.1 How Does the Motor Work? 1.2 How Does the Start Capacitor Work? 1.3 Reasons for the Burning of the Start CapacitorII Troubleshooting of Start Capacitor in Air Conditioner 2.1 Functional Characteristics of Start Capacitor 2.2 How to Quickly Figure out Whether the Start Capacitor is Damaged 2.3 Why is the Start Capacitor of the Air Conditioner Outdoor Unit Easy to Damage? 2.4 Symptoms and Test MethodsIII How Test the Motor Start Capacitor of a Fan?IV Relevant Knowledge in the Step of Start Capacitor Test 4.1 How to Choose Start Capacitor? 4.2 Precautions for Replacing the Start CapacitorV How to Test the Motor Start Capacitor without a MultimeterVI Frequently Asked Questions about Start Capacitors 6.1 Which Motors are Served by the Start Capacitor? 6.2 Why Does a Three-phase Motor do not Need a Start Capacitor? 6.3 The Relationship Between Start Capacitors and Motor 6.4 What are the Functions of Start Capacitors, Run Capacitors, and Centrifugal Switches in Single-phase Motors?VII QuizⅧ FAQI The Principle and Causes of Damage of Start Capacitor1.1 How Does the Motor Work?The single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field, and a capacitor is needed to separate the phases. The purpose is to make the current in the two windings produce a phase difference of nearly 90 ゜ to generate a rotating magnetic field. The capacitive induction motor has two windings, namely the starting winding and the running winding. The two windings are 90 degrees apart in space. A large-capacity capacitor is connected in series to the starting winding. When the running winding and the starting winding pass single-phase alternating current, the current in the starting winding is 90 degrees ahead of the current in the running winding due to the action of the capacitor, thus reaching the maximum value first. Two identical pulsed magnetic fields are formed in time and space so that a rotating magnetic field is generated in the air gap between the stator and the rotor. Under the action of the rotating magnetic field, an induced current is generated in the motor rotor, and the current interacts with the rotating magnetic field. The electromagnetic field torque causes the motor to rotate.Figure1. Electric-Motor1.2 How Does the Start Capacitor Work?A single-phase power supply is different from a three-phase power supply in that a three-phase rotating magnetic field is generated in the motor. The principle of capacitor starting of single-phase motor is: using the principle that the current of the capacitor in the circuit is advanced by 90 degrees so that a magnetic field of 90 degrees ahead of the main winding is generated in the starting winding so that there will be an alternating 90-degree angle in the motor. The magnetic field, to put it plainly, uses the phase-shifting principle of the capacitor to transform a single-phase power supply into a two-phase power supply of 90 degrees to each other, and a rotating magnetic field of 90 degrees to each other is generated in the motor. Maybe it is easier to understand to explain from this aspect. The start capacitor is to give the motor a thrust when the motor is started so that the motor can turn from moving to rotating. Without it, when a single-phase AC motor starts, it will shake at the origin instead of rotating; the start capacitor is a two-phase AC motor, so the magnetic field cannot exert force on the rotor without it, and of course, it is impossible to rotate.Figure2. Capacitor Start Run Induction Motor1.3 Reasons for the Burning of the Start CapacitorGenerally, the start capacitor is not easy to burn, because its working time is very short, and it is thrown off by the centrifugal switch at the moment of starting, with no current flowing through the start capacitor. However, not easy to burn does not mean that it will never burn. If the start capacitor burns out, the possible reasons are as follows:① Capacitors have low voltage resistance or poor quality, so it is best to use capacitors with a voltage resistance of 500V. ② The centrifugal switch will often produce an arc when it is turned off. It is possible to burn the switch to the motor. After the switch is started, the switch cannot be turned off. There is always current through the capacitor. It is easy to burn the secondary winding of the motor and the start capacitor within a certain period of time. ③ The capacity of the selected capacitor is too small, and the starting current exceeds the allowable value of the capacitor. ④ The motor is bored or the bearing is damaged. It is difficult for the motor to start the centrifugal switch within a certain period of time and it is difficult to reach the disconnected speed, and the start capacitor is easy to burn.Figure3. Deteriorated CapacitorII Troubleshooting of Start Capacitor in Air Conditioner2.1 Functional Characteristics of Start CapacitorThe start capacitor is an important part of the auxiliary compressor to start. The capacitor is a large-capacity capacitor (1~6uF), which is used to provide starting current for the auxiliary winding of the motor to assist the compressor to start. The start capacitor is generally fixed on the bracket or support plate above the compressor, and the pin is connected to the starting end of the compressor.2.2 How to Quickly Figure out Whether the Start Capacitor is DamagedFirst of all, it depends on what kind of capacitors are used in electrical parts.● If the compressor start/run capacitor is damaged, the compressor cannot start or run intermittently.● If the capacitor of the fan motor is damaged, some malfunctions such as excessive exhaust temperature, excessive exhaust pressure, compressor overload, and small air output will occur during the operation of the air conditioner.2.3 Why is the Start Capacitor of the Air Conditioner Outdoor Unit Easy to Damage?The capacitor of the air conditioner compressor is installed in the outdoor unit. Due to the high temperature of the outdoor unit (ambient temperature + temperature emitted by the condenser), capacitors that have been used for a long time will easily dry up and fail the electrolyte.When replacing capacitors, be sure to use high-quality capacitors. The capacity must be the same, and the withstand voltage must not be lower than the original standard.Figure4. Air Conditioner Outdoor Unit2.4 Symptoms and Test Methods① Smoke.② Cannot operate normally.③ The display cannot be displayed normally.④ The power supply cannot be charged and discharged normally.⑤ Can not heat normally.⑥ The power factor compensates the capacitor damage, resulting in a waste of electricity. Different symptoms of damage to compressor start capacitor and external motor start capacitor:There are two capacitors in the outdoor electromechanical packaging, the larger one is the compressor capacitor, and the smaller one is the external motor capacitor. Different capacitors have different failure phenomena. ① Compressor capacitor damagethe performance phenomenon is that normal compressor startup will be accompanied by loud noise and jitter. If the capacitor is damaged, you will feel a slight jitter in the compressor by pressing the casing above the compressor, and the sound is like the sound of current passing. The compressor will stop running after a period of time. ② The damage of the external motor capacitorAfter the compressor is working normally, the external motor stops working for a period of time. If there is a fault code, there will be high-pressure protection and compressor exhaust pipe temperature protection.Figure5. Basic Electrical Controls of Air-Conditioning UnitsTest methods of air conditioner start capacitor● Method 1:The start capacitor of the air-conditioning compressor is a large-capacity electrolytic capacitor. When testing, using the capacitance setting of the digital multimeter to determine whether there is any abnormality.Under normal circumstances, the capacitance of the capacitor used to detect the capacitance of a multimeter should be the same or very close to the nominal capacitance, otherwise, the start capacitor is mostly deteriorating, such as dry electrolyte, leakage, etc., which should be replaced. ● Method 2:In addition to using a multimeter to test its capacitance, the ohmic setting of a pointer multimeter can also be used to test the charge and discharge performance of the start capacitor.Steps:① Connect the red and black test leads to the two poles of the compressor start capacitor.② The multimeter gear is set in ohm gear.③ Under normal circumstances, the pointer of the multimeter first swings to the right to a position, then slowly swings to the left, and finally stops at a fixed position.④If the pointer does not swing or the swing range is small, it indicates that the performance of the compressor start capacitor is poor.Suggested Reading: 5 Ways to Test Capacitors How to replace the start capacitor?If it is ensured that the air conditioner failure is caused by the damage of the compressor start capacitor itself, the damaged compressor start capacitor needs to be replaced. Replacing the start capacitor can generally be divided into three steps: removing the start capacitor, finding a replaceable start capacitor, and replacing the start capacitor. ①Remove the start capacitorThe compressor start capacitor is located on the circuit support board above the compressor. When disassembling, unplug the connecting wire and use a screwdriver to remove the fixing screw of the snap ring. ② Looking for replaceable capacitorsAfter removing the damaged compressor start capacitor, then select a suitable new start capacitor to replace it according to the specifications and volume of the damaged start capacitor.The specific content of how to select the start capacitor will be explained in detail below. ③Replace the start capacitorAfter selecting the compressor start capacitor, install the new compressor start capacitor in the outdoor unit, fix the metal fixing ring, reconnect the connecting cable, and then power on and test the machine to complete the replacement.Figure6. Motor Start CapacitorIII How Test the Motor Start Capacitor of a Fan?①Connect all parts on the circuit board of the air conditioner completely.②Turn on the power supply.③Use the remote control to adjust the temperature to make the fan motor rotate.④Connect the ground terminal of the oscilloscope probe to the ground terminal of the circuit board.⑤Use an oscilloscope probe to detect the white lead on the Hall element plug.⑥The oscilloscope shows signal waveform. When testing the start capacitor of the fan motor, a multimeter should be used to measure the resistance of the capacitor. Due to the large size of the capacitor, it is impossible to use the capacitor input jack for testing. At this time, you can use the multimeter pen to test and judge the quality of the capacitor by the change of the value displayed by the multimeter. If the resistance value displayed by the multimeter changes from small to large and then changes to infinity, it means that this is a good capacitor with charging and discharging functions. Check again after changing the test leads, the displayed value still changes from small to large, and changes to infinity. After measuring the capacitor, if the test leads are not replaced when the test is performed again, and the resistance value is displayed as infinite, it means that charging and discharging are not performed, but it does not mean that the capacitor is damaged. Therefore, when testing the capacitance, the test leads must be replaced no matter which multimeter is used.Figure7. Fan MotorRelated recommendation: How to Test Ceramic Disc Capacitor IV Relevant Knowledge in the Step of Start Capacitor Test4.1 How to Choose Start Capacitor?● How to calculate the starting and running capacitance of a single motorrun capacitance C=120000*I/2.4*f*U*cosφWhere: I is current; f is the frequency; U is the voltage; cosφ is the power factor, taking 0.5 to 0.7. The run voltage of the run capacitor is greater than or equal to (2～2.3) U.Start capacitor capacity = (1.5 ~ 2.5) run capacitor capacity.The run voltage of the start capacitor is greater than or equal to 1.42 U.(It is best when the voltage across the capacitor is 311V during operation) The working capacitor is 1-4UF per 100W, and the start capacitor is 4-10 times the working capacitor (the motor requires a larger starting torque). Empirical data, if your motor does not exceed 200W, the start capacitor will not exceed 100uF. If you run the capacitor, you can choose several values for the power-on test, and see which capacitor has the smallest current in the whole machine, then the capacity of the capacitor is the most Good value.) The capacity of the single-phase split-phase motor capacitor can be calculated by the empirical formula C=35000I/2PUfcos&Such as I=250W/220V=1.2AC=35000x1.2/2x1x50x220X0.8=24ufCan choose 350V30uf capacitance.Figure8. Starting Capacitor Table ● How to calculate the voltage across the run capacitor of a single-phase motor?① First of all, you must know the impedance value of the secondary winding. You can measure the resistance value by measuring the DC resistance with a multimeter. Then, the secondary winding is connected to a 12V AC voltage and the current value is measured. According to the winding impedance equal to the resistance and reactance in series, it can be calculated by phasor Out the winding inductance value. ② In normal operation, the capacitor is connected in series on the secondary winding, that is, the three equivalent parameters of winding resistance, winding reactance, and capacitance are connected in series and then connected to 220V voltage. It is easy to calculate the phasor according to the formula of the series circuit. Calculate the voltage on the capacitor. ③ When a single-phase motor is running, the voltage at both ends of the capacitor is generally above 300VAC, so the capacitor voltage is generally selected for a capacitor with a withstand voltage of 400V or more, and a capacitor with a voltage of more than 450V is better. ④ For the calculation of the capacitance withstand voltage, please refer to Article 2. First, measure the resistance R and reactance XL of the secondary winding, and then select the capacitance C according to the power of the motor to calculate the capacitive reactance Xc.The actual voltage across the capacitor during operation: Uc= Xc*220/(R+jXL-jXc); the withstand voltage value of the capacitor: Uce=1.3~1.5Uc.Figure9. Single Phase Motor Starting ● Detailed selection guide of start capacitor and run capacitorSingle-phase motor capacitor selection.Withstand voltage formula: U (capacitance) is greater than or equal to 1.5*USingle-phase run capacitor formula: C=1950×I/U×cosφ (using a capacitor, which is both a start capacitor and a run capacitor, is commonly used for small-capacity motors such as electric fans and washing machines)Start capacitor capacity formula: C=3500*I/U*cosφ (a capacitor is only used when starting, disconnected during normal operation, and switched with a transfer switch or a centrifugal switch.Dual-value capacitor run capacitor capacity formula: C=1200*I/U*cosφ (use 2 capacitors, one for operation and one for startup)Dual-value capacitor start capacitor capacity formula: C=(2～3)*C (run capacitor) C: Capacitor capacity: I: motor rated current, U: motor rated voltage, cosφ: power factor 0.7.Generally, there is no need to calculate. The run capacitor is 2~3μF per 100W, and the start capacitor is 2~3 times the run capacitor. The capacitor selection of the motor has strict requirements on the voltage, and it must be equal to or greater than 1.5 times the rated voltage of the motor. For a power supply with a rated voltage of 220V, the rated voltage of the capacitor cannot be lower than 400V. The capacitance value has a certain broadness, it doesn't matter if it is larger or smaller, especially the start capacitor, which can be selected at 2-6 times the working capacitor. ● How to choose the capacitor of single-phase asynchronous capacitor start the motorWe can calculate according to the following formulaPhase start capacitor capacity:C=350000*I/2p*f*U*cosφIn the formula: I---current;f-frequency;U---voltage;2p-the larger power factor is 2, and the smaller power factor is 4; cosφ---power factor (0.4～0.8).Split-phase start capacitor withstand voltage:The capacitor withstand voltage is greater than or equal to 1.42*U. Run capacitor capacity:C=120000*I/2p*f*U*cosφIn the formula: I---current;f-frequency;U---voltage;2p-take 2.4;cosφ---power factor (0.4～0.8). Run capacitor withstand voltage:The withstand voltage of the capacitor is greater than or equal to (2～2.3)*U.start capacitor capacity of double-value capacitor motor:C=(1.5～2.5)*operating capacitor capacity.Withstand voltage of start capacitor:The capacitor withstand voltage is greater than or equal to 1.42*U.4.2 Precautions for Replacing the Start CapacitorThe start capacitor is an important part of the electronic circuit. Once the start capacitor is broken, the motor cannot be started. The damaged start capacitor will only make a buzzing sound when it is energized for a short time, causing the current to surge, and long-time energization will cause severe overheating and even burn the motor, so it should be replaced immediately. And it is not difficult to judge that the start capacitor is broken. Most of the damaged start capacitors are bulging, and the surface will be burnt due to excessive current, and the rotor speed will be slow and weak. Of course, the most intuitive and accurate way is to use the capacitance setting of a multimeter to measure the quality. Once we have confirmed that the start capacitor has failed, the things that should be noted when replacing the start capacitor:① After the start capacitor is discharged, there will still be part of the residual charge that cannot be discharged for a while, and an artificial discharge should be performed again. ② Since the failed start capacitor may have poor lead contact, internal disconnection or fuse blown, etc., part of the charge may not be discharged. Therefore, the maintenance personnel should wear insulating gloves before touching the failed start capacitor. Using the short-circuit wire to short the two poles of the faulty capacitor first, and then it can be removed and replaced by hand. ③ If multiple start capacitors are used in series, they should be discharged separately.Figure10. Replace the Start Capacitor④ When handling or replacing a malfunctioning start capacitor, disconnect the power supply of the start capacitor, disconnect the switch or unplug the plug, and discharge the start capacitor. ⑤ When discharging, first connect the grounding terminal of the grounding wire, and then use the grounding rod to discharge the start capacitor several times until there is no discharge spark or discharge sound, and then fix the grounding terminal. ⑥ It should also be noted that general users often ignore the instruction manual, and the precautions for use must be carefully understood and followed during installation. As we all know, the impedance of a capacitor is inversely proportional to frequency. As the frequency increases, the loss also increases. Measures should be taken to limit the harmonics and inrush current in the circuit. Capacitors always generate heat, so pay special attention to ventilation and cooling. After the reactive power compensation device is installed, during the trial operation, the system should be tested, and measures should be taken in time if over-voltage, over-current, oscillation, and harmonics are found, which is very necessary for the normal operation of the capacitor. V How to Test the Motor Start Capacitor without a MultimeterA DC voltmeter can be connected to the capacitor in parallel, and an insulating shaker can be used to charge the capacitor (note the + and-poles)(1) See if the voltage can rise to the rated voltage of the capacitor:① 0V, the capacitor is short-circuited.② Slowly rise to the rated voltage of the capacitor, it proves that the capacitor is good.③ Raised quickly to the rated voltage of the capacitor, and the insulation resistance is about the internal resistance of the DC voltmeter, then the capacitor is open.(2) When it is stable at the rated voltage value of the capacitor, look at the insulation resistance of the capacitor:①The insulation resistance is close to the internal resistance of the DC voltmeter, so the capacitance is good.②If the insulation resistance is less than the internal resistance of the DC voltmeter, it means that the leakage of the capacitor is large, and it is easy to generate heat and cannot be used.VI Frequently Asked Questions about Start Capacitors6.1 Which Motors are Served by the Start Capacitor?Although some electrical appliances seem to have similar principles, they are different in the selection of motors, such as electric fans and air conditioners. Most electric fans use single-phase motors. Single-phase motors have only one 220v live wire and one neutral wire, while air conditioners often uses the three-phase motor, which has three wires, 220v live wire, neutral wire, and 380v live wire. The most obvious difference between a single-phase motor and a three-phase motor is that the number of start capacitors is different. A single-phase motor is equipped with a start capacitor, while a three-phase motor has no start capacitor.6.2 Why Does a Three-phase Motor do not Need a Start Capacitor?Because the three-phase motor itself has three running windings and can generate a magnetic field by itself, the appearance of the magnetic field can effectively replace the start capacitor, so the three-phase motor is generally not equipped with a start capacitor. However, the start capacitor still plays an irreplaceable role in a single-phase motor, because there is only one running winding in a single-phase motor, which cannot form a rotating magnetic field, and the operation of electrical appliances can only rely on the start capacitor. In addition to the start capacitor in a single-phase motor, there is also a run capacitor. Although these two capacitors work together, the function of the start capacitor is much greater than that of the run capacitor, so once they start capacitor is damaged, the fan will make a lot of noise, The blade speed is reduced. If this happens to your electric fan, you might as well try to replace a start capacitor, the problem should be solved.6.3 The Relationship Between Start Capacitors and MotorAt present, in single-phase motors with low-power motors, the start capacitor is connected in series with the starting coil and then connected in parallel with the running coil to work at the same time. In order to speed up the start-up time of the high-power motor, a large capacitor to help start is added. After the motor is started, the additional large start capacitor is disconnected by the centrifugal switch. The smaller capacitor connected in series with the starting coil is responsible for the phase shift required during normal operation. Electric current, the power supply machine is operating normally. Is there a single-phase motor that is connected to the starting coil and connected in parallel with the running coil in the circuit from start to run and does not require other large capacitors to help start? Low-power motors are always used in the circuit. High-power motors have to add additional capacitors due to their large power and large starting distance.Figure11. Torque-speed characteristic6.4 What are the Functions of Start Capacitors, Run Capacitors, and Centrifugal Switches in Single-phase Motors?The start capacitor is used for phase separation, and the purpose is to make the current in the two windings produce a phase difference close to 90 ゜ to generate a rotating magnetic field, allowing the motor to run quickly in a static state. There is an automatic clutch switch in the motor. When the motor is started, the motor will continue to run due to inertia. When the speed reaches a certain speed, the start capacitor will be separated by centrifugal action and automatically connected to the run capacitor, and the motor will enter the normal working state. The function of the run capacitor is to keep the current in the two windings with a phase difference of 90° to generate a continuous rotating magnetic field. For motors with start capacitors, the rotational torque generated by the start capacitors is larger than that of the run capacitors, which is more suitable for starting with a load. Motors without start capacitors are not suitable for starting with a larger load.VII QuizThe starting capacitor of a single phase motor is(A) Electrolytic capacitor(B) Ceramic capacitor(C) Paper capacitor(D) None of the above.Answer: AⅧ FAQ1. What happens when a start capacitor goes bad?A motor connected to a run and start capacitor may still attempt to start if one or both of the capacitors has failed, and this will result in a motor that hums and will not remain running for long. ... In most cases of capacitor problems, such as damage or a loss of charge, the capacitor will need to be replaced. 2. What's the difference between a run capacitor and a start capacitor?The start capacitor creates a current to voltage lag in the separate start windings of the motor. The current builds up slowly, and the armature has an opportunity to begin rotating with the field of current. A run capacitor uses the charge in the dielectric to boost the current which provides power to the motor. 3. How do you test a start capacitor with an ohmmeter?To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity. 4. How to test a motor start capacitor?Motor run capacitor failure symptoms include warm air flowing from the vents inside the home, the air conditioner taking more time than usual to kick on or it turns off before it is programmed to, or there is a constant low hum emitting from the machine that isn't typical. 5. How do I test a capacitor with a multimeter?To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity. 6. How do you check if a capacitor is bad with a multimeter?If the capacitance value is within the measurement range, the multimeter will display the capacitor's value. It will display OL if a) the capacitance value is higher than the measurement range or b) the capacitor is faulty. 7. How do I test a capacitor without a multimeter?Just connect those two ends of the capacitor to a single-phase supply and switch it ON for a few seconds. Then take that two-terminal and short it, you will get a spark. 8. How can I check a capacitor?Put the Analog Multimeter in the Ohmmeter position and if there are multiple ranges, choose a higher range. Connect the leads of the capacitor to the multimeter probes and observe the readings on the multimeter. For a good capacitor, the resistance will be low in the beginning and will gradually increase. 9. Can I replace a start capacitor with a run capacitor?Start capacitors give a large capacitance value necessary for motor starting for a very short period of time (usually seconds long). A start capacitor can never be used as a run capacitor because it cannot handle current continuously. 10. Which is bigger, start or run capacitor?A lot of torque is necessary to start up an AC system, so a start capacitor will have greater capacitance than a run capacitor. 

Ⅰ IntroductionComputer memory is used to store programs and data. The main function of memory is to read and write. For random access memory RAM, their main functions are to read and write, and for read-only memory ROM, the main function is only to read. In general, the memory can be a card,  a floppy disk, etc., they can be active or fixed, which used to access data.This Video Introduce How Memory Store Data and How CPU Access Them.Program is the basis of computer operation, and data is the object of computer operation. Regardless of whether it is a program or data, it is expressed in binary form in the memory, and is collectively referred to as information. In a computer, the memory capacity represents by byte (abbreviated as B) as the basic unit, a byte is composed of 8 binary bits. In addition to bytes, the storage capacity is expressed in KB, MB, GB, and TB (which may be referred to as K, M, G, and T, respectively. For example, 128MB may be referred to as 128M). Among them, 1KB=1024B, 1MB=1024KB, 1GB=1024MB, 1TB=1024GB.CatalogⅠ IntroductionⅡ How to Store and Access Data?Ⅲ What Computer Memory Do?3.1 Explain Computer Memory3.2 Binary Decoder3.3 Chip Select & BusⅣ Example: 8086 MicroprocessorⅤ Technology Improvement5.1 What is Direct Memory Access (DMA)?5.2 DMA Transfer Modes5.3 DMA Transfer Process5.4 DMA Advantages and DisadvantagesⅥ Questions Related to Computer Memory WorksⅡ How to Store and Access Data?Before you know how the electronic memory works, it is necessary to get a general idea of the normal computer operation.Turn on the computer.First, the computer loads data from ROM and executes a power-on self-test (POST) to ensure that all major components are working properly. As part of this test, fast read/write operations check all memory addresses to ensure that there are no errors in the memory chip by memory controller. Read-write operation means writing data to a certain bit and then reading from it.Second, the computer loads the basic input/output system (BIOS) from the ROM. The BIOS provides the most basic information about storage devices, boot order, security, automatic identification functions, and other basic items.The computer loads the operating system from the hard drive into the system RAM. Normally, as long as the computer is turned on, the critical part of the operating system is kept in RAM. This allows the CPU to immediately access the operating system, thereby enhancing the performance and functionality of the entire system.Third, when an application is opened, it will be loaded into RAM. To save RAM space, only the basic part of the app programs are loaded, and then load other parts as needed. After the application is loaded, all files that have been opened for use in the RAM. When saving files and closing applications, files will be written to the designated storage device, and then the loading will be cleared from the RAM. It should be noted a fact that, if the changed files are not saved to the permanent storage device before being erased, they will be lost.In the above operation, every time the content is loaded or opened, it is put into RAM. This only means that it has been put into the temporary storage area of computer so that the CPU can more easily access the information. The CPU requests the required data from the RAM, processes it, and then writes the new data back to the RAM with successive cycles. In most computers, data processing reached millions of times between the CPU and RAM per second.Ⅲ What Computer Memory Do?3.1 Explain Computer MemoryThe memory is composed of a storage body, an address decoder, a read-write control circuit, an address bus, and a data bus.A semiconductor memory is like a small drawer, and there are eight small grids in it. Each small grid is used to store charge. The charge is transferred in or discharged through the wire connected to it. It is easy to understand, if you think of a wire as a water pipe, and the electric charge in the small grid is like the water. Each small drawer in the memory is a place to store data, which we call a cell.Figure 1. Computer MemoryThere are many cells in a memory, and the lines are connected in parallel. When the charge is applied, the charge will fill all the cells. When the charge is released, the charge in each cell will be discharged. This is of course not what we want. To avoid it, the memory structure should be changed slightly. There is a control line on each cell. Which unit you want to put the data, a signal is given to the control line of this unit. Be specific, this control line is like a switch, the charge can flow freely when switch on, and there is no signal on the control line of other cells to affect each other. So as long as you control the control lines of different cells, you can write different data to each unit. In the same way, if you want to get data from a unit, you only need to turn on the corresponding control line. 3.2 Binary DecoderFigure 2. DecoderFirst, how to control the control lines of each unit is not easy. For example, there are 655,36 units in a 27,512 memory chip, and each wire has to led out. This integrated circuit must have more than 60,000 pins. Obviously, this is clumsy. At this time, a way of decoding is made. Let’s briefly introduce it: a line can represent 2 states, 2 lines can represent 4 states, etc. And so on, 16 lines can be represented for 65536 states. 3.3 Chip Select & BusNext to the last question, let us focus on another problem. Where did the eight wires connect with each cell come from? In general, there are connected from the computer, and they also connect with other parts except for a memory chip. In this case, a problem arises. Since these eight lines are not dedicated between the memory and the computer, if you always connect a unit to these eight lines, this may cause confusion . For example, the value in a memory unit is 0FFH, in other memory cell is 00H, it is hard to figure out the high level or low level which these lines represent. So we have to separate them. The method is, when external wires are connected to the pins of the integrated circuit, they are not directly connected to each unit, and a group of switches is added between chip and computer. If we really want to write data to this memory or read data from the memory, then just turn on the switch. This group of switches is selected by three leads: read control end, write control end and chip select end.To write data to the chip, first is selecting chip, then send out a write signal, and the switch is turn on, therefore, the data is written to the chip. If you want to read, first is selecting chip, then send the read signal, and the switch is closed, the data is sent out. In addition, the read and write signals should be connected to another memory. Since the chip select terminals are different, there is no conflict when having read or write operation. Many people still have a question, will these two chips be selected at the same time? As long as it is a well-designed system, it will not happen, because it is controlled by mathematical calculation, not a manual control. If there is a situation where two chips are selected at the same time, it may be a circuit failure.It can be seen from the above that the eight lines used to transfer data are not dedicated, but are shared by many components, so we call them the data bus. The other 16 address lines are also connected together, called the address bus. Ⅳ Example: 8086 MicroprocessorThe CPU is connected to the storage unit and the I/O interface circuit through an address bus, a data bus, and a control bus. So how does the CPU access a certain address in the memory? Next, we will use the 8086 microprocessor architecture as an example, because its structure is simple and easy to introduce.Figure 3. 8086 MicroprocessorIf the CPU wants to read and write memory data, there must be wires to connect them together. In a computer, this kind of wire is called a bus. If you disassemble the computer case, it is easy to see that there is a collection of wires. These wires send signals at the same time, and each wire is either high or low level. The bus is divided into address bus, data bus and control bus according to different functions. Taking the above diagram as an example, the CPU needs to read the information at address 3, which is roughly divided into the following steps:The CPU outputs the physical address 3 to the address line.The control line needs to select the corresponding storage device, and then inform the device that data will be read from it.The storage device will send data 8 to the data line.Figure 4. 8086 CPUFrom here we see that there is a bus connection between the CPU and the storage device. In fact, there is a bus inside the CPU. It connects different components, such as registers, operators, and controllers. But in the computer, the bit number of different buses is not necessarily the same. For example, the internal bus of the 8086 is 16-bit, the address bus is 20-bit, and the data bus is 16-bit.Another question, since the 8086 CPU is a 16-bit structure, how can it output a 20-bit physical address? In fact, it is very simple. The address adder solves this way: segment address (16 bits) x 16 + offset address (16 bits) = physical address (20 bits), such as B800H x 16 + 1111H = B9000H. If you are not familiar with hexadecimal, then we can use decimal to describe this calculation. For example, home, school, and library are on a straight line. The distance between them is as shown in the following figure. Show the distance of the library: the library is 200m away from home, which is actually its physical address. But now there are some limits. We can only communicate with our friends through paper slips. Unfortunately, we can only write 2 digits on the papers, and the number of paper is not limited, so we agreed on the rule: paper 1x10+paper 2 = physical address, for example, write 11 on the paper 1 and 90 on the paper 2, which means that the school is 110m away from home and the library is 90m away from the school. The maximum distance that these two pieces of paper can represent is 99x10+99=1089.The above mentioned is the concept of offset address of the segment address. Let's imagine that if the internal bus of the 8086 CPU is 20-bits, it can directly represent the physical address. Therefore, the concept of segment address is not critical here. What's more, the CPU hardware design will change in the future, and it may be completely different.What we should know is how the CPU reads information from memory, and how does the CPU know whether the read information is ordinary data or a program that needs to be executed? We know that there are many registers in the CPU (that is used to store information), it specifies 2 registers, called CS, IP (CS is used to store the segment address, IP is used to store the offset address). They represent the physical address of the current machine code that needs to be executed. During the execution of the code, the CPU will maintain the values of CS and IP. For example, for each execution of the machine code, IP will increase the corresponding value to point to the next instruction. By analogy, we can use other registers to represent the physical address of the data. Therefore, the binary information in the memory has no difference to the CPU. Distinguishing the program and the data depends on the register. Ⅴ Technology ImprovementWhether it is a PC card or the high-speed read-write disk in the storage system, we can not do data operation without the support of a hardware DMA.Figure 5. 8237 DMA Controller5.1 What is Direct Memory Access (DMA)?DMA refers to the interface technology that the external device directly exchanges data with the system memory without going through the CPU. It is a high-speed data transfer method that allows direct reading and writing of data between external devices and memory, neither through the CPU nor CPU intervention.To read the data of the peripherals into the memory or transfer the data of the memory to the peripherals, it is generally done through CPU control, such as CPU program query or interrupt mode. Using interrupts for data transfer can greatly increase the CPU utilization. But it has shortcomings. For a high-speed I/O device and the case of batch exchange of data, the DMA method can be used to solve the efficiency and speed problems. DMA directly exchanges data between peripherals and memory, so the speed of data transfer depends on the working speed of the memory and peripherals.The data transfer operation is performed under a "DMA controller". In addition to the CPU doing a little processing at the beginning and end, the CPU can execute other tasks during the transfer. In this way, the CPU and I/O are in parallel operation. Therefore, the efficiency of the computer system is greatly improved.Figure 6. DMA ControllerWhen realizing DMA transmission, the bus is directly controlled by the DMA controller. Therefore, there is a problem of bus control transfer. That is, before the DMA transfer, the CPU should hand over the bus control to the DMA controller, and after the transfer is done, the DMA controller should immediately return the bus control to the CPU. 5.2 DMA Transfer ModesPeripherals can directly access the memory through the DMA controller, and at the same time, the CPU can continue to execute programs. So how does the DMA controller and CPU use memory in the same time? The following three modes are generally used:(1) Burst mode: Stop the CPU access.(2) Cycle stealing mode: DMA return the control of buses to CPU after transfer of one word at a time.(3) Transparent mode: DMA and CPU access memory alternately. Burst ModeWhen the peripheral device requests to transfer a batch of data, the DMA controller sends a stop signal to the CPU, requesting the CPU to give up to use the address bus, data bus, and related control bus. After the DMA controller obtains the bus control right, it starts the data transfer. After a batch of data has been transferred, the DMA controller informs the CPU that it can use the memory and returns the bus control to it. Figure (a) is a time chart of this transmission method. Obviously, in this DMA transfer process, the CPU is basically in a non-working state or stands by.Advantage: It is suitable for group transmission of equipment requiring high data transmission rate.Disadvantages: In the internal access stage of the DMA controller, the memory performance is not fully utilized, that is to say, a considerable part of the memory work cycle is idle. This is because the interval between two data transmitted by a peripheral device is generally always greater than the memory storage period, even for high-speed I/O devices. For example, a floppy disk requires about 32us to read an 8-bit binary number, and the storage period of semiconductor memory is less than 0.5us, so many idle storage periods cannot be used by the CPU. Cycle Stealing ModeWhen the I/O device has no DMA request, the CPU accesses the memory as required by the program. Once the I/O device executes a DMA request, one or several memory cycles will be embezzled.The time sharing of this transmission mode is as follows:　1) At this time, the CPU does not need to access RAM, for example, the CPU is executing a multiplication instruction. Due to the long execution time of this instruction, there is no conflict between the I/O access and the CPU access, that is, the I/O device stealing one or two memory cycles has no effect on the CPU execution.2) When the I/O device accesses, so does the CPU, which creates an access violation. In this case, the I/O device takes first, because it has an access time requirement, the previous I /O data must be accessed before the next request arrives. Obviously, the I/O device steals one or two memory cycles, which means that the CPU has delayed the execution of the instruction. More specifically, inserting a DMA request during the CPU's execution of the in-access instruction embezzles one or two memory cycles. Compared with the method of stopping CPU access, cycle stealing not only achieves I/O transfer, but also utilizes the efficiency of memory and CPU greatly. It is a win-win  method. However, I/O device diversion has the process of applying for bus control, establishing line control, and returning bus control. Transferring a word takes one cycle for RAM, but it is generally 2-5 memory cycles for DMA controllers (depending on the delay of the logic line). Therefore, the method is suitable for the case where the read/write cycle of the I/O device is greater than the RAM storage cycle. Transparent ModeIf the CPU's work cycle is much longer than the memory access cycle, this method can make the highest efficiency of CPU and DMA access at the same time. Assuming that the CPU work cycle is 1.2μs and the memory access cycle is less than 0.6μs, then a CPU cycle can be divided into two sub-cycles, C1 and C2, where C1 is for DMA controller access and C2 is for CPU access.The time sharing of this transmission method is as follows: The following figure is the detailed time of DMA and CPU alternate accesses. C1 is dedicated to the DMA controller and C2 is dedicated to the CPU. This method does not require the bus usage right. It is allocated through C1 and C2. The CPU and the DMA controller each have their own control registers such as internal address registers, data registers, and read/write signals. In the C1 cycle, if the DMA controller has an access request, it can send signals such as address and data to the bus. In the C2 cycle, if the CPU has an access request, it also do the same process. In fact, for the bus, this is a multiplexer controlled by C1 and C2. This transfer of bus control power takes almost no time, so the efficiency of DMA transfer is very high.It is like transparent glass to the CPU, without any influence. Working in a transparent DMA mode, the CPU neither stops the main program running nor enters the stand-by state. It is an efficient working method, and the corresponding hardware logic is more complicated. 5.3 DMA Transfer ProcessFigure 7. DMA Working ProcessRequestThe CPU initializes the DMA controller and gives an operation command to the I/O interface, then the I/O interface issues a DMA request. ResponseThe DMA controller determines the priority and shielding of the DMA request, and makes a bus request to the bus adjudication logic. When the CPU executes the current bus cycle, the bus control can be released. At this time, the bus arbitration logic outputs a bus response, indicating that the DMA has responded, and notifies the I/O interface to take the DMA transfer through controller. TransferAfter the DMA controller obtains the bus control right, the CPU immediately stops s or only performs internal operations. The DMA controller outputs read and write commands to control the RAM and I/O interface directly. Under the control of the DMA controller, the data is directly transferred between the memory and the external device. In addition, it is necessary to provide the starting position and length of the data to be transferred. Rising an InterruptWhen the specified batch of data transfer is finished, the DMA controller releases the bus control right and sends an end signal to the I/O interface. When the I/O interface receives it, on the one hand, it stops the I/O device, on the other hand, it makes an interrupt request to the CPU. The CPU is free from the state of non-intervention, and performs a section to check the correctness of the DMA transfer operation code. Finally, the CPU will show the transfer result and carry out the original program.It can be seen that the DMA transfer method does not require the CPU to directly control the transfer, nor does it have the process of retaining and restoring the scene when having the interrupt process. Through the hardware, a direct path for data transfer is opened for the RAM and I/O devices, that is DMA. 5.4 DMA Advantages and DisadvantagesA:DMA reduces the clock cycle requires to read or write a patch of data, which improve the system operation efficiency.D:As a hardware device, running DMA control will increase cost.DMA can cause cache coherence problem. Ⅵ Questions Related to Computer Memory Works1. What is the purpose of computer memory?Computer random access memory (RAM) is one of the most important components in determining your system's performance. RAM gives applications a place to store and access data on a short-term basis. It stores the information your computer is actively using so that it can be accessed quickly. 2. What is the role of memory in a computer system?Computer memory or random access memory (RAM) is your system's short-term data storage; it stores the information your computer is actively using so that it can be accessed quickly. The more programs your system is running, the more memory you'll need. 3. Where is 8086 microprocessor used for?8086 Microprocessor is an enhanced version of 8085Microprocessor that was designed by Intel in 1976. It is a 16-bit Microprocessor having 20 address lines and16 data lines that provides up to 1MB storage. It consists of powerful instruction set, which provides operations like multiplication and division easily. 4. How does direct memory access DMA work?With DMA, the CPU first initiates the transfer, then it does other operations while the transfer is in progress, and it finally receives an interrupt from the DMA controller (DMAC) when the operation is done. ... DMA can also be used for "memory to memory" copying or moving of data within memory. 5. Why is DMA faster than CPU?The direct memory access or DMA mode of data transfer is faster amongst all the mode of data transfer . ... The device request the cpu through a DMA controller to hold its data ,address and control bus so that the device may transfer data directly to or from memory.

I IntroductionSummary: A photoresistor, or light-dependent resistor (LDR), is a passive electronic component that decreases in resistance as light intensity increases. Driven by the expansion of IoT and smart home automation, the global photoresistor market is projected to reach $553.75 million by 2025. This guide covers LDR working principles, circuit diagrams, types, and step-by-step Arduino integration.Photoresistor or light-dependent resistor (abbreviated as LDR) or photoconductor is a special resistor made of semiconductor materials such as cadmium sulfide or cadmium selenide. Its working principle is based on the internal photoelectric effect. The stronger the light, the lower the resistance value. With the increase of the light intensity, the resistance value decreases rapidly, and the bright resistance value can be as small as 1KΩ or less. The photoresistor is very sensitive to light, and it shows a high resistance state when there is no light, and the dark resistance can generally reach 1.5MΩ.This article includes an overview of the basic information of the photoresistor and two Arduino tutorials for the photoresistor. The content is very comprehensive and detailed. You can choose the part you want to read or read the full text. We hope this article is helpful to you!II What is a Photoresistor?2.1 What is the Definition of a Photoresistor?A photoresistor (also known as a light-dependent resistor, LDR, or photo-conductive cell) is a passive electronic component that decreases its electrical resistance as the luminosity on its sensitive surface increases.2.2 What is the Symbol for a Photoresistor?The standard schematic symbols for a photoresistor are generally represented by the designators "RL", "RG", or "R", often accompanied by a resistor icon enclosed in a circle with incoming arrows indicating light. The following figure shows the schematic symbols of the photoresistor.Figure1. Photoresistor SymbolRecommended Reading: To learn more about Resistor Symbol.2.3 What is the Composition of a Photoresistor?(1) The structure of the photoresistorA photoresistor is primarily composed of a photosensitive semiconductor layer, a glass substrate or moisture-proof film, and comb-shaped ohmic electrodes.Figure2. The Structure of PhotoresistorMaterials for manufacturing photoresistorsThe materials used for manufacturing photoresistors are primarily semiconductors such as metal sulfides, selenides, and tellurides. Usually, coating, spraying, sintering and other methods are used to make a very thin photoresistor and comb-shaped ohmic electrode on the insulating substrate, and then the lead is taken out and encapsulated in a sealed housing with a light-transmitting mirror to prevent moisture from affecting its sensitivity.III How Does the Photoresistor Work?3.1 What is the Working Principle of Photoresistors?How does a photoresistor workThe working principle of a photoresistor is based entirely on the internal photoelectric effect, where incident light energy excites electrons into the conduction band, thereby lowering the component's electrical resistance.A voltage is applied to the metal electrodes at both ends of the photoresistor, and a current flows through it. When irradiated with light of a certain wavelength, the current will increase with the increase of light intensity, thereby achieving photoelectric conversion. After the incident light disappears, the electron-hole pairs generated by the photon excitation will recombine, and the resistance of the photoresistor will return to its original value.The photoresistor has no polarity and is purely a resistive device. It can be used with either DC voltage or AC voltage. The conductivity of a semiconductor depends on the number of carriers in the semiconductor conduction band. Why is the value of the photoresistor related to the wavelength of the incident light?Simply put, it is the effect of transitions between energy levels. Photons at different wavelengths have different energies, and an electron can only absorb one photon. After an electron absorbs a photon, whether it can be converted from non-conductive to conductive electrons depends on the photon’s Energy, and the number of electrons that can conduct electricity determines the resistance of the photoresistor. Therefore, the light wavelength also affects the resistance of the photoresistor.Recommended Reading: See more about light sensor, wavelength, spectrum and photometric physical quantity.3.2 What is the Internal Photoelectric Effect?The internal photoelectric effect is a phenomenon where the absorption of photons causes a change in the electrical conductivity of a semiconductor material. This differs from the external photoelectric effect, which involves the complete escape of electrons from the material's surface.The internal photoelectric effect can be divided into two main categories:Photoconductive effect: The change in electrical resistance due to light exposure.Photovoltaic effect: The generation of a photo-induced electromotive force (voltage) across a P-N junction. PhotoconductivityThe photoconductive effect is one of two internal photoelectric effects. The internal photoelectric effect refers to the phenomenon that the electrical conductivity of a semiconductor exposed to light changes or a photo-induced electromotive force is generated. Among them, the phenomenon that the conductivity of the semiconductor changes due to light is called the photoconductivity effect.Figure3. Energy Level of AtomPhotovoltaic EffectThe photovoltaic effect occurs when a P-type and N-type semiconductor are combined, creating a P-N junction. The process unfolds as follows:Electrons in the N-type semiconductor and holes in the P-type semiconductor diffuse, forming a depletion region near the interface.When light irradiates the P-N junction, photons with sufficient energy generate minority carriers (electron-hole pairs).Under the action of the internal electric field, excited electrons flow to the N-type region, and holes flow to the P-type region.This accumulation generates an additional electromotive force (voltage) at both ends of the P-N junction.If connected to an external circuit, current flows from the P region to the N region, producing usable electrical power.Figure4. Photovoltaic EffectIV How Do You Wire a Photoresistor Application Circuit?Schematic circuit for conventional applicationsFigure5. Schematic CircuitModule parameters:Working voltage: DC3.3-5VPhotoresistor Model: 5516Module pins: 3-pin or 4-pin (an additional analog output AO for 4-pin)Common circuit diagram （3）Photoresistor application circuit diagramThe following figure is a schematic diagram of the application of the photoresistor in the light control switch. The photoresistor is connected in series with the resistor R1. When there is no light, that is, the voltage across R1 does not reach the turn-on voltage of the Q1 transistor. Once exposed to light, the resistance of the photoresistor drops rapidly. The voltage across R1 rises and the transistor turns on, which causes the transistor Q2 in the subsequent stage to turn on, and finally, the switch K opens and the bulb works.Figure6. Common Photoresistor Application Circuit Diagram(4) Photoresistor dimming circuitThe following figure is a typical light-controlled dimming circuit. Its working principle is: when the surrounding light becomes weak, the resistance of the photoresistor RG increases, which increases the partial voltage added to the capacitor C, which in turn makes the thyristor's conduction angle is increased to achieve the purpose of increasing the voltage across the lamp. Conversely, if the surrounding light becomes brighter, the resistance of RG decreases, resulting in a smaller conduction angle of the thyristor, and the voltage across the lamp decreases at the same time, dimming the light, thereby controlling the illuminance of the lamp.Figure7. Photoresistor Dimming CircuitNote: The rectifier bridge in the above circuit must be a DC pulsating voltage, and it cannot be converted into a smooth DC voltage by capacitor filtering, otherwise the circuit will not work properly. The reason is that the DC pulsating voltage can not only provide the basic conditions for the zero-crossing shutdown of the thyristor, but also enable the charging of the capacitor C to start from zero every half cycle, and accurately complete the synchronous phase-shift triggering of the thyristor.V What Are the Types of Photoresistors?5.1 Classification by MaterialsBased on material composition, photoresistors are classified into intrinsic (pure semiconductor) and extrinsic (doped semiconductor) types. Polycrystalline and single crystal photoresistors can also be divided into cadmium sulfide (CdS), cadmium selenide (CdSe), lead sulfide (PbS), lead selenide (PbSe), indium antimonide (InSb) photoresistors, etc. 5.2 Classification by Spectral CharacteristicsSpectral TypeCommon MaterialsPrimary Applications (2026)Ultraviolet (UV)Cadmium sulfide, Cadmium selenideUV detection, environmental monitoringInfrared (IR)Lead sulfide, Lead telluride, Indium antimonideAstronomical detection, non-contact measurement, IR communicationVisible LightSelenium, Silicon, Germanium, Zinc sulfideIoT smart lighting, automatic street lights, exposure devices(1) Ultraviolet photoresistor: sensitive to ultraviolet rays, including cadmium sulfide, cadmium selenide photoresistors, etc., used to detect ultraviolet rays.(2) Infrared photoresistors: mainly lead sulfide, lead telluride, and lead selenide. Photoresistors such as indium antimonide are widely used in missile guidance, astronomical detection, non-contact measurement, human disease detection, infrared spectroscopy, infrared communication and other national defense, scientific research, and industrial and agricultural production.(3) Visible light photoresistors: including selenium, cadmium sulfide, cadmium selenide, cadmium telluride, gallium arsenide, silicon, germanium, zinc sulfide photoresistors, etc. Mainly used in various photoelectric control systems, which account for a significant portion of the projected $553.75 million global photoresistor market in 2025. Applications include IoT smart lighting, automatic turning on and off of navigation lights, street lights and other lighting systems, automatic water supply and automatic water stop devices, automatic protection devices on machinery and "position detectors" Thickness detectors for thin parts, automatic exposure devices for cameras, photoelectric counters, smoke alarms, photoelectric tracking systems, etc.Figure8. Light Dependent ResistorVI The Main Parameters and Basic Characteristics of the Photoresistor6.1 What Are the Main Parameters of a Photoresistor?The main parameters of a photoresistor define its operational limits, sensitivity, and response time in various lighting conditions.1) Bright resistance (kΩ): refers to the resistance value of the photoresistor when exposed to light.2) Dark resistance (MΩ): refers to the resistance value of the photoresistor when there is no light exposure (dark environment).3) Maximum working voltage (V): refers to the highest voltage the photoresistor is allowed to withstand under the rated power.4) Bright current: refers to the current that the photoresistor passes when it is irradiated by light under the specified applied voltage.5) Dark current (mA): refers to the current that the photoresistor passes under the specified applied voltage when there is no light.6) Time constant (s): refers to the time required for the photoresistor to start from the light jump to stabilize 63% of the bright current.7) Resistance temperature coefficient: refers to the relative change of the resistance value of the photoresistor when the ambient temperature changes by 1°C.8) Sensitivity: refers to the relative change of the resistance value of the photoresistor with and without light irradiation.Figure9. LDR6.2 Basic Characteristics(1) Dark resistance and bright resistanceThe stable resistance value measured by the photoresistor under room temperature and total darkness is called dark resistance. The current flowing at this time is called dark current. For example, MG41-21 type photoresistor dark resistance is greater than or equal to 0.1M.The stable resistance value measured by the photoresistor at room temperature and under certain lighting conditions is called bright resistance. The current flowing at this time is called the bright current. The bright resistance of MG41-21 type photoresistor is less than or equal to 1k.    The difference between bright current and dark current is called photocurrent.    Obviously, the larger the dark resistance of the photoresistor, the better, and the smaller the bright resistance, the better, that is, the dark current should be small and the bright current should be large, so the sensitivity of the photoresistor is high.Figure10. Bright Current and Dark Current(2) Volt-ampere characteristicsUnder a certain illuminance, the relationship between the voltage applied across the photoresistor and the current flowing through the photoresistor is called the volt-ampere characteristic. The volt-ampere characteristic of the photoresistor is approximately a straight line, and there is no saturation phenomenon. Due to the limitation of power dissipation, the voltage across the photoresistor cannot exceed the maximum operating voltage during use. The dotted line in the figure is the allowable power consumption curve, from which the normal operating voltage of the photoresistor can be determined. (3) Photoelectric characteristics    The relationship between the photocurrent of the photoresistor and the illuminance is called the photoelectric characteristic. The photoelectric characteristics of the photoresistor are nonlinear. Therefore, it is not suitable as a detection element, which is one of the shortcomings of the photoresistor. In automatic control, it is often used as a switching photoelectric sensor.Figure11. Characteristics of the Photoelectric Effect(4) Spectral characteristicsFor incident light of different wavelengths, the relative sensitivity of the photoresistor is different. The spectral characteristics of various materials are shown in Figure 2.6.4. It can be seen from the figure that the peak value of cadmium sulfide is in the visible light region, and the peak value of lead sulfide is in the infrared region. Therefore, when selecting the photoresistor, the types of components and light sources should be considered in order to obtain satisfactory results. (5) Frequency characteristicsWhen the photoresistor is exposed to pulsed light, the photocurrent will reach a steady-state value after a period of time. When the light suddenly disappears, the photocurrent will not be zero immediately. This shows that the photoresistor has time-delay characteristics. Because different materials have different time delay characteristics of photoresistors, their frequency characteristics are also different. Figure 2.6.5 shows the relationship between the relative sensitivity Kr and the light intensity change frequency f. It can be seen that the use frequency of lead sulfide is much higher than that of thallium sulfide. However, most photoresistors have large time delays, so they cannot be used in situations where fast response is required. This is a defect of photoresistors. (6) Temperature characteristicsLike other semiconductor devices, the photoresistor is greatly affected by temperature. When the temperature increases, its dark resistance will decrease. Changes in temperature also have a great influence on spectral characteristics. Figure 2.6.6 is the spectral temperature characteristic curve of the lead sulfide photoresistor. It can be seen from the figure that its peak value moves to the short wavelength direction as the temperature rises. Therefore, in order to improve the sensitivity, or in order to receive far-infrared light, cooling measures are taken.Figure12. Temperature CharacteristicsSpectral Temperature Characteristics of Lead Sulfide Photoresistor A commonly used photoresistor is a cadmium sulfide photoresistor, which is made of semiconductor material. The resistance of the photoresistor changes with the intensity of the incident light (visible light). Under dark conditions, its resistance (dark resistance) can reach 1~10MΩ; under strong light conditions (100LX), its resistance (Bright resistance) Only a few hundred to thousands of ohms. The sensitivity of the photoresistor to light (the spectral characteristics) is very close to the human eye's response to visible light (0.4~0.76) μm. As long as the human eye can sense the light, it will cause its resistance to change. Therefore, when designing the light control circuit, the incandescent bulb (small electric bead) light or natural light is used as the control light source, which greatly simplifies the design.Figure13. Photoresistor Characteristic CurveThe corresponding resistance change of the photoresistor with the intensity of the incident light is not linear, so it cannot be used for the linear conversion of the photoelectricity. This is where the user should pay attention. Beginners can purchase a photoresistor (MG45 type), at night a 60~100W incandescent lamp, use a multimeter to directly measure the resistance of the photoresistor. When measuring, the photoresistor should be aimed at the light of the incandescent lamp, and then gradually distance from the lamp (from near to far), observe the change of the resistance value indicated by the multimeter, and the special characteristics of the photoresistor can be visually verified.Commonly used photoresistor models are sealed MG41, MG42, MG43 and unsealed MG45 (cheap price). Their rated power is below 200mW.VII How to Use a Photoresistor with Arduino?7.1 LED Control with Photoresistor and ArduinoLED Control with LDR (Photoresistor) and Arduino7.2 How to Measure Light Intensity Using a Photoresistor (Arduino)In the data collection of modern smart home systems, the measurement of light intensity is highly necessary. For example, indoor IoT lighting can be automatically adjusted according to the intensity of the light to provide users with the most comfortable environment. The tutorial here will use a photoresistor to cooperate with Arduino to complete the light data collection.(1) MaterialsArduino UNO development boardBreadboardPhotoresistor1K-10K resistance(2)Wiring method Figure14. Wiring MethodThe resistance of photosensitive resistors is very high in the condition of no light. The stronger the light, the smaller the resistance. By measuring the voltage variation on both sides of the photosensitive resistance, the variation of the photosensitive resistance can be known and the light intensity can be obtained. In the connection diagram, we find that a partial voltage resistor is connected in series for the photosensitive resistor.Figure15. CircuitIn the above figure, RL is a photoresistor, R1 is a series resistor, Vout=RLR1+RL∗Vin, in the dark, the resistance of RL will be very large, so Vout is also very large, close to 5V. Once the light is irradiated, the value of RL will decrease rapidly, so Vout will decrease accordingly. It can be seen from the above formula that R1 should not be too small, preferably around 1k~10k, otherwise the ratio will not change significantly. (3) CodeThe code part is very simple, just read the analog value of the interface connected to the photoresistor.1 light = analogRead(0);Open the serial monitor of Arduino, illuminate the photoresistor with the flashlight of the mobile phone, and observe the result:2 Serial.println("lignt :");3 Serial.println(light);7.3 Use Experiment of Arduino Photoresistor(1) MaterialsArduino UNO x1Photoresistor x1resistance 10K, 4.7K, 1K x several (or need one, but you can test the difference between different resistance values and data)(2)Wiring method Figure16. Wiring Method(3)Program#define AD5 A5 //Define analog port A5#define LED 13 //Define digital port 13 int Intensity = 0;//Illuminance value void setup() //Program initialization{   pinMode(LED, OUTPUT);//Set LED to output mode   Serial.begin(9600);//Set baud rate 9600} void loop() // Program body loop{   Intensity = analogRead(AD5); //Read the value of analog port AD5 and save it in the Intensity variable   Serial.print("Intensity = "); //Serial output "Intensity = "   Serial.println(Intensity); //The serial port outputs the value of the Intensity variable and wraps   delay(500); //Delay 500ms}(4) Power on, view serial dataTest Results:Figure17. Test ResultsThe above data is the change of the value with the flashlight and no light.(5) SummaryThe positive and negative poles are reversed and the values are reversed. The larger the resistance value, the larger the change range. Using 5V, the range is larger than 3.3V.Recommended Reading: Arduino&mBlock light sensorVIII How to Use Multimeter to Detect the Quality of Photoresistor?Measure the dark resistance: Use a black piece of paper to cover the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter remains basically unchanged, and the resistance value is close to infinity. The larger the value, the better the performance of the photoresistor. If this value is very small or close to zero, it means that the photoresistor has been burnt through and damaged and can no longer be used.Measure the bright resistance value: Point a light source to the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should have a large amplitude swing, and the resistance value is significantly reduced. The smaller the value, the better the photoresistor performance. If this value is large or even infinite, it indicates that the internal open circuit of the photoresistor is damaged and can no longer be used.Test intermittent light response: Align the light-transmitting window of the photoresistor with the incident light, and use a small piece of black paper to shake the upper part of the light-shielding window of the photoresistor to make it receive light intermittently. At this time, the pointer of the multimeter should swing left and right with the black paper. If the pointer of the multimeter always stops at a certain position and does not swing with the shaking of the paper, it means that the photosensitive material of the photoresistor has been damaged.IX A Quiz about the PhotoresistorPhotoresistors, potentiometers, and thermistors are all ________.A. OutputsB. Digital inputsC. Analog inputsD. ThroughputsAnswer: CFrequently Asked QuestionsWhat is the difference between a photoresistor and a photodiode?A photoresistor is a passive component that changes resistance based on light intensity, making it slower but easier to use. A photodiode is an active semiconductor with a P-N junction that converts light into current, offering much faster response times for high-speed applications.Is a photoresistor an analog or digital component?A photoresistor is fundamentally an analog component. Its resistance changes continuously in response to varying light levels. However, when paired with a microcontroller like an Arduino and a voltage divider, its analog signal can be easily converted into digital data.What are the main types of photoresistors?Photoresistors are primarily categorized into intrinsic and extrinsic types. Intrinsic photoresistors use pure semiconductors like silicon, while extrinsic types use doped materials to detect longer wavelengths, such as infrared light, making them ideal for specialized sensors.How is a photoresistor used in smart home circuits?In modern IoT and smart home systems, photoresistors act as ambient light sensors. They automatically trigger actions like turning on outdoor security lights, adjusting indoor smart bulb brightness, or activating motorized blinds when sunlight reaches a specific threshold.{ "@context": "https://schema.org", "@graph":[ { "@type": "Article", "headline": "Photoresistor Basics: Working Principle, Types, and Arduino Tutorial", "datePublished": "2020-06-19T00:00:00Z", "dateModified": "2026-03-14T15:57:00+08:00", "author": { "@type": "Organization", "name": "ApogeeWeb" }, "publisher": { "@type": "Organization", "name": "ApogeeWeb" } }, { "@type": "FAQPage", "mainEntity":[ { "@type": "Question", "name": "What is the difference between a photoresistor and a photodiode?", "acceptedAnswer": { "@type": "Answer", "text": "A photoresistor is a passive component that changes resistance based on light intensity, making it slower but easier to use. A photodiode is an active semiconductor with a P-N junction that converts light into current, offering much faster response times for high-speed applications." } }, { "@type": "Question", "name": "Is a photoresistor an analog or digital component?", "acceptedAnswer": { "@type": "Answer", "text": "A photoresistor is fundamentally an analog component. Its resistance changes continuously in response to varying light levels. However, when paired with a microcontroller like an Arduino and a voltage divider, its analog signal can be easily converted into digital data." } }, { "@type": "Question", "name": "What are the main types of photoresistors?", "acceptedAnswer": { "@type": "Answer", "text": "Photoresistors are primarily categorized into intrinsic and extrinsic types. Intrinsic photoresistors use pure semiconductors like silicon, while extrinsic types use doped materials to detect longer wavelengths, such as infrared light, making them ideal for specialized sensors." } }, { "@type": "Question", "name": "How is a photoresistor used in smart home circuits?", "acceptedAnswer": { "@type": "Answer", "text": "In modern IoT and smart home systems, photoresistors act as ambient light sensors. They automatically trigger actions like turning on outdoor security lights, adjusting indoor smart bulb brightness, or activating motorized blinds when sunlight reaches a specific threshold." } } ] }, { "@type": "HowTo", "name": "How to Use Multimeter to Detect the Quality of Photoresistor", "step":[ { "@type": "HowToStep", "name": "Measure the dark resistance", "text": "Use a black piece of paper to cover the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter remains basically unchanged, and the resistance value is close to infinity. The larger the value, the better the performance of the photoresistor. If this value is very small or close to zero, it means that the photoresistor has been burnt through and damaged and can no longer be used." }, { "@type": "HowToStep", "name": "Measure the bright resistance value", "text": "Point a light source to the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should have a large amplitude swing, and the resistance value is significantly reduced. The smaller the value, the better the photoresistor performance. If this value is large or even infinite, it indicates that the internal open circuit of the photoresistor is damaged and can no longer be used." }, { "@type": "HowToStep", "name": "Test intermittent light response", "text": "Align the light-transmitting window of the photoresistor with the incident light, and use a small piece of black paper to shake the upper part of the light-shielding window of the photoresistor to make it receive light intermittently. At this time, the pointer of the multimeter should swing left and right with the black paper. If the pointer of the multimeter always stops at a certain position and does not swing with the shaking of the paper, it means that the photosensitive material of the photoresistor has been damaged." } ] } ]}