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Ⅰ IntroductionInvented by Boykin in 1959, resistors are today commonly used in almost all electronic circuits. Resistors can be described as a device that resists the flow of current flowing through itself, back when the resistor size was very huge and the tolerance value reached as high as 10 percent when it was implemented. Besides, they are usually made of compressed carbon. Resistors are mostly made from metal films and are available in small SMD packets with a tolerance value of as little as 2%, or even less, in the case of precision resistors. Carmet, KWK, Epcos India Pvt Ltd. and more are some of the leading manufacturers of resistors in India. If you didn't know, India accounts for some 34% of the market for passive components such as resistors by importing them, the remainder being imported. If you are interested in learning more about the work and characteristics of resistors, then you can try reading this article. We will address the difference between carbon film resistors and metal film resistors in this article.CatalogⅠ IntroductionⅡ Brief Intro to the ResistorsⅢ Carbon Film ResistorsⅣ Metal Oxide Film ResistorsⅤ Carbon Film Resistors VS Metal Oxide Film ResistorsⅥ Voltage and Temperature CoefficientⅦ SizeⅧ FAQⅡ Brief Intro to the ResistorsThe word "resistor" is born from the word "resist," meaning to withstand the impact. A resistor resists the movement of electrons that move through it, guides it, or controls it. With the support of the conductive material that it is made of, this is achieved. Now, the name makes sense, does it not? In parallel and series, resistors are connected according to the specifications for current and voltage. These small devices monitor, attenuate or decrease voltage and current, but do not have a power source of their own. The current flows through them in a controlled manner, resulting in a heat-like loss of energy. Only when there is a potential difference do two resistors bind and carry on a current between them. Yeah, they obey the Rule of Ohm. You must have heard, we're sure, of this statute. Oh, in the field of electronics and electrics, it is something to swear by. Moving on, depending on their characteristics, there is an infinite list of various types of resistors including composition form, film type, and wire-wound type of resistors. Physical size, durability, temperature rating, noise, temperature coefficient, and voltage coefficient, to name a few of these features. Well, the drill is known to you. We are here, however, to address two very significant types of resistors that are capable of transforming your electronic circuits.Ⅲ Carbon Film ResistorsLet us first contemplate what film resistors are before we begin talking about this. Well, after depositing oxide film or pure metals on a substrate or some insulating ceramic, these are simply those resistors that are formed. The layer is extremely thin and sputtering is known as the entire process. By depositing carbon film on the ceramic substrate that is an insulator, the carbon film resistor is prepared. The electric current is blocked by the carbon film to a certain degree. The insulating ceramic, on the other hand, does not allow heat to move through it, which in turn allows the carbon film resistor to withstand massive temperatures without being harmed. Carbon film resistors have a good tolerance rating, available from 1 ohm to 1 megaohm. Speaking of the resistance coefficient of negative temperature - the property of observing a decrease in resistance in response to a rise in temperature, these have a high coefficient of negative temperature that makes them susceptible to decreasing resistance as the temperature increases.These resistors are also available and have a very low tolerance at a low cost. They have a large variety of activities. Carbon film resistor applications are commonly used in X-Rays, power supplies and RADAR.Ⅳ Metal Oxide Film ResistorsMetal oxide film resistors use thin metal oxide films to coat an insulating ceramic rod, in contrast to carbon film resistors. Informing a coating film, the compound made from oxygen atoms and other atoms performs wonders. Using tin oxide, however, metal oxide film resistors are made. To produce better resistance, antimony oxide is also added. Because of the existence of an insulating ceramic rod that does not let heat pass through itself, these resistors are capable of withstanding high temperatures. Metal oxide resists the current at the same time. The greater the sum of antimony, the greater the resistance. But that doesn't even stop here, for good resistance metal oxide film resistors rely heavily on the thickness of the metal oxide and the width of the helical metal oxide film cut. The helical metal oxide film cut width and metal oxide thickness are inversely proportional to the resistance.Wondering what makes them special? Resistors come at a very low cost and withstand high temperatures while making much less sound. Also, along with high reliability and stability, they are small in scale.Ⅴ Carbon Film Resistor VS Metal Oxide Film ResistorWell, engineers are still in a dilemma about which to use one. Whether to use the resistor for the carbon film or the resistor for the metal oxide film. All right, let us break it down, bit by bit, for you. You want your experiments, after all, to go spot on. According to our contrast between Metal Film and Carbon Film Resistors, due to certain properties they possess that are listed below, we feel that metal-oxide film resistors prevail over carbon film resistors.Ⅵ Voltage and Temperature CoefficientThere are a stronger voltage coefficient and temperature coefficient for Metal Oxide film resistors than for carbon film resistors. The coefficient of voltage is the change in resistance concerning the change in voltage. In short, it is the ratio of the resistance change to the voltage change. Metal oxide film resistors operate in a wide range of resistors and can withstand a higher temperature than the resistors of the carbon film. Noise Design In contrast to carbon film resistors, metal oxide film resistors has a low noise design. They keep the minimum current. Therefore, it ensures less noise. If you didn't know, metal oxide film resistors, relative to carbon film resistors, make up for stronger resistors for radio frequency or high-frequency applications. Tolerance The 2 percent minimum carbon film resistor tolerance level does not stand a chance against metal oxide film resistors that can go as low as 0.1 percent.Ⅶ SizeFinally, compared to the carbon film resistors, the size of the metal oxide film resistors is smaller, making them a safer choice to go for. Now that we've done our bit to make you see the complexities of the resistors of both kinds, you can take your pick.Ⅷ FAQ1. What is a carbon film resistor?The resistive film deposited on the glass or ceramic rod is of pure carbon that is why they are called carbon film resistors. The thickness of the film will decide the value of the resistor. Spiralling is done on it in order to adjust the value of resistance.Some important features:• Tolerance =0.5% to 10%• Negative temperature coefficient of resistivity.• Wide temperature range from 55°c to 155°c• These are low power resistors typically of 1/8W, 1/4W, or 1/2W capacity. 2. What are the advantages of using metal film resistors versus carbon composition resistors?Metal film resistors produce less thermal noise than carbon composition resistors. Metal film resistors also typically have a much lower inductance/capacitance than carbon comp resistors so they (metal film) work better at higher frequencies. Carbon composition resistors have no real performance advantage over metal film resistors except that they are cheaper. 3. Are carbon film and metal film resistors interchangeable?No. They simply have less noise and do not drift in value. In short, they behave more like an ideal resistor. You can find metal films in 1% tolerance so the amps with those are very consistent in sound from amp to amp. 4. What is the advantage of a metal film resistor over a carbon resistor?The advantages that a metal film resistor has over a carbon composition resistor is that they don't change their value with age and their tolerance is better than the carbon resistor. 5. How do you identify a metal film resistor?Common carbon film resistors are mostly yellow or pink due to their low accuracy and low production costs, while most metal film resistors are blue. There is a layer of black protective paint on the surface of carbon film resistors, while metal film resistors are usually coated with bright white protective films. 6. What is the advantage of a metal film resistor?Metal film resistor has a low-temperature co-efficient of resistance. The rate at which the resistance of the material changes with a change in temperature is called the temperature coefficient of resistance. Metal film resistors have a low-temperature coefficient of resistance. 7. What is a carbon film resistor used for?The carbon film resistor is a type of fixed resistor that uses carbon film to restrict the electric current to a certain level. These types of resistors are widely used in electronic circuits. 8. Are metal film resistors inductive?Film resistors may be approximately classified as follows: values < 100Ω are inductive. values between 100Ω and 470Ω are practically true resistive. 9. What is a film resistor?Film Resistor is a general term referring to different types such as Carbon Film, Metal Film, and Metal Oxide Film resistors. They are generally manufactured by depositing pure metals (e.g., nickel) or oxide film (e.g., tin-oxide) onto an insulating ceramic or substrate. 10. What are carbon film resistors made of?Carbon film resistors are a fixed form type resistors. They are constructed out of a ceramic carrier with a thin pure carbon film around it, that functions as resistive material.

CatalogCatalogⅠ Oscillation DefinitionⅡ Definition of Voltage Controlled OscillatorⅢ Types of Voltage Controlled Oscillator  3.1 Harmonic Oscillators  3.2 Relaxation OscillatorsⅣ Working Principle of Voltage Controlled OscillatorⅤ Voltage Controlled Oscillator Requirements  5.1 VCO tuning range  5.2 VCO tuning gain  5.3 VCO V/f slopeⅥ Voltage Controlled Oscillator FeedbackⅦ Colpitts & Clapp Voltage Controlled Oscillator CircuitsⅧ Voltage Controlled Oscillator Varactor Issues  8.1 Abrupt  8.2 Hyper-abruptⅨ FAQⅠ Oscillation DefinitionAn oscillator is a circuit that, without any input, generates a continuous, repeated, alternating waveform. Basically, oscillators transform unidirectional current flow from a DC source into an alternating waveform that, as determined by its circuit components, is of the desired frequency. By observing the behavior of the LC tank circuit shown in Figure 1 below, which uses an inductor L and a completely pre-charged capacitor C as its components, the basic theory behind the operation of oscillators can be understood. In this case, the capacitor initially begins discharging through the inductor, which results in the conversion of its electrical energy into an electromagnetic field that can be stored in the inductor. There will be no current flow in the circuit until the capacitor discharges fully.The stored electromagnetic field, however, would have created a back-emf by then, which results in the flow of current through the circuit in the same direction as before. This flow of current through the circuit continues until the electromagnetic field collapses, resulting in the electromagnetic energy back-conversion into electrical form, allowing the cycle to repeat. Now, however, the capacitor would have been charged with the opposite polarity, because of which an oscillating waveform is obtained as the output. However, because of the resistance of the circuit, the oscillations that occur due to the inter-conversion between the two energy-forms will not continue indefinitely as they will be subject to the impact of energy loss. The amplitude of these oscillations gradually decreases to zero as a result, making them damp. This means that the energy loss needs to be balanced to achieve continuous oscillations and constant amplitude. However, in order to achieve oscillations of constant amplitude, it should be noted that the energy supplied should be precisely regulated and must be equal to that of the energy lost.Ⅱ Definition of Voltage Controlled OscillatorA voltage-controlled oscillator (VCO) is an output signal oscillator whose output can be varied over a particular frequency range that is controlled by the DC voltage input. It is an oscillator whose output frequency is directly connected by its input to the voltage applied (FM control).A main parameter of the VCO is the sweeping time: this is the minimum time required to turn or sweep from minimum frequency to maximum frequency or reverse. From an external analog signal, the VCO can be modulated by amplitude (AM). To produce the requested RF power level, an external power amplifier may be required.Ⅲ Types of Voltage Controlled OscillatorThe VCOs can be categorized based on the output waveform:• Harmonic Oscillators• Relaxation Oscillators3.1 Harmonic OscillatorsThe output waveform that harmonic oscillators generate is sinusoidal. This can also apply to the oscillator that regulates the linear voltage. The LC and Crystal oscillators are examples. Here, the capacitance of the diode varies according to the voltage around the diode. This in turn alters the LC circuit's capacitance. Hence, the frequency of the output will change. The advantages are frequency stability in terms of power supply, noise and temperature, and frequency control precision. The only downside is that this form of the oscillator on monolithic ICs can not be implemented effortlessly.3.2 Relaxation OscillatorsThe waveform output produced by harmonic oscillators is a screwed tooth. Using the decreased amount of components, this type may provide a wide range of frequency. It can primarily be used in ICs that are monolithic. The oscillators for relaxation may have the following topologies: • Delay-based ring VCOs • Grounded capacitor VCOs • Emitter-coupled VCOs Here: In delay-based ring VCOs, in a ring shape, the gain stages are connected. As the name implies, in every single point, the frequency is connected to the delay. The VCOs of the second and third types act almost equally. The time taken in each stage is directly linked to the capacitor's charging and discharging time.Ⅳ Working Principle of Voltage Controlled Oscillator Using several voltage regulation electronic components such as varactor diodes, transistors, Op-amps, etc., VCO circuits can be built Here, using Op-amps, we are going to address the function of a VCO. Below, the circuit diagram is shown.A square wave is going to be the output waveform of this VCO. The output frequency is, as we know, connected to the control voltage. The first Op-amp will act as an integrator inside this circuit. The arrangement of the voltage divider is applied here. Because of this, half of the control voltage given as input is supplied to the Op-amp 1 positive terminal. At the negative terminal, the same voltage level is retained. This is to maintain the voltage drop, R1 as half of the control voltage, across the resistor. The current flowing from the R1 resistor passes through the MOSFET when the MOSFET is in good condition. The R2 has half of the resistance, the same drop in voltage and twice the current as of the R1's. So, the attached capacitor is charged by the extra current. To supply this current, the Op-amp 1 should have a gradually increasing output voltage. The current flowing from the R1 resistor passes through the capacitor and gets discharged when the MOSFET is out of order. The output voltage obtained at this time from the Op-amp 1 will decrease. A triangular waveform is therefore produced as the output of Op-amp 1. The Op-amp 2 will act as a catalyst for Schmitt. A triangular wave that is the output of the Op-amp 1 is the input to this Op-amp. If the input voltage is greater than the threshold level, VCC will be the output from the Op-amp 2. If the input voltage is lower than the threshold level, the Op-amp 2 output is zero. The output of the Op-amp 2 is therefore going to be square waves. LM566 IC or IC 566 is an instance of VCO. In fact, it is an integrated 8-pin circuit that can generate double-square wave and triangular wave outputs. Below, the internal circuit is depicted.Ⅴ Voltage Controlled Oscillator RequirementsThere are several parameters that must be considered before the design begins when designing a voltage-regulated oscillator, VCO. These describe the parameters of key performance required for the VCO. 5.1 VCO tuning rangeIt is clear that the oscillator that is powered by voltage must be able to tune over the range that the loop is supposed to work over. This requirement is not always simple to satisfy and, in certain extreme situations, can require the VCO or resonant circuit to be switched. 5.2 VCO tuning gainThe gain of the oscillator regulated by voltage is important. It is calculated per Hz (or V/MHz, etc) in terms of volts. It is the tuning shift for a given change in voltage, as indicated by the units. Any of the overall loop design factors and measurements are influenced by the voltage-controlled oscillator gain.At lower frequencies, the VCO response curves can be shown to be relatively straight. They typically flatten out at higher voltages, however, where the capacitance changes from the variable diodes decrease. 5.3 VCO V/f slopeFor any voltage-driven oscillator used in a phase-locked loop, it is a crucial requirement that the voltage to frequency curve is monotonic, i.e. it always shifts in the same context, usually increasing voltage frequency. If ti alters, as can generally occur in some instances due to spurious resonances, etc., this can cause the loop to become unstable. This must therefore be avoided if the phase-locked loop is to work satisfactorily. This curve shows a slight dip which will result in an unstable phase-locked loop.Phase noise efficiency: In some PLL applications, the phase noise performance of the voltage regulated oscillator is of particular importance - particularly where it is used in frequency synthesizers. Outside of the PLL loop bandwidth, the phase noise output of the voltage-regulated oscillator is the dominant factor in phase noise. While the operation of the PLL reduces close-in noise, there is no reduction in VCO phase noise outside the loop bandwidth. These are some of the main specifications that must be understood from the outset of the VCO design. Careful optimization of the tuned circuit Q, especially the use of variable diodes with as high a Q as possible, selection of the active system, optimization of the oscillator feedback.Ⅵ Voltage Controlled Oscillator FeedbackA VCO can be considered, like any oscillator, as an amplifier and a feedback loop. It is possible to denote the amplifier's gain as A and the feedback as B. For the circuit to oscillate, 360 ° must be the complete phase shift around the loop and unity must be the gain. Signals are fed back around the loop in this manner so that they are addictive and, as a result, any slight disturbance in the loop is fed back and builds up. Because the feedback network is frequency-dependent, the signal is based on one frequency, the feedback network is resonant, and a single frequency signal is produced. A typical emitter circuit is used by many oscillators and thus by VCOs. This in itself generates a 180° phase shift, leaving a further 180° to be given by the feedback network. A typical base circuit where there is no phase shift between the emitter and collector signals (assuming a bipolar transistor is used) can be used by other oscillator or VCO circuits and the phase shift network must provide either 0 ° or 360 °. The device requires a resonant circuit for the oscillator to oscillate on a given frequency to ensure that the oscillation happens on a given frequency. The resonant circuit may be one of a variety of LC resonant circuit configurations, depending on the circuit, or a quartz crystal, etc., in either series or parallel resonance.Ⅶ Colpitts & Clapp Voltage Controlled Oscillator CircuitsThe Colpitts and Clapp oscillator circuits are two commonly used formats for the VCO. Of the two, the most commonly used is the Colpitts circuit, but both are somewhat similar in their configuration. These circuits serve as oscillators because an active device such as a bipolar transistor with capacitors positioned between the base and the emitter (C1) and the emitter and the ground (C2) has been found to fulfill the requirements needed to provide adequate feedback for the output of the oscillator in the correct step. The C1:C2 ratio must be greater than one for the oscillation to take place. The resonant circuit is rendered between the base and ground by adding an inductive function. This consists of only an inductor in the Colpitts circuit, while an inductor and capacitor in series are used in the Clapp circuit.The resonance conditions are that:The capacitance for the overall resonant circuit consists of a series of combinations of the two C1 and C2 series capacitors. The capacitor in the series with the inductor is also used in the series with C1 and C2 in the case of the Clapp oscillator.The capacitance of the series is thus:It is important to change the resonant point of the circuit to make the oscillator tune. This is better accomplished in the case of the Colpitts oscillator, by inserting a capacitor across the indicator. Alternatively, the capacitor may be in series with the inductor for the Clapp oscillator. A circuit where the inductive reactance is located between the base and ground is often favored for high-frequency applications because it is less vulnerable to spurious oscillations and other anomalies.Ⅷ Voltage Controlled Oscillator Varactor IssuesIn order to ensure that the drive frequency in the tuned circuit is not too high, caution must be taken in the design of the circuit when varactor diodes are used inside a voltage-driven oscillator. If this is the case, then the varactor diodes, reducing the Q and increasing the number of spurious signals, can be forced into forwarding conduction. Within a VCO, there are two main types of varactor diode that can be used-the name refers to the diode junction and this impacts their output.8.1 Abrupt: Abrupt diodes have a relatively sharp transition between the areas of the diode, as the name implies. They are able to give a higher Q than their hyper-abrupt relatives, while abrupt varactor diodes do not offer such a high tuning range or linear transfer characteristic. This results in a better oscillator phase noise output regulated by voltage. The other point to note is that in order to have the appropriate tuning range, abrupt varactor diodes may need a high tuning voltage, as certain diodes may need a tuning voltage for the VCO to differ up to 50 volts or slightly more. This can cause problems with supplying the drive circuits with a voltage supply with a sufficiently high voltage.8.2 Hyper-abrupt: There is a relatively linear voltage for hyper-abrupt diodes: the capacitance curve. As a consequence, in some applications, they give a very linear tuning characteristic that may be needed. They can also tune over a wide range, and can normally tune over an octave range with less than a 20-volt tuning voltage shift. They do not give an especially high Q standard, however. Since this will deduct from the tuned circuit's overall Q, this will mean that the output of the phase noise is as good as that which can be obtained using an abrupt varactor diode. Despite the apparent simplicity of the circuit, the voltage-controlled oscillator design is far from trivial. A design would also involve careful optimization of the levels of input coupled with the system and layout. The VCO's design will need to carefully balance the requirements of sometimes conflicting requirements, such as a large tuning range and low noise phase. The standards of efficiency that can be achieved are surprisingly good once the design has been completely configured and the design has been completed.Ⅸ FAQ1. What is a Voltage Controlled Oscillator?A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. 2. What is the use of VCO in PLL?VCO stands for Voltage Controlled Oscillator. PLL operation is simple. VCO creates a high-frequency clock that is divided by some factor. This divided frequency is compared against a stable, reference, frequency using a phase comparator and difference (in-phase or frequency) is converted into voltage and fed back into VCO.Depending on voltage difference VCO frequency will be higher or lower.For example, let’s suppose we have VCO generating 10000 at 5V and divide by 100 dividers. The reference frequency is 90. The phase comparator will subtract two frequencies, 100 - 90 = 10 and will produce some voltage proportional to the frequency difference. This voltage is fed back into VCO and will increase 5V to 6V. Voltage increase will result in frequency drop. The process will continue as long as VCO generated frequency is equal to reference, in our case 9000.From above we see PLL output frequency is: Out = Ref * DividerVCO in RF is produced using varicap diodes - diodes which capacity depends on reverse voltage. Varicap diodes are available with capacities ranging from 1pF up to 500pF and capacity change 2 - 20. How PLL is stable depends on the reference clock and a phase comparator. In the simplest case phase comparator are the XOR gate and RC filter. 3. Why is VCO better than DCO?Of course, the real answer depends on the application. But one important application for a VCO is to implement a so-called phase-lock-loop. In that application, the smoothly continuous frequency vs voltage characteristic of a VCO would allow the VCO to track some variable reference frequency much more precisely. A 'typical' DCO in the same application could only achieve a step-wise approximation to tight tracking. Another, historically more important, application of a VCO is as the primary component of an FM broadcast transmitter. Using a conventional DCO in this application would typically produce an unacceptable amount of weird, noisy distortion in the demodulated audio as the DCO control input attempted to track the audio signal. But yes, it is possible to conceptualize, and even practical to design, a DCO whose frequency control steps are so fine and rapid that, used in an FM broadcast transmitter, the listener would not notice the step-wise tuning of the carrier. 4. What is the function of a VCO voltage-controlled oscillator?A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. 5. How does voltage control oscillator work?A voltage-controlled oscillator is an oscillator with an output signal whose output can be varied over a range, which is controlled by the input DC voltage. It is an oscillator whose output frequency is directly related to the voltage at its input. The oscillation frequency varies from few hertz to hundreds of GHz. 6. How do you make a voltage-controlled oscillator?To make a VCO, the oscillator needs to be tuned by a voltage. This can be achieved by making the variable capacitor from varactor diodes. The tuning voltage for the VCO can then be applied to the varactors. 7. What is the output of VCO?The VCO has an output power level of -3 dBm into 50 Ω with phase noise of -101 dBc/Hz typical at 100 kHz offset. The control voltage range is 0.4 to 2.4 volts, and load pulling is typically 0.75 MHz, pk-pk. Power supply pushing is 280 kHz/volt (typical). 8. What is the VCO tuning range?The VCO is linearly tunable from 806 to 1,113 MHz with a 34% tuning range controlled linearly by the tuning voltage. The phase noise of the VCO is -100.4 dBc/Hz at 100-kHz offset frequency from a 903 MHz carrier. 9. What is VCO phase noise?Characterizing Phase Noise. The term phase noise is widely used for describing short-term random frequency fluctuations of a. signal. Frequency stability is a measure of the degree to which an oscillator maintains the same value. 10. Which is the input terminal in a VCO?It generates the square wave at the output whose frequency is determined by a control voltage. The first op-amp works as an integrator. The control voltage is applied at the input terminal and due to the voltage divider arrangement, half the control voltage is applied at the positive terminal of the first op-amp. 

Introduction: A Comprehensive Guide to Using a Multimeter in 2025When it comes to measuring instruments, the question often arises: how do you use a multimeter effectively? A Multimeter, also known as a VOM (volt-ohm-milliammeter) or DMM (Digital Multimeter), is the indispensable "Swiss Army Knife" for electronics. It is used to measure two or more electrical values—principally voltage (volts), current (amps), and resistance (ohms) in electronic and electrical circuits.In 2025, modern multimeters have evolved significantly. Beyond basic measurements, many now include features like capacitance testing, transistor checks, temperature sensing, and even Bluetooth connectivity for smartphone data logging. Whether you are troubleshooting a smart home device, checking automotive wiring, or testing a battery, the multimeter is your primary diagnostic tool.Multimeters are generally divided into two types: analog multimeters and digital multimeters (DMM). While analog meters use a moving needle, digital versions are now the industry standard, offering high accuracy, auto-ranging capabilities, and easy-to-read LCD or OLED displays. This guide is designed for beginners and will cover the basic parts, working principles, functions, and safety tips for using a multimeter in the modern era.Ⅰ What are the Parts of a Multimeter?The anatomy of a multitester generally includes a display (scale or screen), a selection knob, ports (jacks), and test probes. However, the interface differs between the older analog style and modern digital tools.Analog Multimeters: Use a microammeter with a moving pointer (needle) to display readings against a printed scale. They require the user to interpret the scale based on the selected range.Digital Multimeters (DMM): The current mainstream choice. They feature a numeric digital display, often with a backlight for dark environments. Modern DMMs may also include "True RMS" for accurate AC readings and auto-ranging features that automatically select the correct measurement scale for you.A common digital multimeter is composed of three main interface parts:1.1 The Display (Meter Head)On a digital multimeter, this is the LCD or LED screen where measurement values are shown. It often displays icons for battery life, high voltage warnings, and the unit of measurement (e.g., V, A, Ω). High-end models in 2025 may feature high-resolution color screens or bar graphs that mimic analog needles to show fluctuating signals.1.2 Measuring Circuit (Internal)Hidden inside the case, the measuring circuit converts incoming analog signals (voltage, current, resistance) into digital data. It is composed of precision resistors, an Analog-to-Digital (A/D) converter, and protective fuses to ensure safety during overload.1.3 Rotary SwitchThe central dial allows you to select the function (Volts, Amps, Ohms) and the range. In Auto-ranging multimeters, you simply select the function (e.g., DC Voltage), and the meter automatically adjusts to the scale of the signal. In Manual-ranging meters, you must select the specific range (e.g., 20V, 200V) yourself.Ⅱ Working Principle of MultimeterA Digital Multimeter (DMM) works by converting an analog input signal into a digital value using an A/D converter. The core logic involves comparing the input voltage against a reference voltage.To measure current and resistance, the multimeter actually uses voltage as the base reference:Current: The meter measures the voltage drop across a known internal shunt resistor.Resistance: The meter outputs a small, known current through the component being tested and measures the resulting voltage to calculate resistance (Ohm's Law).Modern DMMs use complex logic control circuits to manage the display, auto-ranging, and safety checks, ensuring that the reading on the screen is stable and accurate. Ⅲ Multimeter Operating ProceduresTo ensure accuracy and safety—especially with modern high-energy circuits—follow these steps:Safety Check: Inspect your test leads for damaged insulation. Ensure the multimeter is rated for the voltage you are testing (e.g., CAT III or CAT IV ratings for household mains).Select Function: Turn the rotary dial to the correct setting (Voltage, Current, or Resistance) before connecting the probes.Select Range: If using a manual-ranging meter and the value is unknown, always start at the maximum range and switch down to lower ranges to get a precise reading. Auto-ranging meters handle this automatically.Zero Calibration: For resistance measurements on analog meters, touch the probes together and zero the needle. Digital meters usually auto-zero, but you can check the "Rel" (Relative) mode on advanced models to null out lead resistance.Power Off for Resistance: Never measure resistance or continuity on a live circuit. Turn off the power to the device before testing.Probe Handling: Keep your fingers behind the finger guards on the test probes to avoid electric shock.Ⅳ Common Multimeter FunctionsMeasure Resistance (Ohms - Ω)Ensure the circuit power is OFF. Connect the test leads across the component (resistor, coil, etc.). Note: Measuring a component while it is soldered to a PCB may give inaccurate results due to parallel paths from other components.Measure Current (Amps - A/mA)Critical Step: You must physically move the red probe to the specific "Amps" or "mA" jack on the multimeter. The meter must be connected in series with the circuit (you must break the circuit and let current flow through the meter). Warning: Connecting a meter in current mode across a voltage source (parallel) will blow the meter's fuse.Measure Voltage (Volts - V)Connect the meter in parallel (across the component or power source). Ensure you select AC (V~) for wall outlets or DC (V=) for batteries.Detect Short Circuit (Continuity)Set the dial to the continuity mode (often marked with a sound wave symbol). Touch probes to two points. If resistance is near zero (a short), the multimeter will beep. This is excellent for checking broken wires or fuses.Non-Contact Voltage (NCV)Many modern multimeters (2020s era and later) include an NCV sensor at the tip. This allows you to detect the presence of live AC voltage in a wire or outlet without touching the metal contacts.Test ICs and ComponentsWhile multimeters can check supply voltage (VCC) and ground pins on Integrated Circuits (ICs), troubleshooting internal IC logic is often better done with an oscilloscope or logic analyzer. However, a multimeter is perfect for finding shorted pins. Ⅴ Multimeter Usage Tips1. Probe Safety: Never touch the metal tips of the probes during a test. Always hold the insulated handles behind the finger guards.2. Switching Gears: Never rotate the selection dial while the probes are connected to a live high-voltage circuit. This can cause internal arcing. Disconnect probes, switch modes, then reconnect.3. Battery Maintenance: If the multimeter will not be used for a long period, remove the internal battery to prevent leakage and corrosion.4. High Voltage Awareness: Be aware of the "CAT" rating of your meter. Do not use a CAT II meter for main service panels (which require CAT III or CAT IV ratings).Video: How to Use a Multimeter?Ⅵ How to Find Circuit Faults with Multimeter6.1 General Troubleshooting Strategy1. Visual InspectionBefore measuring, look for charred components, swollen capacitors, or loose wires. Touching components (cautiously) to check for overheating can also identify shorts.2. Voltage TracingMeasure the voltage at various points in the circuit starting from the power source. If voltage is present at point A but missing at point B, the fault lies between them (e.g., a broken trace or failed component).3. Hidden TroubleshootingIntermittent failures are often caused by "cold" solder joints or loose connectors. Continuity mode is vital here. Wiggle wires while testing continuity; if the beep stops, you have found a loose connection. 6.2 Measuring Techniques6.2.1 Voltage Measuring PointsTesting voltage against a "Ground" reference is the standard method. Connect the black probe to the circuit Ground (GND) and use the red probe to touch various test points (VCC, output pins). Deviations from the expected voltage (e.g., getting 2V on a 5V line) usually indicate a problem.6.2.2 Measuring Parameters Selection① Always check if you are measuring AC or DC. Measuring AC voltage while in DC mode (or vice versa) will give false readings.② For signal circuits, remember that standard multimeters measure "Average" or "RMS" voltage. They may not accurately show fast data pulses—an oscilloscope is required for that.③ Polarity: In DC mode, if you reverse the probes (Red on negative, Black on positive), the screen will simply show a negative sign (e.g., -12V). This is normal and safe for digital meters. Ⅶ How to Measure Current with Digital MultimeterMeasuring DC Current1. Insert the Red test lead into the jack marked "mA" (for low current) or "10A/20A" (for high current). Insert the Black lead into "COM".2. Turn the dial to the DC Current setting (A=).3. Break the circuit: You must physically interrupt the wire and place the multimeter probes in line with the circuit so current flows through the meter.4. Power on the circuit and read the value.Figure 1. Measuring Operating Current of a DC Relay If the reading is negative, your probes are reversed (current is entering the black probe). This does not damage a digital meter.Figure 2. Measuring DC Current in Series Measuring Current with an Analog MultimeterWith analog meters, polarity matters. If you connect probes backward, the needle will deflect to the left (below zero), potentially damaging the mechanism. Always ensure Red connects to the more positive side and Black to the negative side.Figure 3. Series connection with Analog Multimeter Figure 4. Reading the Scale on an Analog Multimeter Measuring AC CurrentThe process is similar to DC, but you must select the AC Current setting (A~). For high currents (e.g., household mains), it is highly recommended to use a Clamp Meter instead of a standard multimeter for safety. A clamp meter measures current via magnetic fields without breaking the circuit.Figure 5. AC Measurement Diagram Ⅷ Which is Better: Analog or Digital Multimeter?For 99% of users in 2025, the Digital Multimeter (DMM) is superior. It offers higher accuracy, higher input impedance (which means it doesn't affect the circuit you are testing), and easier readability. Analog meters are now a niche tool, primarily used by technicians who need to see real-time trends or rapid fluctuations that a digital digital might miss (though modern DMMs with "bar graphs" solve this).Analog MultimeterDigital MultimeterVisualizes trends/fluctuations easily via needle movement.Easy to read exact numbers; no parallax errors.Lower input impedance (can load down sensitive circuits).High input impedance (better for delicate electronics).Susceptible to damage from physical shock (drops).generally more rugged and durable.Often requires manual zeroing and range selection.Includes Auto-ranging, Auto-polarity, and specialized functions.Both meters have their place, but if you can only afford one, buy a Digital Multimeter. They are now available at very affordable price points without sacrificing essential accuracy. Ⅸ Best Multimeter Brands in 2025When selecting a multimeter, reliability and safety are paramount. Brands differ by their target audience: Professionals (industrial use) vs. Hobbyists (home use). Below are reputable brands recognized in the industry today.*Etekcity (Budget/Home)Mastech GroupInnova (Automotive)Klein ToolsKeysight Technologies (formerly Agilent)BrymenKaiweets / AstroAI (Budget)Fluke (Professional Gold Standard)ExtechAmprobeHioki Ⅹ Frequently Asked Questions1. What is a multimeter primarily used for?A digital multimeter is a diagnostic tool used to measure electrical values—principally voltage (Volts), current (Amps), and resistance (Ohms). It is standard equipment for electricians, automotive mechanics, and HVAC technicians. 2. What is "Auto-Ranging"?Auto-ranging is a feature on digital multimeters where the meter automatically selects the correct measurement range for the signal it detects. You simply select "Voltage," and the meter decides if it's millivolts or hundreds of volts. This saves time and prevents setup errors. 3. Why is "True RMS" important?True RMS (Root Mean Square) allows a multimeter to accurately measure AC voltage even when the sine wave is distorted (common in motor drives, HVAC systems, and computers). Non-True RMS meters are only accurate on perfect sine waves. 4. Can I use a multimeter to test a battery?Yes. Set the multimeter to DC Voltage (V=). Connect the red probe to the positive (+) terminal and black to negative (-). A fully charged 1.5V AA battery should read greater than 1.5V (usually 1.6V). If it reads below 1.2V, it is dead. 5. What are CAT ratings?CAT ratings (Category I, II, III, IV) define the safety level of the meter. CAT III or CAT IV is recommended for testing household wiring and mains power to protect against dangerous transient voltage spikes.

IntroductionIn electronics, what is clipper? A circuit which removes the peak of a waveform is known as a clipper. Clipper circuit is designed to prevent a signal from exceeding a predetermined reference voltage level. The clipper circuit can be designed by utilizing both the linear and nonlinear elements such as resistors, diodes, or transistors. The diode clipper, also known as a diode limiter, is a wave shaping circuit that limits positive or negative amplitude, or both. In electronics, diode clipper circuits are commonly used to process various signals. It is is a circuit designed to prevent a signal from exceeding a predetermined reference voltage level. Clipping changes the shape of the waveform and alters its spectral components.Clipper Circuits IntroductionCatalogIntroductionⅠ Clipper Circuit Types1.1 Positive Clipper Circuit1.2 Negative Clipper Circuit1.3 Combinational Limiter CircuitⅡ Clipper Circuits Analysis2.1 Clipper Circuit Structure2.2 Clipper Circuit ProblemsⅢ General Forms of Clipper Circuits3.1 Clipper Circuit Description3.2 Common Clipper Circuit ExamplesⅠ Clipper Circuit TypesDiode clipper is a limiting circuit which limits the output voltage. In electronics, a clipper is a circuit designed to prevent a signal from exceeding a predetermined reference voltage level. A basic diode limiter circuit is composed of a diode and a resistor. It is divided into three types: positive clipper circuit, negative clipper circuit and combinational clipper circuit. The positive clipper circuit produces a clipping effect when the input voltage is higher than a certain upper limit value; the negative clipper circuit produces a limit effect when the input voltage is lower than a certain lower limit value; the combinational clipper circuit produces a limit effect when the input voltage is too high or too low. In a positive clipper, the positive half cycles of the input voltage will be removed. During the negative half cycle of the input, the diode is forward biased and so the negative half cycle appears across the output. The clipper circuits are described as following.1.1 Positive Clipper CircuitThe diode in clipper circuit is connected in series to the input signal and that attenuates the positive portions of the waveform. The positive clamping circuit blocks the input signal when the diode is forward biased. During the negative half cycle of an AC signal, the diode is forward biased and allows electric current through it. In following figure, when the input signal voltage is lower than a preset upper limit voltage, the output voltage will change with the input voltage, however, when the input voltage reaches or exceeds the upper limit, the output voltage will remain at a fixed value, so that the signal amplitude is limited at the output.1.2 Negative Clipper CircuitThe diode in clipper circuit is connected in series to the input signal and that attenuates the negative portions of the waveform, is termed as negative series clipper. For the figure below, the diode is series to the input and output. If the diode has ideal switching characteristics, when iu is lower than E, D will not conduct, ou=E; when ui is higher than E, D will conduct, ou=iu. The limiting characteristic of this limiter circuit is shown in the figure.1.3 Combinational Limiter CircuitThis kind of circuit combines the positive and negative limiters together which shows in the following figure. Ⅱ Clipper Circuits Analysis2.1 Clipper Circuit StructureIn the circuit, Al is an integrated circuit (a common component), VT1 and VT2 are transistors, Rl and R2 are resistors, and VDl to VD6 are diodes.Analyzing the effect of VD1 and VD2 in the circuit mainly explains the following points.1) It can be seen from the circuit that the circuit structure of the two groups of diodes are the same. Both play the same role in this circuit, so the working principle of them are the same.2) The pin ① is connected to the base of the transistor VT1 through a resistor Rl. Obviously Rl is a signal transmission resistor. The signal output on the pin ① is added to the base of VT1 through Rl (there is no DC blocking capacitor between pin ① and VT1). From this circuit structure, it can be judged that the pin ① is an output signal pin, and it outputs a composite signal of DC and AC. The purpose of determining that the pin ① is to figure out the specific function of the diode VD1 in the circuit.3) The DC voltage output by pin ① is not high enough to make the external diode in a conducting state. The analysis is: if the DC voltage output by the pin ① is high enough, then VD1, VD2 and VD3 conduct, and the internal resistance becomes small. This will shunt the AC signal output by the pin ① to the ground, so the signal will be attenuated. However, this circuit does not need such attenuation. Therefore, the conclusion drawn from this: VD1, VD2 and VD3 are not turned on by pin ① DC voltage output.4) The output from pin ① is the superimposed signal of DC and AC, which is added to the base of the transistor VT1 through the resistor Rl. VT1 is an NPN transistor. If the amplitude of the positive half-cycle AC signal added to the base of VT1 is very large, which may burn the VT1. When the negative half-cycle signal added to the base of VT1 is large, which has no effect on VT1, because the negative signal on the base of VT1 reduces current.Follow the above circuit analysis, it can be judged that VD1, VD2, and VD3 in the circuit has clipper function, to prevent VT1 from burning out. 2.2 Clipper Circuit ProblemsIn the figure, Ul is the DC voltage in the output of pin ①, U2 is the limiting voltage value.When the AC voltage in the output signal of pin ① is relatively small, the positive half cycle of the AC signal plus the DC output voltage does not make the VD1, VD2 and VD3 conduction. Therefore, all diodes are cut off, which has no effect on the AC signal output by pin ①. Assuming that the positive half-cycle output AC signal by pin ① is very large during a certain period, as shown in the signal waveform, at this time it plus the DC voltage can conduct VD1, VD2 and VD3. If the conduction voltage of each diode is 0.7V, then three diodes is 2.1V. Since the tube voltage drop after conduction is basically the same, that is, the maximum voltage of pin ① is 2.1V. So the excess part of the positive half cycle of the AC signal is limited by the resistor. When the DC and AC output signals at pin ① is less than 2.1V, diodes will not conduct and keep cutoff state, which has no clipping effect on the signal.For the specific details of clipper circuit, there are several explanations as follows.1) The negative half cycle large signal output by the pin ④ will not cause VT1 overcurrent, because it will decrease the base voltage of the NPN transistor and the base current, so there is no need to add the limiter circuit.2) The one-way limiter circuit mentioned above, it can only limit the large signal part of the positive or negative half of the signal, and does not limit the signal in the other half. The other is the combinational limiter circuit, which can limit the positive and negative half-cycle signals at the same time.3) There are many reasons for the abnormal increase of the signal amplitude. For example, the fluctuation of the power supply voltage cause it to increase a lot at a certain moment, and the large-scale interference pulse from the outside into the circuit also causes a certain increase.4) After the three diodes VD1, VD2 and VD3 conduct, the sum of the DC and AC voltages on pin ① is 2.1V. This voltage added to the base of VT1 through resistor Rl is maximum, so as to the current of VT1.5) Since the pin ① is the same as the external circuit of pin ②, the working principle of the limiter circuit is the same. So only one circuit needs to be analyzed when analyzing the circuit.6) According to the characteristics of the series circuit, the current in the series circuit is equal everywhere. It can be known that the three series diodes VD1, VD2 and VD3 are turned on at the same time, or they will be turned off at the same time. Therefore, in the series circuit, a diode is turned on and other diodes are turned on.Ⅲ General Forms of Clipper Circuits3.1 Clipper Circuit DescriptionThere are two types of clippers namely series and parallel. In series clipper, diode is connected in series with the load. In parallel clipper, diode is in parallel to the load.1) Series clippers: if the diode is connected in series with load resistanceUnbiased series clipper: in that case the circuit diode is connected in series with load resistance and no external voltage is applied to the circuit.+ve unbiased series clipper: if the +ve portion of output is clipped its called +ve unbiased series clipper.-ve unbiased series clipper: if the -ve portion of output is clipped its called +ve unbiased series clipper.Biased series clipper: if in the circuit, diode is connected in series with load resistance and external voltage is applied to the circuit+ve biased series clipper: if the +ve portion of output is clipped its called +ve biased series clipper.-ve biased series clipper: if the -ve portion of output is clipped its called +ve biased series clipper.2) Parallel clippers: if the diode is connected in parallel with load resistanceUnbiased parallel clipper: in that case the circuit diode is connected in parallel with load resistance and no external voltage is applied to the circuit.+ve unbiased parallel clipper: if the +ve portion of output is clipped its called +ve unbiased parallel clipper.-ve unbiased parallel clipper: if the -ve portion of output is clipped its called +ve unbiased parallel clipper.Biased parallel clipper: if in the circuit, diode is connected in parallel with load resistance and external voltage is applied to the circuit+ve unbiased parallel clipper: if the +ve portion of output is clipped its called +ve biased series clipper.-ve unbiased parallel clipper: if the -ve portion of output is clipped its called +ve biased series clipper. 3.2 Common Clipper Circuit ExamplesIn general, clippers circuit are classified into two types: Series Clippers, Shunt Clippers, and Dual (Combination) Clippers.Series Clipper: The diode is connected in series with the load resistance. 👇Figure 1. Series Positive ClipperThe positive amplitude waveform is cut, and the negative amplitude waveform is retained, as follows:Figure 2. Series Positive Clipper with Positive BiasThe positive amplitude waveform is cut, and the offset positive voltage is retained on the negative amplitude waveform, as follows:Figure 3. Series Positive Clipper with Negative BiasThe waveform of positive amplitude is cut, and the negative voltage is shifted based on the waveform of negative amplitude, as follows:Figure 4. Series Negative ClipperThe negative amplitude waveform is cut, and the positive amplitude waveform is retained, as follows:Figure 5. Series Negative Clipper with Positive BiasThe negative amplitude waveform is cut, and the positive voltage is offset on the positive amplitude waveform, as follows:Figure 6. Series Negative Clipper with Negative BiasThe negative amplitude waveform is cut, and the negative voltage is offset on the positive amplitude waveform as follows: Shunt Clipper: Diode is in parallel with load resistance in circuit. 👇Figure 7. Shunt Positive ClipperFigure 8. Shunt Positive Clipper with Positive BiasFigure 9. Shunt Positive Clipper with Negative BiasFigure 10. Shunt Negative ClipperFigure 11. Shunt Negative Clipper with Positive BiasFigure 12. Shunt Negative Clipper with Negative Bias Dual (Combination) Clipper: It is desired to remove a small portion of both positive and negative half cycles. 👇Figure 13. Combination ClipperWhen the positive and negative waveforms must be limited, a combinational limiter circuit is required, as follows:Images Reference: Clipper Circuits - Series Clipper, Shunt Clipper, and Dual Clipper Frequently Asked Questions about Diode Limiter and Clipper Circuit1. What is Clipper and clamper?The major difference between clipper and clamper is that clipper is a limiting circuit which limits the output voltage while clamper is a circuit which shifts the DC level of output voltage. ... While clamper is used when we need multiples of the input voltage at the output terminal. 2. What is the function of clipper circuit?In electronics, a clipper is a circuit designed to prevent a signal from exceeding a predetermined reference voltage level. A clipper does not distort the remaining part of the applied waveform. 3. What is Clipper circuit and its types?A clipper is a device which limits, remove or prevents some portion of the wave form (input signal voltage) above or below a certain level, in other words, the circuit which limits positive or negative amplitude ,or both is called chipping circuit. The clipper circuits are of the following types. Series positive clipper. 4. What is the difference between a positive clipper and a negative Clipper?Positive Clipper and Negative Clipper. In a positive clipper, the positive half cycles of the input voltage will be removed. ... During the negative half cycle of the input, the diode is forward biased and so the negative half cycle appears across the output. 5. How does diode clipping work?The Diode Clipper, also known as a Diode Limiter, is a wave shaping circuit that takes an input waveform and clips or cuts off its top half, bottom half or both halves together. This clipping of the input signal produces an output waveform that resembles a flattened version of the input. 6. What is the main purpose of a diode limiter?The diode limiter also called Clipper as it is used to limit the input voltage. A basic diode limiter circuit is composed of a diode and a resistor. Depending upon the circuit configuration and bias, the circuit may clip or eliminate all or part of an input waveform. It limits the output voltage to a specific value. 7. What is the purpose of a clamping diode?The clamping circuit fixes the voltage lower limit to zero, that is, the start of the signal is 0 V. The positive clamping circuit blocks the input signal when the diode is forward biased. During the negative half cycle of an AC signal, the diode is forward biased and allows electric current through it. 8. What is a diode clamping circuit?A clamper circuit shifts the DC level or the reference level of the signal to the desired level without changing the shape of the waveform. The clamper circuit can be designed using the diode, resistor, and the capacitor.

IntroductionAs we all know, the most basic passive linear components are resistors (R), capacitors (C) and inductive components (L). These components can be used to form 4 different circuits: RC circuit, RL circuit, LC circuit and RLC circuit. They have some important properties for analog electronics, and can be used as passive filters. In practice, capacitors (and RC circuits) are usually used instead of inductors to form filter circuits. This is because capacitors are easier to manufacture with smaller size. This article mainly introduces the RC Circuit in series and parallel state.RC circuit (resistor–capacitor circuit), also called RC filter or RC network, has a resistor and a capacitor in series connection. When connected to a DC voltage source, the capacitor charges exponentially in time. That is, a capacitor can store energy, and when a resistor placed in series with it will control the rate at which it charges or discharges. This produces a characteristic time dependence that turns out to be exponential.RC Circuits Basic ExplainedCatalogIntroductionⅠ RC Circuit Basics1.1 What is RC Circuit?1.2 RC Circuit CharacteristicsⅡ How to Calculate RC Circuit?Ⅲ RC Circuits Classification3.1 Series and Parallel Circuits3.2 Example: RC Low Pass FilterⅣ Visualizing Filter Response4.1 Frequency Response4.2 Low Pass Filter Phase Shift4.3 Second-order Low-pass FilterⅤ ConclusionⅠ RC Circuit Basics1.1 What is RC Circuit?For a RC circuit (resistor-capacitor circuit), the primary composes of a resistor and a capacitor. According to the arrangement of resistors and capacitors, it can be divided into a RC series circuit and a RC parallel circuit. In addition, simple RC parallel circuits cannot resonate, because resistor does not store energy. However, LC parallel circuits can resonate. RC circuits are widely used in analog circuits and pulse digital circuits. If a RC parallel circuit connected in series in the circuit, it can attenuate low-frequency signals, and if it connected in parallel in the circuit, it can attenuate high-frequency signals. That is filtering.RC circuit is common element in electronic devices. It also play an important role in the transmission of electrical signals in nerve cells. A capacitor can store energy and a resistor placed in series with it will control the rate at which it charges or discharges.Figure 1. Passive Low-pass RC Circuit1.2 RC Circuit CharacteristicsIn the analog circuit, the passive RC filter circuit can be divided into a low-pass filter circuit and a high-pass filter circuit according to the connection and size of the capacitor.The low-pass filter circuit is somewhat equal to the integrator circuit (capacitor C is in parallel at the output.), but both circuits are applied to different requirements. The integrator circuit mainly uses the integration effect of the capacitor C when it is charged. In the case of square wave input, periodic sawtooth wave (triangular wave) will generate, so the capacitor C and resistor R are selected according to the square wave. While the low-pass filter circuit bypasses the higher frequency signal (because XC=1/( 2πfC), when f is larger, XC is smaller, which is equivalent to a short circuit), so the value of capacitor C is determined by referring to the value of the low frequency. For the filter circuit of the power supply, theoretically the larger the value of C, the better.Figure 2. Low Pass Filter CircuitThe high-pass filter circuit has the same form as the differential circuit or the coupling circuit. In the pulse digital circuit, due to the different relationship between RC and pulse width, it is divided into a differential circuit and a coupling circuit. In an analog circuit, choosing an appropriate capacitance C value can pass higher frequency signals selectively, even block DC and low-frequency signals. For example, a capacitor connected in series with a tweeter, is to prevent the low pitch from entering the tweeter to avoid burnout. What’s more, in the multi-stage AC amplifier circuit, the high-pass filter circuit is also a coupling circuit.Figure 3. High Pass Filter CircuitⅡ How to Calculate RC Circuit?From a mathematical point of view, suppose that the RC circuit has been connected to a DC power supply with a voltage value of U0. The voltage on the capacitor is equal to the power supply’s, and at a certain moment t0 the left end S of the resistor is grounded, then the capacitor discharges. In the theoretical analysis, the time t0 is taken as the zero point of time.According to KVL's law, establish the circuit equation: The initial condition is .This is a first-order homogeneous differential equation, and its general solution is .After substituting into the original equation: The characteristic equation is .The characteristic root is .According to , get .Therefore, the required initial value of the differential equation is It can be seen that the voltage attenuation speed on the capacitor depends on the , and its size only depends on the circuit structure and component parameters.When the unit of resistance is Ω and the unit of capacitance is F, the unit of product RC is seconds (s), which is represented by τ, then the capacitor voltage can be written as .tτ2τ3τ4τ5τ...∞uc(t)Uo0.368Uo0.135Uo0.05Uo0.018Uo0.0067Uo...∞0The τ time constant is the time it takes for the capacitor voltage to drop to 1/e=36.8% of the initial value. Specifically, it is the time required to charge the capacitor, through the resistor, from an initial charge voltage of zero to approximately 63.2% of the value of an applied DC voltage, or to discharge the capacitor through the same resistor to approximately 36.8% of its initial charge voltage. When t=4t, the capacitor voltage is very small, and it is generally considered that the circuit enters a steady state, which is also called the zero input response of the RC first-order circuit. Ⅲ RC Circuits Classification3.1 Series and Parallel CircuitsRC Series CircuitIn circuit, the capacitor cannot flow DC current, and R & C have an obstructive effect on the current. So the total impedance is determined by the resistance and capacitive reactance, and it changes with frequency. RC series circuit has a turning frequency: f0=1/2πR1C1. When the input signal frequency is greater than f0, the total impedance is basically unchanged, and it is equal to R1.RC Parallel CircuitThe RC parallel circuit can pass both DC and AC signals. It has the same turning frequency as the RC series circuit: f0=1/2πR1C1. On the one hand, when the input signal frequency is less than f0, the total impedance of the circuit is equal to R1, on the other hand, when the input signal frequency is greater than f0, the capacitive reactance of C1 is relatively small, and the total impedance is the sum of resistance and capacitance. In addition, when the frequency is high to a certain level, the total impedance is zero.Introduction to Parallel RC CircuitWhat’s more, as frequency increases, the capacitor will act like a short circuit to high frequency current in its path. At low frequencies, the capacitor tends to block current flow.3.2 Example: RC Low Pass FilterCircuit AnalysisTo create a passive low-pass filter, we need to combine the resistor elements with the reactance elements. That is a circuit consisting of a resistor and a capacitor or an inductor. Theoretically speaking, the RL low-pass topology is equivalent to the RC low-pass topology in terms of filtering ability. However, in practice, RC circuits are more common.Figure 4. RC Low-pass FilterAs shown in the figure, connecting a resistor in series with the signal path and a capacitor in parallel with the load, an RC low-pass response can be generated. In the figure, the load is a single part, but in actual circuits, it may be more complicated, such as the input stage of an analog-to-digital converter, amplifier, or oscilloscope to measure the response of the filter.If a resistor and a capacitor form a frequency-dependent voltage divider circuit, we can intuitively analyze the filtering function of the RC low-pass circuit.Figure 5. Change RC Low-pass Filter into a Voltage DividerWhen the frequency of the input signal is low, the impedance of the capacitor is high than the resistor. Therefore, most of the input voltage will drop on the capacitor (and both ends of the load, which is in parallel with the capacitor). When the input frequency is higher, the impedance of the capacitor is lower than the impedance of the resistor, which means that the resistor voltage decreases and less voltage is transferred to the load. Therefore, low frequencies pass and high frequencies are blocked.Cutoff FrequencyWhere the filter does not cause significant attenuation for a frequency range is called the passband, and the opposite is called the stopband. Analog filters, such as RC low-pass filters, always gradually transit from the passband to the stopband. This means that it cannot be recognized that the filter stops passing the signal and starts blocking one frequency of the signal. This is why the cutoff frequency concept introduced.When checking the frequency response graph of the RC filter, the signal spectrum is "cut" into two halves of the image, one of which is retained and one is discarded. Because as the frequency moves from below the cutoff point to above the cutoff value, the attenuation gradually increases.The cut-off frequency of the RC low-pass filter is actually the frequency at which the input signal amplitude is reduced by 3dB (this value is chosen because a 3dB reduction is equal to a 50% reduction in power). Therefore, the cutoff frequency is also called -3dB frequency. The term bandwidth refers to the width of the passband of the filter. For a low-pass filter, its bandwidth is equal to the -3dB frequency (as shown in the figure below).Figure 6. Cutoff Frequency -3dBFilter Response CalculationWe can discuss the theoretical behavior of the low-pass filter by a typical voltage divider. The output of the resistor divider is expressed as following:The RC filter uses an equivalent structure, using a capacitor XC replace R2. Then we need to calculate the total impedance and place it in the denominator, so there is The reactance of a capacitor represents the opposite amount of current, but unlike resistance, the opposite amount depends on the frequency of the signal passing through the capacitor. Therefore, we must calculate the reactance at a specific frequency. The equation we use for this as follows: In the above design example: R≈160Ω and C=10nF. We assume that the magnitude of VIN is 1V, so we can simply remove VIN from the calculation. First, let's calculate the amplitude of VOUT with a sine wave frequency: While suppressing noise, the amplitude of the sine wave is basically unchanged. Because the cutoff frequency (100kHz) we chose is much higher than the sine wave frequency (5kHz).Let’s see how the filter successfully attenuates the noise component.The noise amplitude is only about 20% of its original value. Ⅳ Visualizing Filter Response4.1 Frequency ResponseThe most convenient way to assess the effect of a filter on a signal is to examine the frequency response graph. That is Bode plot, which has amplitude (in decibels) on the vertical axis and frequency on the horizontal axis; the horizontal axis usually has a logarithmic scale so that the physical distance between 1Hz and 10Hz is the same as 10Hz to 100Hz and 100Hz to 1kHz. This configuration allows us to quickly and accurately evaluate the behavior of the filter over a large frequency range.Figure 7. Bode PlotEach point on the curve represents the amplitude that the output signal is 1V and the frequency is equal to the corresponding value on the horizontal axis. For example, when the input frequency is 1MHz, the output amplitude (assuming the input amplitude is 1V) will be 0.1V (because -20dB corresponds to a tenfold reduction factor).The curve in the passband is almost completely flat, and then as the input frequency approaches the cutoff frequency, it starts to drop faster. Finally, the rate of change of attenuation becomes stable, that is, for every ten times the input frequency increases, the amplitude of the output signal decreases by 20dB. 4.2 Low Pass Filter Phase ShiftThe way in which the filter modifies the amplitude of various frequency components in the signal has been discussed above. However, in addition to amplitude effects, reactive circuit elements always involve phase shifts.The concept of phase refers to the value of the periodic signal at a specific moment in the cycle. Therefore, when we say that a circuit causes a phase shift, we mean that it creates a misalignment between the input signal and the output signal. That is the input and output signals no longer start and end their periods at the same time. The phase shift value, such as 45° or 90°, indicates how much misalignment has been created.Each reactance element in the circuit introduces a 90° phase shift, but this phase shift does not occur at the same time. The phase of the output signal is the same as the amplitude of the output signal, and it changes gradually as the input frequency increases. In the RC low-pass filter, we have a reactive element (capacitor), so the circuit will eventually introduce a 90° phase shift.As with the amplitude response, the phase response can be most easily evaluated by examining the graph on the horizontal axis which represents the logarithmic frequency. The following description is the general pattern.The phase shift is initially 0°, and it gradually increases until it reaches 45° at the cutoff frequency. During this part of the response, the rate of change is increasing. With time, the phase shift continues to increase, but the rate of change is decreasing. As the phase shift approaches 90°, the change of rate becomes very small.Figure 8. Phase Shift4.3 Second-order Low-pass FilterAs above mentioned, we have assumed that the RC low-pass filter consists of a resistor and a capacitor. This configuration is a first-order filter. The "order" of passive filters is determined by the number of reactive components (ie capacitors or inductors) in the circuit. Higher-order filters have more reactive components, which lead to more phase shift and steeper roll-off.Second-order filters are usually built a resonant circuit consisting of inductors and capacitors (this topology is called "RLC", or resistor-inductor-capacitor circuit). However, it is also possible to create a second-order RC filter. As shown in the figure below, all we need to do is to cascade two first-order RC filters.Figure 9. Second-order Filter CircuitAlthough this topology can produce a second-order response, it is not widely used. Because its frequency response is usually not as good as a second-order active filter or a second-order RLC filter.Frequency ResponseWe can try to create a second-order RC low-pass filter by designing a first-order filter based on the required cutoff frequency, that is connecting two first-order stages in series. This set has a similar overall frequency response, with a maximum roll-off of 40dB/decade instead of 20dB/decade.However, we cannot simply connect these two stages together and analyze the circuit as a second-order low-pass filter. In addition, even if we insert a buffer between the two stages so that the first RC stage and the second RC stage can be used as independent filters, the attenuation at the original cut-off frequency will be 6dB instead of 3dB. Because the two stages work independently.Figure 10. Frequency Response of RC-RC FilterA limitation of the second-order RC low-pass filter is that the designer cannot tune the conversion from passband to stopband by adjusting the Q factor (this parameter indicates the degree of damping of the frequency response.) of the filter. If two identical RC low-pass filters are cascaded, the overall transfer function corresponds to the second-order response, but the Q factor is always 0.5. When Q = 0.5, the filter is at the boundary of over-damping, which results in a "sag" frequency response in the transition region. While second-order active filters and second-order resonant filters do not have this limitation, designers can control the frequency response of the transition region. Ⅴ ConclusionAll electrical signals contain a mixture of requiring frequency and unwanted ones. Undesirable frequency components are usually caused by noise and interference, and in some cases they have a negative impact on the performance of the system.Filters are circuits that react to different parts of the signal spectrum in different ways. The low-pass filter is designed to pass low frequency components and block high frequency components. The output voltage of an RC low-pass filter can be calculated by considering the circuit as a voltage divider (frequency-independent) composed of resistance and reactance.The graph of amplitude (in dB, on the vertical axis) vs. log frequency (in Hz, on the horizontal axis) is a convenient and effective way to check the theoretical behavior of the filter. You can also use phase and log frequency graph determines the amount of phase shift that will be applied to the input signal.The second-order filter provides a steeper roll-off, and its response is useful when the signal cannot provide broadband separation between the desired frequency and the unwanted one. You can make a second-order RC low-pass filter by connecting two identical first-order RC low-pass filters, but the overall -3 dB frequency will be lower than expected.In RC filtering circuit, the capacitor can store energy, and the resistor placed in series with it can control the charge-discharge rate. And this produces a characteristic time dependence that turns out to be exponential. Frequently Asked Questions about RC Filter Circuit1. What does an RC filter do?RC circuits can be used to filter a signal by blocking certain frequencies and passing others. The two most common RC filters are the high-pass filters and low-pass filters; band-pass filters and band-stop filters usually require RLC filters, though crude ones can be made with RC filters. 2. What is RC filter in electronics?A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors. ... A first order RC circuit is composed of one resistor and one capacitor and is the simplest type of RC circuit. 3. How do you calculate RC circuit?The (real value) impedance is the real part of the complex impedance Z. For a series RC circuit, we get Z=√R2+(1ωC)2 Z = R 2 + ( 1 ω C ) 2 . We see that the amplitude of the current will be V/Z=V√R2+(1ωC)2 V / Z = V R 2 + ( 1 ω C ) 2. 4. What is RC circuit used for?The RC circuit has thousands of uses and is a very important circuit to study. Not only can it be used to time circuits, it can also be used to filter out unwanted frequencies in a circuit and used in power supplies, like the one for your computer, to help turn ac voltage to dc voltage. 5. What is RC series circuit?A circuit that contains pure resistance R ohms connected in series with a pure capacitor of capacitance C farads is known as RC Series Circuit. A sinusoidal voltage is applied and current I flows through the resistance (R) and the capacitance (C) of the circuit.

IntroductionSemiconductor diode, also known as crystal diode, has obvious unidirectional conductivity. It is a kind of electronic components widely used in electrical equipment for protection, rectification, switching, and many other applications. So it is pretty common to see diodes in daily electronic circuits, such as Zener diodes, light-emitting diodes, photodiodes, etc. Therefore, it is necessary to know how to test whether a diode is properly working or not.How to Test a Diode Using a MultimeterCatalogIntroductionⅠ Diode Basics1.1 To Figure Out Diode Anode and Cathode1.2 What Would Cause a Diode to Fail?1.3 Common Diode Failures AnalysisⅡ How to Test Diode with a Multimeter?2.1 Digital Multimeter and Analog Multimeter2.2 Common Diodes Testing Rules2.3 Testing Methods of Types of DiodesⅢ Example Analysis3.1 Test a Diode in Circuit3.2 Power-off and Power-on Testing Methods3.3 ConclusionⅠ Diode Basics1.1 To Figure Out Diode Anode and CathodeThe anode and cathode of diode can be distinguished by screen printing on PCB board, which are as shown in the following:1) The notched end is the cathode of diode.2) The end with a horizontal bar is the cathode.3) The end with white parallel bars is the cathode.4) One end of the triangle arrow is the cathode.5) The small end of the plug-in diode is the cathode, and another big end is the anode.1.2 What Would Cause a Diode to Fail?The common reasons for a diode failure are open circuit, short circuit and unstable voltage regulation. Among these three types of failures, there may be signs. For example, the power supply voltage rises, the supply voltage drops to zero or the output is unstable. Therefore, specific problems must be analyzed in detail for the diode test.The common measurement tool for diode is a multimeter, including on-circuit measurement (the diode is on the circuit board) and off-circuit measurement (the diode is not on the circuit board). As for the basic principle of diode measurement, the forward resistance and the reverse resistance of the PN junction are measured, and the basic judgment is based on the values of them. Therefore, to do a good job in diode test, it is necessary to understand the basic structure and working principle of diodes, and then to understand the main fault characteristics of the diode. 1.3 Common Diode Failures Analysis1) open circuitThis means that the positive and negative electrodes of the diode have been disconnected, and the forward and reverse resistance of the diode have become infinite. After the diode is open, the circuit is in an open state.2) voltage breakdownThis means that there is a path between the positive and negative electrodes of the diode, and the forward and reverse resistance are as large as or close to each other(but not infinite). After a diode breaks down, the action between the positive and negative electrodes may always exit, because there are different manifestations in different circuits.3) forward voltageIf the forward resistance of the diode is too large, the voltage drop of the signal on the diode will increase, which will cause the output signal to decrease, and the diode will be damaged due to the heat. After the forward resistance becomes larger, the unidirectional conductivity of the diode will become poor.4) reverse voltageThe reverse resistance of the diode becomes smaller, which means the unidirectional conductivity of the diode be effected.5) performance degradationUnder this circumstance, the diode does not have obvious failures such as open circuit or breakdown. However, when the situation is getting worse, the stability of the circuit will deteriorate or the output signal voltage of the circuit will drop. Ⅱ How to Test Diode with a Multimeter?2.1 Digital Multimeter and Analog MultimeterWhen using a digital multimeter to test a diode, the red probe connects with the anode and the black probe connects with the cathode. At this time, the measured resistance is the forward conduction resistance of the diode, which is just the opposite of the test result of an analog multimeter. 2.2 Common Diodes Testing Rules(1) The forward resistance of the low-power germanium diode is 300Ω～500Ω, and the silicon diode is lkΩ or more. The former reverse resistance is tens of thousand ohms, and the latter is above 500kΩ (the value of high-power diode is smaller).(2) The polarity of the diode can be judged according to the resistance values (small forward resistance and large reverse resistance). Set the multimeter to the ohm block (Usually use R×100 or R×1k block, do not use R×1 block or R×10k block. The R×1 block is in a large current, it is easy to burn the tube, while using R×10k block may cause the tube broken down with high voltage). Connect the two polarities of the diode with the test probes respectively, and measure the two resistance values. When the measured resistance value is smaller, the end connected to the black lead is the anode. In the same way, when the measured resistance value is larger, the end connected to the black probe is the cathode. If the measured reverse resistance is small, it means that the diode is short-circuited, on the contrary, if the forward resistance is large, it means that the tube is open. In both cases, the diode can’t be work normally.(3) Silicon diodes generally have a forward voltage drop of 0.6V～0.7V, and the forward voltage drop of a germanium diode is 0.IV～0.3V. By measuring the forward voltage of the diode, it can be judged that the tested diode is a silicon tube or a germanium tube. This method is to connect a resistor (lkΩ) behind the power supply, and then connect with the diode according to the polarity characteristic to make the diode forward conducting. At this time, use a multimeter to measure the tube voltage drop. In addition, it is more convenient if it is used in energized dynamic measurement. 2.3 Testing Methods of Types of DiodesZener DiodesHow to test a Zener diode? The following here is to give some ideas.(1) Generally use the low-resistance block to test the Zener diode with a multimeter. Since the battery in the meter is 1.5V, this voltage is not enough to make the Zener diode reverse breakdown. So the forward and reverse resistance should be the same as a normal diode.(2) Measurement of the voltage stabilization value Vz of the Zener diode. When measuring diode, the power supply voltage must be greater than the stable voltage of the tube under test. In this way, the high-resistance block of the multimeter (R×10k) must be used. At this time, the battery in the meter has a higher voltage. When the multimeter's range is set to high barrier, measure diode reverse resistance. If the measured resistance is Rx, the voltage regulation value of the Zener diode is:In the formula, n is the override of the gear used. For example, if the highest electrical barrier ofR0 is the central resistance of the multimeter.E0 is the highest battery voltage value of the multimeter used.Example: Use an MF50 multimeter to measure a 2CW14 diode.R0=10Ω, the highest electrical barrier is R×10k.E0=15V, the measured reverse resistance is 75kΩ, then its voltage regulation value is:If the measured resistance is very large (close to infinite), it means that the voltage Vz under test is greater than E0, therefore, tube will not break down. If the measured resistance is very small (0 or only a few ohms), it means that the test probes are connected reversely, and then just swap the test probes. Light-emitting Diodes (LED)A light-emitting diode is a semiconductor device that converts electrical energy into light energy. It has the characteristics of small size, low working voltage and low working current.(1) There is a PN junction inside the light-emitting diode, so LED has the same characteristic of unidirectional conductivity. Its detection is similar to the measurement of ordinary diodes.(2) Use the R×1k or R×10k gear, and the forward and reverse resistance values are measured. Generally, the forward resistance is less than 50kΩ, and the reverse resistance is greater than 200kΩ.(3) The working current of the light-emitting diode is an important parameter. If the working current is too small, the light-emitting diode will not light up, and it is too large, the light-emitting diode will be easily damaged.(4) The forward turn-on voltage of the light-emitting diode is 1.2V ~ 2.5V, and the reverse breakdown voltage is about 5V. PhotodiodesPhotodiode is a semiconductor device that can convert light intensity into electrical signals.(1) There is a window on the top of the photodiode that can inject light, and the light irradiates the die through it. Under the excitation of the light, a large number of photoelectric particles are generated in the photodiode, which greatly enhances its conductivity and reduces internal resistance.(2) The photodiode is similar to the Zener diode. It also works in the reverse state, with reverse voltage.(3) The forward resistance of the photodiode does not change with the light. Its reverse resistance is larger when there is no light, and becomes smaller when it is exposed to light. That is, the stronger the light, the smaller the reverse resistance. Without light, the reverse resistance will return to the original value.(4) According to the related principle, use a multimeter to measure the reverse resistance of the photodiode. Change the light intensity when measuring, and observe the change of the reverse resistance of the photodiode. If there is no change or less change of the reverse resistance when light changes, it indicates that the tube has failed. High-speed Switching DiodesThe method of detecting high-speed silicon switching diodes is the same as that of ordinary diodes. The difference is that the forward resistance of this tube is relatively large. Measuring with Rxlk block, the forward resistance value is 5k ~ 10k in general, and the reverse resistance value is infinite. Fast Recovery Diodes / Ultrafast Recovery DiodesDetecting fast recovery and ultra-fast recovery diodes with a multimeter is basically the same as that of detecting plastic-encapsulated silicon rectifier diodes. That is, first use the Rxlk block to test its unidirectional conductivity. Generally, the size of forward resistance is about 4 ~ 5k, and the reverse resistance is infinite. And then use the Rxl block to repeat the test, at this time, the forward resistance is several ohms, and the reverse resistance is still infinite. DIAC (Diode for Alternating Current) DiodesUse the Rxlk block, and measure the forward and reverse resistance values of diac, which should be infinite. If the test probes are exchanged to measure, the pointer swings to the right, which indicates that the test tube has a leakage fault. Another method is placing the multimeter in the DC voltage block. During the test, shake the megohmmeter, and the voltage value indicated by the multimeter is the VBO value of the tube. Then change the two pins of the tested tube, and measure the VBR value in the same way. Finally, compare VBO and VBR. The smaller the difference between the absolute values of the two, the better the symmetry of the diac diode. TVS DiodesFor the dual TVS, resistance values between the two pins should be infinite when the red and black test probes of multimeter are exchanged at random. Otherwise, the tube has poor performance or has been damaged. High-frequency Varistor Diodesa. Identify Diode PolarityThe difference between high-frequency varistor diodes and ordinary diodes is that their color code is different. It is generally black of ordinary diodes, while high-frequency varistor diodes’ is light. Its polarity rule is similar to that of ordinary diodes. That is, the end with the green ring is the cathode, otherwise it is the anode.b. Measure Forward and Reverse ResistanceThe specific method is the same as the method of measuring ordinary diodes. Using the Rxlk block of a AM-500 multimeter, the forward resistance is 5k～55k, and the reverse resistance is infinite. Varactor DiodesUsing Rx10k block, no matter how the red and black test leads are exchanged for measurement, the resistance between the two pins of the varactor diode should be infinite. During the measurement, if the multimeter swings slightly to the right or the resistance value is zero, it means that the varactor diode under test has a leakage fault or has been broken down. No matter the loss of varactor diode capacity or internal open-circuit fault, it is impossible to detect them with a multimeter. When necessary, the replacement method can be used for inspection to make judgment. Infrared Light Emitting Diodes (IRED)Put the multimeter in the Rxlk block and measure the forward and reverse resistance of the IRED diode. Generally, the forward resistance should be about 30k, and the reverse resistance should be above 500k. It means the tube can work normally. The larger the reverse resistance, the better. IR Receiver Diodesa. Appearance Identification: Diode Cathode / Anode(1) Common infrared receiving diodes are black in appearance. In addition, there is a small oblique plane at the top of the tube body of the infrared receiving diode. Usually, the pin with one end of the oblique plane is the negative pole and the other end is the positive pole.(2) Use the Rxlk block to test the resistances between two pins. When a diode works normally, the resistance value of two pins are different. And exchange the test leads several times to get some pairs of values. According to the smaller resistance value, the pin connected to the red probe is the cathode, and the pin connected to the black probe is the anode.b. Performance DetectionUse a multimeter to measure the forward and reverse resistance of the infrared receiving diode. According to the resistance values, whether the diode is damaged can be judged preliminarily. Laser DiodesUse the Rxlk block of multimeter, and determine the order of the pins of the laser diode according to the method of detecting ordinary diodes. Because the forward voltage drop of the laser diode is larger than that of the ordinary diode, when detecting the forward resistance, the pointer of the multimeter is slightly deflected to the right, and the reverse resistance is infinite. Unijunction Transistor (UJT)a. Discrimination of ElectrodesBased on the R×1k block, use two meter pens to measure the forward and reverse resistance between any two of the three electrodes ( base B1 and base B2, and emitter E) of the ujt diode. The measured resistance values between the two electrodes are both 2~10kΩ, in addition, B1 and B2 will be different.b. Performance JudgmentThe performance of an ujt diode can be judged by measuring whether the resistance between its pins is normal. Use the R×1k barrier, the black test lead connect to the emitter E, and the red test lead connect to the two base electrodes in turn. Normally, a resistance value should be several thousand to ten thousand ohms. On the contrary, the red test lead connects to the emitter E, and the black test lead connects to the two base electrodes in turn, and the resistance should be infinite under normal conditions. The forward and reverse resistance values between the two bases are both in the range of 2~10kΩ. If they differ greatly from the normal value, the diode is damaged. Ⅲ Example Analysis3.1 Test a Diode in Circuita. Diode Test UsingAnalog MultimeterThe following measurements are all based on silicon diodes. If it is a germanium diode, the forward and reverse resistance of the diode will decrease.1) Measure forward resistance FRThe following figure is a wiring schematic diagram for measuring the forward resistance of a diode with an analog multimeter:Give the result as follows:IndicatorDescriptionUse the R×1k block to measure the diode, the forward resistance is several thousand ohms, and the pointer indicates stability. If the pointer swings slightly, it indicates that the thermal stability of the diode is poor.If the pointer indicates hundreds of kiloohms when measuring the forward resistance, it means that the diode is open.If the pointer indicates tens of kiloohms, it indicates that the diode has a large forward resistance and poor diode performance. Description of measurement of forward resistance:Forward Resistance (FR)DescriptionThousands of ohmsNormalZero or much less than a few thousand ohmsBreakdownHundreds of kilosLarge FR, the diode is openDozens of kilohmsLarge FR, bad forward characteristicsThe pointer is unstablePoor stability 2) Measure reverse resistance RRThe following figure is a wiring schematic diagram for measuring the reverse resistance of a diode with an analog multimeter:Give the result as follows:IndicatorDescriptionWhen measuring the reverse resistance, the value should be several hundred kiloohms. The larger the resistance value is, the better the indicator should be stable.If the reverse resistance is only a few thousand ohms, it means that the diode has broken down and has lost its unidirectional conductivity. Description of measurement of reverse resistanceReverse ResistanceDescriptionHundreds of kilosNormalZeroBreakdownMuch less than a few hundred thousand ohmsDiode’s reverse characteristic is not good.Pointer does not moveThe diode is open. Note: The reverse resistance of some diodes is very large, at this time, it is not certain that the diode is open, so that its forward resistance should be measured. If the value is normal, it means that the diode is not open.Pointer is unstableThe pointer cannot be stabilized at a certain resistance value during measurement, indicating that the diode has poor stability. 3.2 Power-off and Power-on Testing MethodsDiode in-circuit measurement is divided into two situations: Power-off and Power-on statea. Power-off MeasurementThere are something should be noted the method of this test.The influence of the external circuit on the test result is the same as the resistance and capacitance measured of internal circuit. And the influence of the measured forward resistance by the external circuit is lower than the reverse resistance.If there is any doubt about the measuring result, the diode should be removed from the circuit and measured separately. b. Power- on MeasurementWhen the circuit board is powered on, the test point is the tube voltage drop. Because the diode has a very important characteristic: when it is turned on, the tube voltage drop is basically unchanged. So the voltage drop is normal after being turned on, that is to say, the diode is normal.Measurement method: The diagram below shows the connection diagram of the tube voltage drop after the diode in the DC circuit. Setting multimeter in DC voltage 1 V block, the red probe is connected to the cathode of the diode, and the voltage indicated is the forward voltage drop of the diode. Diode forward voltage drop measurement results are analyzed as follows:DiodeDescriptionSilicon diode0.6VThe diode is normal and in a forward conducting state.> 0.6VThe diode is not in the conducting state.Close to 0The diode is in a breakdown state, the current in the loop will increase.Germanium diode0.2VThe diode is normal and in a forward conducting state.> 0.2VThe diode is off or is faulty.Close to 0In the breakdown state, the loop current increases significantly, without unidirectional conductivity. 3.3 ConclusionThe following points should be noted when measuring diodes:1) The diode in AC is in the cut-off state, because the diode is in the reverse state, and the reverse voltage at both ends is very large. The average voltage across the diode measured by the DC block is negative at this time.2) Use different blocks of the same multimeter to measure positive and negative resistance of one diode, their values will different. The forward and reverse resistances of the same diode measured with different multimeters are also different.3) When measuring the forward resistance of a diode, if the pointer cannot stop at a certain resistance value and constantly swings, it indicates that the thermal stability of the diode is not good.4) Some multimeters will provide a “diode check” function that displays the actual forward voltage of the diode when its conducting current. Such meters typically indicate a slightly lower forward voltage than what is “nominal state” for a diode, due to the very small amount of current used during the measurement. Frequently Asked Questions about Diode Test1. What is a diode test?A diode is best tested by measuring the voltage drop across the diode when it is forward-biased. ... A multimeter's Diode Test mode produces a small voltage between test leads. The multimeter then displays the voltage drop when the test leads are connected across a diode when forward-biased. 2. How do you test a rectifier diode?Touch the red (positive) probe of the multimeter to the positive terminal of the diode closet to the welder case interior. Touch the black (negative) probe of the multimeter to the negative terminal of the same diode. The multimeter should read a resistance between 0 and 1 ohm, or the diode is faulty. 3. How can you tell if a diode is positive or negative?Sometimes it's easiest to just use a multimeter to test for polarity. Turn the multimeter to the diode setting (usually indicated by a diode symbol), and touch each probe to one of the LED terminals. If the LED lights up, the positive probe is touching the anode, and the negative probe is touching the cathode. 4. How do you test a Schottky diode?Connect the red positive test lead to the anode of the Schottky diode and the black common test lead to the cathode of the diode. Listen for a “beep” or a “buzz” from the multimeter. If the Schottky diode responds as expected, the multimeter will sound a tone. 5. Can I test a diode in circuit?A diode is best tested by measuring the voltage drop across the diode when it is forward-biased. A forward-biased diode acts as a closed switch, permitting current to flow. A multimeter's Diode Test mode produces a small voltage between test leads. ... Voltage may be present in the circuit due to charged capacitors. 6. How do you check a diode?Diode PolarityThe polarity of both diodes is indicated with a stripe on one end of the body. The stripe corresponds to the line in the schematic symbol, indicating the cathode. The other end (no stripe) is the anode, indicated by the triangle in the schematic symbol. 7. What happens when a diode fails?However, a failed diode can short out too. In this case, the diode will exhibit a small resistance in both directions. The common reasons for a diode failure are excessive forward current and a large reverse voltage. Usually, large reverse voltage leads to a shorted diode while overcurrent makes it fail open. 8. How can you tell if a diode is blown?Turn the dial to “diode test” mode.This level of current is high enough to produce a reading, yet not so high that the diode will fail. It may also be labeled as “diode check” on your multimeter and is usually indicated by a small diode symbol. The diode symbol will look like a triangle pointing towards a line.