Introduction

Using transistors to build a constant current and constant voltage power supply is less practical, slightly more difficult than the circuit in the textbook, and involves a wider range of knowledge.

Involving knowledge points

1. Bridge rectifier and filter

2. Single key switch

3. Positive and negative feedback

4. Current sampling

working principle

Bridge rectifier filter

D1~D4 form a bridge rectifier, which rectifies alternating current into pulsating direct current.

C1 filters and smoothes the output voltage to maintain the no-load voltage near the peak value.

Single key switch

The initial state of power-on is Q1 and Q2 ends. The current passes through R1, R2, and R6 to charge C2, making C2 close to the power supply voltage.

At this time, press S1, the capacitor charge passes through S1, R7 limits the current, and Q2_BE. Turn on Q2.

Q2 is turned on, and the Q2_C voltage is pulled low. After R1 and R2 divide the voltage, the Q1_B voltage is pulled low, and Q1 is turned on, forming a positive feedback and the circuit is self-locking.

At this time, Q2_C is low and the LED emits light. Q1_C is high, and current is output through R8, and the control part of the subsequent voltage stabilizing circuit is powered to operate.

The C1 charge is released through R6, Q2_CE and is approximately zero. When S1 is pressed again, the Q2_B voltage is pulled low and Q2 is turned off. In the same way, due to positive feedback, the circuit releases its self-locking and returns to the initial state.

voltage stabilization

When current flows into R8 alone, Q4 and Q3 form a Darlington tube to increase the amplification factor. A common amplifier circuit is formed.

The upper half of the output voltage RV1 and the lower half of RV1 are divided in series by R10. If U1_REF is greater than 2.5V, U1_CA is turned on.

The potential of Q4_B is reduced, and the output voltage is reduced, forming a negative feedback to stabilize the output voltage.

Constant current/OCP

The load current flows through Q6_EC, and a small current flows out of Q6_B (the ratio can be calculated according to the transistor amplification formula). The current flows through R9 and RV2 to form a voltage drop.

When the voltage at both ends is greater than Q5_EB, Q5_CE is turned on. If S2 is on the right, Q4_B is pulled down, forming negative feedback to keep the output current constant.

If S2 is on the left, pull Q2_B low to cut off Q2 and release the self-locking circuit. The voltage stabilizing module stops running and forms OCP (over current protection).

Component selection

Some people may think that these circuits are a bit extravagant compared to Taobao kits.

But in fact, if the quantity is large, placing orders yourself will not be much more valuable than Taobao.

As a soldering learning board, it is recommended to choose the cheapest electronic components. Can support domestic chips.

Because the component fails, the process of troubleshooting further tests whether you have a thorough grasp of the principles.

related suggestion

The potentiometers and chips between these circuits are universal. If a certain circuit is damaged, the east wall can be removed to repair the west wall.

Sorted from simple to difficult, teachers are welcome to use them in teaching, but we hope that the URL of this open source page will be added to the courseware to show respect, thank you.

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