DETAILED EXPLANATION OF FIVE CLASSIC CIRCUITS

The circuit diagram is one of the basic skills that electronics engineers must learn. This article combines the classic integrated circuit data related to regulated power supply, DC to DC conversion power supply, switching power supply, charging circuit and constant current source to provide engineers with the most fresh circuit diagram reference materials.
 
First, the power supply
 
1, 3 ~ 25V voltage adjustable voltage regulator Electronic circuit diagram
 
The adjustable power supply can be adjusted between 3.5V and 25V, the output current is large, and the adjustable voltage regulator circuit is used to obtain a satisfactory and stable output voltage.
 
Working principle: After rectification and filtering, the DC voltage is supplied to the base of the adjusting tube by R1, so that the adjusting tube is turned on. When V1 is turned on, the voltage passes through RP and R2 to make V2 turn on, and then V3 is also turned on. At this time, V1. The emitter and collector voltages of V2 and V3 are no longer changing (the effect is exactly the same as the Zener). Adjusting the RP can obtain a smooth output voltage. The ratio of R1, RP, R2 and R3 determines the voltage value output by this circuit.
 
Component selection: Transformer T selects 80W ~ 100W, input AC220V, output double winding AC28V. FU1 selects 1A, FU2 selects 3A~5A. VD1 and VD2 use 6A02. RP selects 1W or so common potentiometer, the resistance is 250K~330K, C1 selects 3300μF/35V electrolytic capacitor, C2, C3 selects 0.1μF monolithic capacitor, C4 selects 470μF/35V electrolytic capacitor. R1 selects 180~220Ω/0.1W~1W, R2, R4, R5 selects 10KΩ, 1/8W. V1 selects 2N3055, V2 selects 3DG180 or 2SC3953, V3 selects 3CG12 or 3CG80.
 
2, 10A3 ~ 15V regulated power supply circuit diagram
 
Whether it is maintenance computer or electronic production is inseparable from the regulated power supply, the following describes a DC power supply from 3V to 15V continuously adjustable power supply, the maximum current can reach 10A, the circuit uses temperature compensation characteristics, high precision The standard voltage source integrated circuit TL431 makes the voltage regulation higher. If there is no special requirement, it can basically meet the normal maintenance and use. The circuit is shown in the figure below.
 
The working principle is divided into two parts. The first part is a fixed 5V1.5A regulated power supply circuit, and the second part is another high-precision high-current voltage regulator circuit with 3 to 15V continuous adjustment.
 
The circuit of the first circuit is very simple. The DC voltage rectified by the transformer secondary 8V AC voltage through the silicon bridge QL1 is filtered by the C1 electrolytic capacitor, and then the 5V three-terminal regulator block LM7805 is not used for any adjustment and can be generated at the output. A fixed 5V1A regulated power supply that can be used as an internal power supply when servicing a computer board.
 
The second part is basically the same as the ordinary series-type regulated power supply. The difference is that the high-precision standard voltage source integrated circuit TL431 with temperature compensation characteristics is used, so the circuit is simplified, the cost is reduced, and the voltage regulation performance is high. .
 
In the figure, the resistor R4, the voltage regulator TL431, and the potentiometer R3 form a continuously adjustable constant voltage source, which provides a reference voltage for the BG2 base. The voltage regulation value of the voltage regulator TL431 is continuously adjustable, and this regulation value determines the stability. The maximum output voltage of the voltage supply. If you want to expand the adjustable voltage range, you can change the resistance of R4 and R3. Of course, the secondary voltage of the transformer also needs to be increased.
 
The power of the transformer can be flexibly controlled according to the output current, and the secondary voltage is about 15V. The rectifier tube QL used for bridge rectification uses a 15-20A silicon bridge. It has a compact structure and a fixing screw in the middle. It can be directly fixed to the aluminum plate of the casing to facilitate heat dissipation.
 
The adjustment tube uses a high-current NPN-type metal-shell silicon tube. Because of its large heat generation, if the chassis allows, try to buy a large heat sink to expand the heat-dissipation area. If you do not need a large current, you can also use a smaller power. Silicon tube, which can be made smaller.
 
Filtering 50V4700uF electrolytic capacitors C5 and C7 are connected in parallel with three, which makes the large current output more stable. In addition, this capacitor should be purchased in a relatively large volume. Those smaller in size are also marked with 50V4700uF as much as possible. When encountering frequent voltage fluctuations, or It is easy to fail after a long time of use.
 
Finally, let’s talk about the power transformer. If you don’t have the ability to wrap it yourself, you can’t buy a ready-made one. You can buy a ready-made switching power supply of more than 200W instead of a transformer. This can further improve the voltage regulation performance, but the production cost is not too bad. Other electronic components have no special requirements, and they can work normally without much adjustment after installation.
 
Second, switching power supply
 
1, PWM switching power supply integrated control IC-UC3842 working principle
 
The following figure shows the internal block diagram and pin diagram of UC3842. The UC3842 adopts a fixed operating frequency pulse width controllable modulation method. There are 8 pins in total, and the functions of each pin are as follows:
 
Pin 1 is the output of the error amplifier, and the external resistor-capacitor is used to improve the gain and frequency characteristics of the error amplifier;
 
The 2 pin is the feedback voltage input terminal, and the voltage of the pin is compared with the 2.5V reference voltage of the non-inverting terminal of the error amplifier to generate an error voltage, thereby controlling the pulse width;
 
Pin 3 is the current detection input terminal. When the detection voltage exceeds 1V, the pulse width is reduced to make the power supply in intermittent operation.
 
The 4th pin is the timing terminal, and the operating frequency of the internal oscillator is determined by the external resistance time constant, f=1.8/(RT×CT);
 
5 feet are public places;
 
The 6-pin is a push-pull output terminal, and the inside is a totem pole type. The rising and falling time is only 50 ns and the driving capability is ±1 A;
 
The 7-pin is a DC power supply terminal with under- and over-voltage lockout function. The power consumption of the chip is 15mW. The 8-pin is a 5V reference voltage output terminal with a load capacity of 50mA.
 
UC3842 is a PWM switching power supply integrated controller with excellent performance, wide application and simple structure. Because it has only one output, it is mainly used for switching power supply for sound terminal control.
 
The UC3842 7 pin is a voltage input terminal with a startup voltage range of 16-34V. When the power is turned on, VCC<16V, the input voltage trimmer comparator output is 0. At this time, no reference voltage is generated, and the circuit does not work. When Vcc>16V, the input voltage Schmitt comparator sends a high level to 5V fern. The regulator generates a 5V reference voltage, which is supplied on the one hand to the internal circuit and on the other hand
 
Pin 8 provides a reference voltage to the outside. Once the Schmitt comparator is flipped high (after the chip has started operating), Vcc can be varied from 10V to 34V without affecting the operating state of the circuit. When Vcc is below 10V, the Schmitt comparator flips low again and the circuit stops working.
 
When the reference regulator has a 5V reference output, the reference sense logic comparator reaches a high level signal to the output circuit. At the same time, the oscillator will generate an oscillating signal of f=/Rt.Ct according to the external Rt and Ct parameters of the 4-pin. This signal is directly applied to the input terminal of the totem pole circuit.
 
The other circuit is applied to the set terminal of the PWM pulse width commercial RS flip-flop, and the R terminal of the RS type PWN pulse width modulator is connected to the current detecting comparator output. The R terminal is the duty control terminal. When the R voltage rises, the Q pulse is widened, and the pulse width of the 6 pin is also widened (the duty ratio is increased); when the R terminal voltage drops, the Q pulse is narrowed. At the same time, the pulse width of the 6-pin feed is also narrowed (the duty ratio is reduced).
 
The timing of each point of UC3842 is shown in the figure. Only when E point is high level, there is signal output, and when a and b points are all high level, d point will send high level, c point will send low level, otherwise d The point is sent low and c point is sent high. The 2 pin is generally connected to the output voltage sampling signal, also called the feedback signal. When the voltage of the 2 pin rises, the voltage of the 1 pin will drop, and the voltage of the R terminal will also drop, so the 6-pulse pulse will be narrowed; otherwise, the 6-pulse pulse will be widened.
 
The 3 pin is a current sensing terminal. Usually, a small resistance sampling resistor is connected in the source or the emitter of the power tube, and the current flowing through the switching tube is converted into a voltage, and the voltage is introduced into the foot. When the load is short-circuited or other causes the power tube current increases, and the voltage on the sampling resistor exceeds 1V, the 6-pin stops the pulse output, so that the power tube can be effectively protected from damage.
 
2, TOP224P 12V, 20W switching DC stabilized power supply circuit
 
The 12V, 20W switching DC stabilized power supply circuit composed of TOP224P is shown in the figure.
 
Two integrated circuits are used in the circuit: TOP224P three-terminal single-chip switching power supply (IC1), PC817Alinear optical coupler (IC2). The AC power source is rectified and filtered by UR and Cl to generate a DC high voltage Ui, which supplies power to the primary winding of the high frequency transformer T.
 
VDz1 and VD1 clamp the spike voltage generated by the leakage inductance to a safe value and attenuate the ringing voltage. VDz1 uses a P6KE200 transient voltage suppressor with a reverse breakdown voltage of 200V. VDl uses a 1A/600V UF4005 ultrafast recovery diode.
 
The secondary winding voltage is rectified and filtered, C2, L1 and C3 to obtain a 12V output voltage Uo. The Uo value is set by the sum of the VDz2 stable voltage Uz2, the forward voltage drop UF of the LED in the opt coupler, and the voltage drop across R1.
 
By changing the turns ratio of the high frequency transformer and the regulated value of VDz2, other output voltage values ​​can also be obtained. R2 and VDz2 also provide a dummy load for the 12V output to improve load regulation at light loads. The feedback winding voltage is rectified and filtered by VD3 and C4 to supply the required bias voltage of TOP224P. The control terminal current is regulated by R2 and VDz2, and the regulation is achieved by changing the output duty cycle.
 
The common mode choke L2 can reduce the common mode leakage current generated by the high voltage switching waveform of the primary winding connected to the D terminal. C7 is a protection capacitor that filters out interference caused by the coupling capacitance of the primary and secondary windings. C6 reduces the differential mode leakage current generated by the fundamental and harmonics of the primary winding current. C5 not only filters out the spike current applied to the control terminal, but also determines the self-starting frequency. It also compensates the control loop with R1 and R3.
 
The main technical indicators of this power supply are as follows:
 
AC input voltage range: u=85~265V;
Input grid frequency: fLl=47~440Hz;
Output voltage (Io=1.67A): Uo=12V;
Maximum output current: IOM=1.67A;
Continuous output power: Po=20W (TA=25°C, or 15W (TA=50°C);
Voltage regulation rate: η = 78%;
The maximum value of the output ripple voltage: ±60mV;
Operating temperature range: TA = 0 ~ 50 °C.
 
Third, DC-DC power supply
1, 3V turn +5V, +12V circuit diagram
 
Battery-powered portable electronic products generally use a low power supply voltage, which can reduce the number of batteries and reduce the size and weight of the product. Therefore, 3 to 5V is commonly used as the operating voltage to ensure the stability and accuracy of the circuit operation. Requires a regulated power supply.
If the circuit uses a 5V operating voltage, but requires a higher operating voltage, this often makes the designer difficult. This article describes a circuit that uses two boost modules to solve this problem and is powered by two batteries.
 
The circuit is characterized by few external components, small size, light weight, and the output of +5V and +12V are stable, meeting the requirements of portable electronic products. The +5V power supply can output 60mA, and the +12V power supply has a maximum output current of 5mA.
 
The circuit is shown above. It consists of the AH805 boost module and the FP106 boost module. The AH805 is a boost module with an input of 1.2 to 3V and an output of 5V. It can output 100mA when it is powered by 3V. The FP106 is a chip-type boost module with an input of 4 to 6V, a fixed output voltage of 29±1V, and an output current of up to 40mA. The AH805 and FP106 are both level-controlled shutdown power control terminals.
 
Two 1.5V alkaline battery output 3V voltage input AH805, AH805 output +5V voltage, one way for 5V output, the other input FP106 to produce 28 ~ 30V voltage, after the voltage regulator is regulated, output +12V voltage.
 
As can be seen from the figure, as long as the voltage regulator voltage is changed, different output voltages can be obtained, which is very flexible. The fifth pin of the FP106 is the control power off terminal. When the power is turned off, the power consumption is almost zero. When the 5th pin is high level 2.5V, the power is turned on; when the 5th pin is low level <0.4V. When the power is turned off. It can be controlled by circuit or manually. If no control is needed, pin 5 and pin 8 are connected.
 
2, use MC34063 to  3.6V electric 9V circuit diagram
 
Working status:
 
No load: Input: 3.65V, 18uA (approximately 600mAH battery standby for more than three years)
Load: Output: 9.88V, 50.2mA, input: 3.65V, 186.7mA, efficiency is 72%
 
 
working principle:
 
When there is no load, the IC 6 has no power and stops working. The input current of 3.65V is only 18uA (the battery of 600mAH is more than three years standby)!
When there is load (Q1 has Ieb current), the EC pole of 8550 is turned on, and the IC is energized. Whether the IC works depends on whether there is load or not, it is equivalent to a battery. With IC for high voltage conversion efficiency, the output is stable!
This circuit is improved, and the power can be increased to “4.2V to 5V mobile power supply without switching.” You can use a battery box to make backup power for your phone!
 
Fourth, the charging circuit
 
1, lm358 alkaline battery charger circuit diagram
 
There are two different ways to charge an alkaline battery. Some say that they can be charged, and the effect is very good. Some say that it can’t be charged, and the battery description indicates that there is a danger of explosion. In fact, alkaline batteries are indeed rechargeable, and the number of times of charging is generally about 30-50 times.
 
In fact, due to the mastery of the charging method, it has two distinct consequences. First of all, it is undoubted that the alkaline battery can be charged. At the same time, in the description of the battery, it is mentioned that the alkaline battery is not rechargeable, and charging may cause an explosion.
 
This is also true, but note that the wording here is “probably” causing an explosion. You can also understand the manufacturer’s disclaimer of self-protection. The key to alkaline battery charging is temperature. As long as the battery can be charged without high temperature, the charging process can be completed smoothly. There are several requirements for the correct charging method:
 
Small current 50MA
But charge 1.7V, but put 1.3V
 
Some people try to charge the practice, they can not charge, the reason why there is no charge, short time, leakage, explosion, etc., most of the problems with the charger, if the charger charging current is too large, far exceeds 50ma, such as some fast charger charging current is more than 200ma, the direct result is that the battery temperature is very high, touch it hot, light will leak, serious will explode.
 
Some people use a nickel-metal hydride rechargeable battery charger to charge, low-end chargers do not have automatic stop charging function, long-term charging causes battery overcharge will also leak and explode. A better charger has an automatic stop function, but the charge voltage is generally set to 1.42V for a Ni-MH rechargeable battery, while the alkaline battery fill voltage is about 1.7V.
 
Therefore, the voltage is too low, it feels that it is not charged, and the power consumption time is short, which has no effect. Then there is the battery, but the finger does not wait until the battery is completely dead and then recharged. In this way, even the best battery can be charged three or five times, and the effect is poor.
 
It is generally recommended to use a battery voltage not less than 1.3V. Therefore, if you plan to charge alkaline batteries, you must have a qualified charger, charging current of about 50ma, charging cut-off voltage of about 1.7V. Look at your home charger.
 
There are so-called patented products on the market that sell special chargers for alkaline batteries. In fact, it is a simple circuit with a charging voltage of 1.7V and a current of 50ma. Using the existing parts LM358 and TL431 at hand, I made a simple circuit, the cut-off voltage of 1.67V automatically stops charging, For interested friends’ reference.
 
 
Its characteristics:
 
The open circuit voltage is 1.5V;
The operating temperature range is between -20 ° C and 60 ° C, suitable for use in alpine regions;
The current of continuous discharge of large current is about 5 times that of acid zinc-manganese battery;
Its low temperature discharge performance is also very good. The number of times of charging is less than 30 times, usually 10-20 times. A special charger is required, and it is extremely easy to lose the charging ability.
2, 2.75W medium power USB charger circuit diagram
 
The design uses Power Integrations’ Link Switch family of products, the LNK613DG. This design is ideal for mobile phone or similar USB charger applications, including mobile phone battery chargers, USB chargers, or any application with constant voltage/constant current characteristics.
 
In the circuit, diodes D1 through D4 rectify the AC input and capacitors C1 and C2 filter the DC. L1, C1 and C2 form a π-type filter that attenuates differential mode conducted EMI noise. These transformers E-sheild with Power Integrations? The combination of technologies allows the design to easily meet EN55022 Class B conducted EMI requirements with sufficient headroom without the need for a Y capacitor. The fire-resistant, fusible, wire-wound resistor RF1 provides severe fault protection and limits the inrush current generated during startup.
 
 
Figure 1 shows that U1 is powered by an optional bias supply, which reduces no-load power consumption to less than 40 mW. The value of bypass capacitor C4 determines the amount of cable drop compensation. A value of 1μF corresponds to a compensation for a 0.3 Ω, 24 AWG USB output cable. (The 10 μF capacitor compensates for the 0.49 Ω, 26 AWG USB output cable.)
 
During the constant voltage phase, the output voltage is regulated by switch control. The output voltage is maintained by skipping the switching cycle. The regulation can be maintained by adjusting the ratio of the enable and disable cycles. This also allows the efficiency of the converter to be optimized over the entire load range. Under light load (turbulent charging) conditions, the current limit is also reduced to reduce the transformer flux density, which in turn reduces audible noise and switching losses. As the load current increases, the current limit will also rise and the skip period will be less and less.
 
When no switching cycles are skipped (maximum power point is reached), the controller in Link Switch-II will switch to constant current mode. When the load current needs to be further increased, the output voltage will drop. The drop in output voltage is reflected in the FB pin voltage. In response to a voltage drop on the FB pin, the switching frequency will drop linearly, resulting in a constant current output.
 
D5, R2, R3, and C3 form an RCD-R clamp circuit that limits the drain voltage spike caused by leakage inductance. Resistor R3 has a relatively large value for avoiding oscillation of the drain voltage waveform caused by leakage inductance, which prevents excessive oscillation during turn-off, thereby reducing conducted EMI.
 
Diode D7 rectifies the secondary and C7 filters it. C6 and R7 together can limit transient voltage spikes on D7 and reduce conducted and radiated EMI. Resistor R8 and Zener Diode VR1 form an output dummy load that ensures that the output voltage at no load is within acceptable limits and that the battery does not fully discharge when the charger is disconnected from AC mains. Feedback resistors R5 and R6 set the maximum operating frequency and output voltage during the constant voltage phase.
 
Five, constant current source
 
1. How to design a three-wire constant current source driving circuit
 
The constant current source driving circuit is responsible for driving the temperature sensor Pt1000 to convert its sensed temperature-varying resistance signal into a measurable voltage signal. In this system, the constant current source needs to have constant output current, good temperature stability, large output resistance, and output current less than 0.5 mA (Pt1000 has no upper limit of self-heating effect), one end of the load is grounded, and the output current polarity can be Change and other characteristics.
 
Since the influence of temperature on the integrated operational amplifier parameters is not as significant as that of the transistor or FET parameters, the constant current source composed of the integrated operational amplifier has the advantages of better stability and higher constant current performance. Especially in the case where the load end needs to be grounded, it has been widely used. Therefore, the dual op amp constant current source shown in Figure 2 is used. The amplifier UA1 constitutes an adder, UA2 constitutes a follower, and UA1 and UA2 select a low noise, low offset, high open loop gain bipolar operational amplifier OP07.
 
Let the potentials at the upper and lower ends of the reference resistor Reef in Fig. 2 be Va and Vb, respectively, Va is the output of the in-phase adder UA1. When the resistor R1=R2, R3=R4, then Va=VREFx+Vb, so the constant current The output current of the source is:
 
It can be seen that the dual op amp constant current source has the following remarkable features:
 
The load can be grounded;
When the op amp is powered by dual power supplies, the output current is bipolar;
The constant current magnitude is achieved by changing the input reference VREF or adjusting the reference resistor Rref0, making it easy to obtain stable small current and compensated calibration.
Due to the mismatch of the resistor, the voltage across the reference resistor Rref0 will be affected by the terminal voltage Vb of its driving load. At the same time, because it is a constant current source, Vb will definitely change with the load, which will affect the stability of the constant current source. Obviously this is unacceptable for high precision constant current sources. Therefore, the selection principle of the four resistors R1, R2, R3, and R4 is that the mismatch should be as small as possible, and the mismatch size of each pair of resistors should be the same. In practice, a large number of precision resistors of the same batch can be screened, and four resistors with similar resistance values ​​are selected.
 
2, switching power supply type high withstand voltage constant current source circuit diagram
 
The development of the instrument requires a constant current source capable of generating 1MA current on a 0 to 3 mega ohm resistor. The UC3845 is combined with a 12V battery. The transformer is a color TV high voltage package. The L1 is wrapped around the original high voltage core with an enameled wire. L3 uses one coil of the original high voltage package, and L2 uses the high voltage part of the high voltage package. L3 and LM393 form a voltage limiting circuit to limit the output voltage too high. Adjusting R10 can adjust the open output voltage.

Source: https://www.slw-ele.com/detailed-explanation-of-five-classic-circuits.html