30 V / 3 A 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 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: Figure 1 consists 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 fullness of the current is 20.00A.
The control circuit is shown in Figure 2. Constant current control circuit consists of a comparator of voltage comparator 1M393, RP4 is the current adjustment potentiometer, and the precision voltage reference (about 2.5-2.6V) generated by IC5 is sent to the inverting input of IC6 after voltage division by RP3. The current sampling voltage generated by RP4 after dividing is sent to the in-phase input of IC6. If the actual current exceeds the set constant current value, IC6 outputs a high level, Q4 conducts, and the potential of the 1M317 adjustment terminal decreases → the output voltage decreases → the output current decreases until the actual current is equal to the set current value. At the same time, Q3 conducts, and light-emitting diode VD6 shows that it is in constant current state.
Short-circuit protection function: 1M317 itself has a perfect protection function, but the output short-circuit does not protect the current spreading power tube.
Short-circuit output current is much higher than the set current value so that Q4 fully conductive, 1M317 output for the * small value (about 1.2V) at this time the measured short-circuit current value of the display is about 4-5 A. Although the short-circuit current limit, but due to the diffusion of power tubes of the dissipation of power is large, long time is still a danger * good to add the output of short-circuit protection fuse (5A).
J2 is the current setting/display switching relay. 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 across the current adjustment potentiometer RP4 is sent to the ammeter to display the set current value. The ammeter is a voltmeter with a fullness of 2 V. The other comparator of the 1M393 constitutes the fan control circuit, and here a hysteresis comparison circuit is used. The circuit shown in the figure enables the fan to start automatically at 55°C and stop when it falls below 45°C. This reduces noise and improves fan life. rH is a negative temperature coefficient thermistor of 820 ohms and should be fastened to the heat sink. The action temperature can be selected by adjusting the corresponding resistance of the hysteresis comparator.
