30V/3A Constant Voltage/ Constant Current DC Adjustable Regulated Power Supply
Circuit characteristics
(1) digital voltmeter voltage on the map current display, display accuracy 0.1 V on the map 0.01A
(2) overcurrent protection function, limit current through the ammeter settings. That is, with a constant current function. This function in the maintenance, adjustment of short-circuit fault circuit can prevent the current is too large and burn the circuit board or the voltage regulator itself.
(3) has an automatic fan control circuit, the power supply regulator tube heat sink over 55 ℃ automatically start the cooling fan.
Working Principle
Main circuit: composed of 1M31 7, Q1, Q2. It is a typical current expansion application circuit of 1M31 7. Not using the popular high-power voltage regulator IC 1M338, because it is too sensitive to overcurrent protection, instantly more than 5A that is entering the protection state, and small power tools (such as small drills, DC motors) start-up current is often more than 5A and can not take inductive loads, which I have tested. Ammeter sampling resistor R6 if the use of copper wire winding, due to the resistance value is too small, even if the bridge beforehand precision measurement, plus contact (solder joints) resistance will be outside the error range. Here the use of 0.12Ω cement resistor, the voltage drop generated by the current is adjusted by RP3 and sent to the fullness of 2V voltmeter head, the current fullness of 20.00A.
Component selection
Voltmeter using fullness 200V voltmeter head, ten digits after the decimal point is lit. The ammeter adopts a full degree 2V voltmeter head, and the decimal point after the hundredth digit is lit (i.e. full degree 20.00A).
Transformer using more than 150W industrial frequency transformer or ring-type fire cow conversion, power winding with 1.4mm enameled wire.
J1 using contact current greater than 10A 12V relay. J2 using JRX type small relay.
RP1.RP2 due to the passage of the current is large, can not use ordinary carbon film potentiometer, using WX13-1 1-type wirewound potentiometer, can significantly improve the service life. RP3 using multi-turn precision trimming resistor. Q1 using Ic ≥ 10A, BVceo ≥ 60V high-power NPN tube, Q2 with a general PNP ** rate of the tube can be.
12V fan with 10 × 10cm instrument fan.
Circuit adjustment
Assembly is complete. Voltage, ammeter normal display, and voltage adjustable, the following debugging:
10 Ω, 5W load resistor and digital multimeter (in high current gear) connected to the power supply output, adjust the voltage output so that the multimeter shows 1.00A, adjust the RP3 so that the machine ammeter shows 1.00A.
1 0 Ω, 5W load resistor connected to the output, K2 hit in 2 position, set the constant current current at a certain value (for example, 0.50A), and then K2 hit in 1 position, slowly increase the output voltage, the load current rises to the set value, the voltage, current are no longer rising, maintained at the current set value. At this time the constant current function works.
Short-circuit test: short-circuit the output, the ammeter shows between 4-5A, at the same time, VD6 lights up, prompting overcurrent.
