30V/3A constant voltage/constant current DC adjustable voltage regulated power supply

Circuit Features

(1) The digital voltmeter displays voltage and current with a display accuracy of 0.1 V and 0.01 A.
(2) Overcurrent protection function limits the current through the ammeter setting. That is, it has a constant current function. This function can prevent excessive current from burning the circuit board or the voltage regulator itself when repairing and adjusting a circuit with a short circuit fault.

(3) It has an automatic fan control circuit, which automatically starts the cooling fan when the heat sink of the power regulator tube exceeds 55°C.

Working principle
Main circuit: Figure 1 consists of 1M317, Q1, and Q2. It is a typical current expansion application circuit of 1M317. The popular high-power voltage regulator integrated circuit 1M338 is not used because its overcurrent protection function is too sensitive. It enters the protection state when it exceeds 5A instantly. The starting current of small power tools (such as small electric drills and DC motors) often exceeds 5A and cannot carry inductive loads. I have tested this. If the ammeter sampling resistor R6 is wound with constantan wire, due to its small resistance, even if it is accurately measured with a bridge in advance, the resistance of the contact (solder point) will exceed the error range. Here, a 0.12Ω cement resistor is used. The voltage drop generated by the current is adjusted by RP3 and sent to the voltage meter head with a full scale of 2V. The full scale of the current is 20.00A.

The control circuit is shown in Figure 2. The constant current control circuit is composed of a comparator of the voltage comparator 1M393, RP4 is a current adjustment potentiometer, and the precision voltage reference (about 2.5-2.6V) generated by IC5 is sent to the inverting input terminal of IC6 after voltage division by RP3. The current sampling voltage generated by the voltage division of RP4 is sent to the non-inverting input terminal of IC6. If the actual current exceeds the set constant current value, IC6 outputs a high level, Q4 is turned on, the potential of the adjustment terminal of 1M317 drops → the output voltage drops → the output current drops until the actual current is equal to the set current value. At the same time, Q3 is turned on, and the light-emitting diode VD6 shows that it is in a constant current state.

Short circuit protection function: 1M317 itself has perfect protection function, but it cannot protect the current expansion power tube when the output is short-circuited.

When short-circuited, the output current is much greater than the set current value, making Q4 fully conductive. The output of 1M317 is the minimum value (about 1.2V). At this time, the short-circuit current value measured and displayed is about 4-5A. Although the short-circuit current is limited, it is still dangerous for a long time due to the large power dissipation of the expansion power tube. It is best to install an output short-circuit protection fuse (5A).
J2 is a current setting/display conversion relay. When in position 1, the voltage drop of the current sampling resistor R6 is adjusted by RP3 and sent to the ammeter to display the current actual current. When in position 2, the voltage on the current adjustment potentiometer RP4 is sent to the ammeter to display the set current value. The ammeter is a voltage meter with a full scale of 2V. Another comparator of 1M393 constitutes a fan control circuit, and a hysteresis comparison circuit is used here. The circuit in the figure can realize the automatic start of the fan at 55℃ and the stop of the fan when it is lower than 45℃. Thereby reducing noise and improving the life of the fan. The negative temperature coefficient thermistor with RH of 820 ohms should be fastened to the heat sink. The operating temperature can be selected by adjusting the corresponding resistance of the hysteresis comparator.

Touch button control circuit: Considering the comfortable operation feel, ordinary button/button switch is not used, but touch switch + digital control circuit + relay mode is used. The circuit is shown in Figure 3.

Component selection
The voltmeter uses a full-scale 200V voltage meter head, and the decimal point after the tens digit is lit. The ammeter uses a full-scale 2V voltage meter head, and the decimal point after the hundreds digit is lit (i.e. full-scale 20.00A).

The transformer is modified from a power frequency transformer or a toroidal transformer with a power rating of more than 150W, and the power winding uses 1.4mm enameled wire.

J1 uses a 12V relay with a contact current greater than 10A. J2 uses a JRX type small relay.

RP1 and RP2 cannot use ordinary carbon film potentiometers due to the large current passing through them. WX13-1 1 type wire wound potentiometers are used to significantly increase the service life. RP3 uses multi-turn precision trimming resistors. Q1 uses a high-power NPN tube with Ic ≥ 10A and BVceo ≥ 60V, and Q2 uses a general PNP medium-power tube.

12V fan uses 10×10cm instrument fan.
Circuit adjustment

After the assembly is completed and the voltage and current meters display normally and the voltage is adjustable, perform the following debugging:

Connect a 10Ω, 5W load resistor and a digital multimeter (in the high current range) to the power supply output terminal, adjust the voltage output so that the multimeter displays 1.00A, and adjust RP3 so that the local ammeter displays 1.00A.

Connect a 10Ω, 5W load resistor to the output terminal, turn K2 to position 2, set the constant current to a certain value (for example, 0.50A), then turn K2 to position 1, slowly increase the output voltage, and when the load current increases to the set value, the voltage and current will no longer increase and remain at the current set value. At this time, the constant current function takes effect.

Short circuit test: Short-circuit the output terminal, the ammeter displays between 4-5A, and VD6 lights up, indicating overcurrent.