30V/3A constant voltage/constant current DC adjustable regulated power supply
Circuit Features
(1) Digital voltmeter voltage and current display in the above figure, display accuracy 0.1 V 0.01A in the above figure
(2) Over-current protection function, limit current set by ammeter. That is, it has a constant current function. This function can prevent the circuit board or the regulated power supply itself from being burned due to excessive current when repairing and adjusting a circuit with a short circuit fault.
(3) With an automatic fan control circuit, the heat dissipation fan is automatically activated when the heat sink of the power adjustment tube exceeds 55°C.
working principle
Main circuit: composed of 1M31 7, Q1, Q2. It is a typical current expansion application circuit. The current popular high-power voltage regulator integrated circuit 1M338 is not used because its overcurrent protection function is too sensitive, and it will enter the protection state when it exceeds 5A in an instant, while the starting current of small electric tools (such as small electric drills and DC motors) often exceeds 5A And it can't carry inductive load, which I have tested. If the ammeter sampling resistor R6 is made of constantan wire, the resistance value is too small, even if it is precisely measured with a bridge in advance, the resistance of the contact (solder point) will exceed the error range. A 0.12Ω cement resistor is used here, and the voltage drop generated by the current is adjusted by RP3 and sent to the voltmeter head with a full scale of 2V, and the full scale current is 20.00A.
Control circuit.
The constant current control circuit is composed of a voltage comparator. RP4 is a current adjustment potentiometer. The precision voltage reference (about 2.5-2.6V) generated by IC5 is divided by RP3 and sent to the inverting input of IC6. 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 1M317 adjustment terminal 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: It has a 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, so that Q4 is completely turned on, and the output is the minimum value (about 1.2V). At this time, the measured short-circuit current value is about 4-5A. Although the short-circuit current is limited, due to the large power dissipation of the current-expanding power tube, it is still dangerous for a long time. It is best to install an output short-circuit protection fuse (5A).
J2 is the current setting/display switching relay. When it is in position 1, the voltage drop of 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 voltmeter head with a full scale of 2V. Another comparator constitutes the fan control circuit, which is a hysteresis comparator circuit. The circuit in the figure can automatically start the fan when the temperature is 55°C, and stop the fan when it is lower than 45°C. Thereby reducing the 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.
Light-touch button control circuit: Considering the comfortable feel of the operation, instead of using ordinary buttons/button switches, it adopts light-touch switch + digital control circuit + relay mode.
Component selection
The voltmeter adopts a voltmeter head with a full scale of 200V, and the decimal point after the tens digit lights up. The ammeter uses a voltmeter head with a full scale of 2V, and the decimal point after the hundreds digit lights up (that is, the full scale is 20.00A).
The transformer adopts a power frequency transformer of 150W or above or a ring-shaped fire cow, and the power winding uses 1.4mm enameled wire.
J1 uses a 12V relay with a contact current greater than 10A. J2 adopts JRX type small relay.
RP1 and RP2 cannot use ordinary carbon film potentiometers due to the large current passing through them. Using WX13-1 type 1 wire-wound potentiometers can 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 high-efficiency tube.
The 12V fan uses a 10×10cm meter fan.
circuit adjustment
Assembly is complete. After the voltage and ammeter display normally, and the voltage is adjustable, perform the following debugging:
Connect a 10Ω, 5W load resistance and a digital multimeter (in the high current range) to the output of the power supply, 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, set K2 to 2, set the constant current at a certain value (for example, 0.50A), then set K2 to 1, slowly increase the output voltage, and the load current will increase When the value is set, the voltage and current will no longer rise and remain at the set value of current. At this time, the constant current function comes into play.
Short-circuit test: Short-circuit the output terminal, the ammeter will display between 4-5A, and VD6 will light up at the same time, indicating over-current.
