# How to Read and Understand Schematics in Electrical? Basic Symbols Expressions

Introduction

How to Read an Electrical Diagram Lesson

What is a circuit Diagram?

Circuit diagram is the basic of engineering research and planning. A schematic layout diagram, which is drawn with the standard symbol of physical electricity, can show the working principle of each component and device relationship, Each electronic component has a symbol. After seeing a few circuit diagrams, you’ll quickly learn how to distinguish the different symbols, and provide planning plan for installing electrons or electrical products. Circuit diagram is one of the basic skills that must be learned by electronic engineers. So this paper gathers the classical circuit materials related to regulated voltage power supply, DCDC conversion power supply, switching power supply, charging circuit, constant current source to provide the most practical circuit diagram reference for engineers.

Schematic Symbols

Basic Devices

 A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses.

 An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a coil around a core.

 An electric battery is a device consisting of one or more electrochemical cells with external connections provided to power electrical devices such as flashlights, smartphones, and electric cars.[1] When a battery is supplying electric power, its positive terminal is the cathode and its negative terminal is the anode.[2] The terminal marked negative is the source of electrons that will flow through an external electric circuit to the positive terminal.

 A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.

 An operational amplifier (often op-amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output.[1] In this configuration, an op-amp produces an output potential (relative to circuit ground) that is typically hundreds of thousands of times larger than the potential difference between its input terminals.

 A capacitor is a passive two-terminal electronic component that stores electrical energy in an electric field. The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical conductors in proximity in a circuit, a capacitor is a component designed to add capacitance to a circuit.

Five Parts to Understand Circuit Diagrams

*Regulated Power Supply

1. The voltage adjustable range is between 3.5V~25V, the output current is large, using VR- tube circuit to obtain the stable output voltage.

Working principle: after rectifying and filtering, DC voltage is supplied by R1 to the base of the adjusting tube, so that the adjusting tube can be switched on. When the voltage passes through the RP, R2 of the V1 conduction, V2 switched on, and then V3 is switched on. At this time, the emitter and collector voltage of V1, V2 and V3 do not change (it acts exactly like a voltage stabilizer). A stable output voltage can be obtained by adjusting RP, and the ratio of R1, PR, R2 to R3 determines the output voltage of the circuit.

 T: 80W～100W     Input: AC220V Output Duplex Winding: AC28V RP: 1W (resistance: 250K～330K) FU1: 1A                FU2: 3A~5A VD1 | VD2: 6A02 C4: 470µF/35V(electrolytic capacitor) C1: 3300µF / 35V C2 | C3: 0.1µF (MONO CAP) R1: 180～220Ω / 0.1W～1W

Fig 1. VR-tube Circuit

2. Regulated Voltage Adjustable Power Supply Circuit Diagram

Whether the computer detection or electronic product can not be separated from the regulated power supply(RPS). This paper introduces one kind of RPS: a DC voltage continuously adjustable from 3V to 15V, the maximum current can be up to 10A, and the circuit uses a high precision standard voltage source integrated circuit (TL431) with temperature compensation which makes the voltage stabilizer more accurate. If there is no special requirement, it can basically meet the normal maintenance. The circuit is shown in the figure below.

Fig 2. Regulated Voltage Adjustable Power Supply Circuit Diagram

Its working principle is divided into two parts. The first part is a fixed 5V/1.5A power supply circuit; the second part is a high precision and large current regulator circuit which can be adjusted continuously from 3V to 15V.

The first circuit is very simple. The DC voltage rectified by silicon bridge QL1 is filtered by C1 from the secondary 8V AC voltage of transformer, then the 5V three-terminal stabilizer block LM7805 can produce a fixed 5V | 1A power supply at the output end without any adjustment. This power supply can be used as an internal power source when the computer board is overhauled.

The second part is basically the same as the common series power supply. The circuit is simple, the cost is low, but the voltage stabilizer performance is very high.

The resistor R4, the regulator TL431, potentiometer R3 constitutes a continuously adjustable constant voltage source, which provides the reference voltage for the BG2 base. The regulated voltage value of the regulator TL431 is continuously adjustable, which determines the maximum output voltage of the power supply. If you want to expand the range of adjustable voltage, you can change the resistance values of R4 and R3, of course, the secondary voltage of transformer should also be increased.

The power of the transformer can be controlled flexibly according to the output current, and the secondary voltage is about 15 V. Bridge rectifier QL, using 15A-20A silicon bridge, compact structure, fixed screws in the middle, can be directly fixed on the shell aluminum plate, better for heat sink.

What adjusts the tube is the high current NPN metal shell silicon tube, because it has the very big heat, if the chassis allows, buying the big radiator as far as possible to expand the heat dissipation area; if does not need the big current, a smaller power silicon tube can be used to makes it smaller.

The filter uses three 50V/4700uF electrolytic capacitance C5 and C7 in parallel, respectively, to make the output of large current more stable. In addition, this capacitor should be bought with a relatively larger volume, and those smaller ones will also mark 50V/4700uF, but the voltage fluctuates frequently, Or easy to fail for a long time lay idle.

Finally, the power transformer can buy a ready-made switching power supply of more than 200W instead of the transformer. In this way, the voltage stability can be further improved, but the manufacturing cost is not too high, and other electronic components have no special requirements. After installation is completed, it can work properly without too much adjustment.

*Switched Power Supply(specific examples)

The working principle of integrated control IC-UC3842 for PWM switching power supply

The following is the UC3842 internal block diagram and pin diagram. UC3842 uses a fixed frequency pulse width controllable modulation mode, a total of 8 pins, each foot function as follows:

Pin① is the output of the error amplifier, and the external resistor-capacitor unit is used to improve the gain and frequency characteristics of the error amplifier;

Pin② is the feedback voltage input, which is compared with the 2.5 V reference voltage at the same phase of the error amplifier to generate the error voltage, thus controlling the pulse width;

Pin③ is the current detection input, when detecting voltage exceeds 1V, the pulse width is reduced so that the power supply is in the state of intermittent operation;

Pin④ is the timing end, the operating frequency of the internal oscillator is determined by the external resistor-capacitor time constant, f=1.8 / (RT×CT);

Pin⑤ is the common ground;

Pin⑦ is a DC power supply terminal with the function of undervoltage and overvoltage locking, the chip power consumption is 15mW.

Pin⑧ is the output terminal of 5V reference voltage, its load capacity is 50mA.

Fig 3. IC-UC3842 Electrical Diagram

UC3842 Internal Schematic Diagram

UC3842 is an integrated controller of PWM switching power supply with excellent performance, wide application and simple structure. Because it has only one output, it is mainly used for voice control.

The UC3842 pin7 is a voltage input with a starting voltage range of 16V-34V. When the power supply is on, the VCC is less than 16V, and the output of the Schmidt comparator is 0. At the same time, no reference voltage is generated and the circuit does not work. When Vcc > 16V, the input voltage Schmidt comparator sends out a high voltage to the 5V fern voltage regulator, which generates a 5V reference voltage. On the one hand, this voltage used in internal circuit; on the other hand, it provides a reference voltage to the outside through pin8. Once the Schmidt comparator flips to a high level (when the chip starts working), Vcc can change in the 10V-34V range without affecting circuit; when the Vcc is below 10V, the Schmidt comparator flips to a low level and the circuit stops working.

When the reference voltage stabilizer has a 5V reference voltage output, the reference voltage detection logic comparator outputs a high level signal to the output circuit. At the same time, the oscillator will generate the oscillation signal of the f=Rt/Ct according to the parameters of the pin④ external Rt and Ct, which is added directly to the input of the totem pole circuit, the other is added to the position end of RS flip-flop made by PWM pulse width modulator, and the output end of R connects the output of current-detection comparator. The R-terminal is the control end of the duty ratio. When the R voltage rises, the Q pulse is widened. At the same time, the pulse width of pin⑥ is widened (duty cycle increased); when the R voltage drops, The Q pulse narrows and the pin⑥ pulse width becomes narrow (duty cycle reduced).

The sequence of UC3842 points is as shown in the diagram. Only when the E point is in high level, and meanwhile, a point and b point is all in high level, the d point sends out the high level, the c point sends the low level, otherwise the d point sends the low level, c point sends out the high level. Pin② generally connects the feedback signal. When the pin②voltage increases, the pin① voltage will decrease, and the R-terminal voltage will also decrease, so the pin⑥ pulse will narrow, on the contrary, the pin⑥ pulse will become wider.

Pin③ is a current sensing terminal. Usually, a small sample resistor is inserted into the source or emitter of the power transistor to convert the current passing through the switch to a voltage, and the voltage is introduced into the pin. When the load short circuit or other reasons cause the current of the power transistor to increase and the voltage on the sampling resistance exceeds 1V, the pulse output pin⑥ is stopped, which can effectively protect the power transistor from damage.

Fig 4. UC3842 Internal Schematic Diagram

TOP224P 12V | 20W Switching DC Power Supply Circuit Based on Regulated Voltage

Two integrated circuits are used in the circuit: TOP224P three-terminal monolithic switching power supply (IC1) and PC817A linear optical coupler (IC2). After UR and Cl rectifier filter, AC power supply produces DC high voltage Ui, to supply primary winding of high frequency transformer T.

VDz1 and VD1 can clamp the peak voltage of leakage inductance to the safe value and can attenuate the ringing voltage. VDz1 adopts P6KE200 type transient voltage suppressor with reverse breakdown voltage 200V, and VDl uses UF4005 type UFRD in 1A/600V.

The secondary winding voltage is filtered by V, C2, L1 and C3 rectifier, getting 12V output voltage Uo. Uo value is set by the sum of the forward voltage drop UF, R1 of LED and the value of regulated voltage Uz2.

Other output voltage values can be obtained by changing the turn ratio of high frequency transformer and the regulated voltage value of VDz2. R2 and VDz2 also provide a false load for 12V output to improve the load adjustment rate at light load. The feedback winding voltage is filtered by VD3 and C4 rectifier to supply the bias voltage required by TOP224P. Since the control current is regulated by R2 and VDz2, the output duty cycle is changed to stabilize the voltage.

The common mode choke L2 can reduce the common mode leakage current generated by the waveform of the high voltage switch connected to the D by the primary winding. C7 is a protective capacitor used to filter out interference caused by coupling capacitors of primary and secondary windings. C6 can reduce the differential mode leakage current caused by the fundamental and harmonic waves of the primary winding current. C5 can not only filter the peak current added to the control terminal, but also determine the self-starting frequency, compensating the control loop with R1 and R3.

Fig 5. TOP224P 12V | 20W Switching DC Power Supply Circuit

The Main Technical Specifications of This Power Supply are as Follows

AC Voltage: u=85～265V

Voltage Regulation: η=78％

Grid Frequency: fLl=47～440Hz

Input Voltage (Io=1.67A): Uo=12V

Working Temperature: TA=0～50℃

Maximum Output Current: IOM=1.67A

Maximum Output Ripple Voltage: ±60mV

Continuous Power Output: Po=20W /TA=25℃ or 15W /TA=50℃）

*DC-DC Power Supply

3V→+5V or +12V Circuit

Portable electronic products powered by batteries generally use low power supply voltage, which can reduce the number of batteries and product size. In order to ensure the stability and accuracy of the circuit, it is necessary to use a regulated power supply.

If the circuit uses 5V working voltage, but one component requires a higher working voltage, this often makes the designer feeling hard. In this paper, a circuit composed of two booster modules is introduced to solve this problem, and only two batteries are used to supply power.

The circuit has fewer components, small size, light weight, stable output of 5V or 12V, and meets the requirements of portable electronic products. +5V power supply can output 60mA, and +12 V power supply maximum output current is 5 mA.

Fig 6. 3V→+5V or +12V Circuit

The circuit is shown above. It is composed of AH805 and FP106 booster module. AH805 is a kind of boost module with an input of 1.2V~3V and an output of 5V, which can output 100mA current at 3V. FP106 is a chip boost module with input of 4V~6V and output fixed voltage of 29 ±1V, the output current up to 40 mA.  AH805 and FP106 are both a level-controlled to shut down the power.

The output voltage of two 1.5V alkaline batteries is 3V, inputting to the AH805, and its output voltage is 5V, inputting 5V to the FP106, and the output voltage is 28V~30V, and then the output voltage is 12 V after through the voltage stabilizer.

It can be seen from the diagram that different output voltages can be obtained by changing the stabilizer voltage. Pin⑤ of FP106 is the closing end of controlling the power supply. When Pin⑤ is added a high level > 2.5V, the power supply is switched on; When adding the low level is less than 0.4V, the power supply is off. It can be controlled by circuit or manually. If it is not necessary, Pin⑤ is connected to Pin⑧.

MC34063 3.6V→9V Circuit

Working State:

No-load: Output 3.65V| 18uA  Load: Output 9.88V | 50.2mA; Input 3.65V | 186.7mA, efficiency 72%

Working Principle:

When there is no load, the IC has no power on pin⑥ and stops working. The input current is only 18uA with input 3.65V.

When there is a load (Q1 has Ieb current), the EC pole of 8550 is switched on and the IC is operating. Whether the IC works is determined by whether there is a load or not, it is quite a battery. Using IC has a high voltage conversion efficiency and output stably.

If this circuit adds a point of improvement, for example, when increasing power, it can turn into a power supply from 4.2V to 5V without switch. You can use a battery box as a backup power source for your phone.

Fig 7. MC34063 3.6V→9V Circuit

*Charging Circuit

lm358 basic Battery Charger Circuit Diagram

Fig 8. lm358 basic Battery Charger Circuit Diagram

There are two different arguments about whether alkaline batteries can be recharged. Some can be filled; the other say it has a risk of explosion. In fact, alkaline batteries can be rechargeable, generally 30-50 times of its service life.

In fact, due to the charging methods, there are two different consequences. First of all, there is no doubt that alkaline batteries can be rechargeable, and in the battery instructions, it is mentioned that alkaline batteries are not rechargeable and that charging can lead to explosions.

That's true, but note that the word is "could". Actually, it can be viewed as a manufacturer's self-protection statement of exemption. The key to charging alkaline batteries is temperature. As long as you can charge the battery without high temperature, you can successfully do it. The right charging method requires several points:

small current: 50mA  charge 1.7V  discharge 1.3V

After some people tried charging practice, they said categorically that they could not recharge. The reason for the problems such as lack of charging, short electricity consumption, leakage, explosion, actually, most are charger problems. If the charging current of the charger is too large, far more than 50 ma, and some fast chargers is above 200ma, the direct result is that the temperature of the battery is very high. If the battery is hot, the batteries will leak, and the serious will explode.

Some people use Ni-MH rechargeable battery charger to charge, low grade charger does not automatically stop charging function, after long time charging will lead to overcharge then causing battery leakage and explosion. A better charger has the function of automatic shutdown, but the stop charge voltage is generally set to 1.42 V of the Ni-MH rechargeable battery, while the voltage of the alkaline battery is about 1.7V when it fully charged.

As a result, the voltage is too low which causing fake charge. And not to wait until the battery is completely out of power to charge, it will lead to poor lifetime of the battery.

It is recommended that the voltage of alkaline battery is not less than 1.3V. Therefore, if you plan to charge the alkaline battery, you must have a qualified charger, charging current around 50mA, and charging cut-off voltage is about 1.7V.

Related Description

Alkaline manganese rechargeable battery: based on alkaline zinc manganese battery, it is also called mercury-free alkaline manganese battery because of the use of mercury-free zinc powder and new additives. The battery can be recharged for dozens to hundreds of times without changing the discharge characteristics of the alkaline battery, which is more economical.

Alkaline zinc-manganese battery was developed in 1882. It was developed in 1912 and put into production in 1949. It has been found that when KOH electrolyte solution replaces NH4Cl as electrolyte, both the electrolyte and the structure change greatly, its performance improved significantly.

Features

Open voltage is 1.5V

Working temperature is between -20℃ to 60℃, it is suitable in alpine region.

The capacity of high current continuous discharge is about 5 times that of acid zinc-manganese battery.

2.75W USB Charger

This design adopts Power Integrations's LinkSwitch series product LNK613DG. This design is well suited for mobile phones or similar USB charger applications, including mobile phone battery chargers, USB chargers, or any application with constant voltage or constant current.

In the circuit, the diode D1 to D4 rectifies the AC input, and the capacitors C1 and C2 filter the DC. The L1, C1 and C2 form a π type filter to attenuate the differential mode conduction EMI noise. These are connected by E-sheild technology of Power Integrations transformers. This design can easily meet the requirements of EN55022 B-type conduction EMI with sufficient margin, and no Y capacitor is required. Fire proof, fusible, winding resistor RF1 provides fault protection and limits surge current generated during startup.

Fig 9. 2.75W USB Charger Circuit

Fig 9 shows that U1 is powered by optional offset power, which reduces no-load power to less than 40 mW. The value of by-pass capacitance C4 determines the number of cable voltage drop compensation. The value of 1μF corresponds to the compensation of a 0.3Ω / 24 AWG USB output cable. (10μF capacitance compensates 0.49 Ω / 26 AWG USB output cable.).

In the constant voltage stage, the output voltage is regulated by switch control. The output voltage is maintained by skipping the switching cycle. By adjusting the ratio of the prohibition period to maintain voltage regularly. This also optimizes the efficiency of the converter throughout the load range. Under the condition of light load (trickle charge), the current limit will be decreased to reduce the magnetic flux density of the transformer, thus reducing the audio noise and switching loss. With the increase of load current, the current limit will increase, and the skipping period will be reduced continuously.

When no longer skipping any switching period (maximum power point), the controller in the LinkSwitch-II switches to constant current mode. When the load current needs to be further increased, the output voltage will decrease, and it reflects in the FB pin voltage. In response to the voltage drop of the FB pin, the switching frequency will decrease linearly to achieve constant current output.

The RCD-R clamping circuit is composed of D5, R2, R3 and C3, which is used to limit the leakage voltage spike caused by leakage inductance. Resistance R3 has a relatively large value to avoid drain voltage waveform oscillations caused by leakage inductance, which prevents excessive oscillation during turn-off, thus reducing EMI conduction.

Diode D7 rectifies secondary and C7 filters it. C6 and R7 together limit the transient voltage spike on D7 and reduce EMI conduction and radiation. The resistor R8 and Zener diode VR1 form an false output load which ensures that the output voltage is within an acceptable limit and that the battery does not discharge completely when the charger is off. Feedback resistors R5 and R6 set maximum operating frequency and output voltage at constant voltage stage.

*Constant-Current Source

1. Discussion on How to Design Three-wire Constant Current Source Driving Circuit

The constant current source drive circuit is responsible for driving the temperature sensor Pt1000, to convert its sensing resistive signal with temperature into measurable voltage signal. In this system, the required constant current source should have good temperature stability, large output resistance, output current less than 0.5mA (upper limit of Pt1000 without self-heating effect), earthing at one end of load, and variable polarity of output current.

Because the influence of temperature on the parameters of integrated operational amplifier is less significant than of the transistor or FET, the constant current source composed of integrated operational amplifier has the advantages of better stability and higher constant current performance. Especially in the case where one end of the load needs grounding, it has been widely used. So use the dual operational amplifier constant current source shown in figure 2. Amplifier UA1 is used as adder, UA2 as follower, UA1 and UA2 are gain bipolar operational amplifier OP07,  which having low noise, low misalignment and high open-loop.

Fig 10. Three-wire Constant Current Source Driving Circuit

Vb and Va are the up and down potential of the reference resistor Rref in figure 2: Va is the output of in-phase adder UA1. When taking the resistor R1= R2 , R3=R4, the output current of the Va=VREFx+Vb.

It can be seen that the dual operational amplifier constant-current source has the following remarkable characteristics:

Load earthing

The output current is bipolar when the operational amplifier is supplied by a dual power source.

The constant current can be achieved by changing the input reference VREF or adjusting the reference resistor Rref0. It is easy to obtain stable small current and compensation calibration.

Because of the mismatch of the resistor, the voltage at both ends of the reference resistance Rref0 will be affected by the terminal voltage Vb of its driving load. At the same time, as a constant current source, Vb will definitely change with the load, which will affect the stability of the constant current source. Therefore, the four resistors R1, R2, R3, R4 are chosen according to the principle that the mismatch should be as small as possible, in addition, the mismatch direction of each pair of resistors should be consistent. In practice, a large number of precision resistors of the same batch can be screened, and 4 resistors with close resistance values can be selected.

2. High Voltage Constant Current Source Circuit Diagram(switch power model)

The instrument needs a constant current source that can generate 1mA current on 0 to 3 megabytes ohmic resistance. A design composed with 12V storage battery and UC3845 has be made: the transformer uses a color TV high voltage packet, in which L1 enamelled wire is wound 24 turns on the core of the original high voltage package; L3 uses a coil of the original high voltage package and L2 with the high voltage part of the high voltage packet; L3 and LM393 constitute a voltage limiting circuit which limits the output voltage too high and adjusts the open-circuit output voltage by adjusting R10.

Fig 11. High Voltage Constant Current Source Circuit Diagram(switch power model)

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