30V/3A regulated voltage and current power supply circuit principle-EEWORLD

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Circuit Working Principle
The op amp IC1 (μA741) and peripheral components form a voltage regulation circuit, and V1//V2 and V3 form a Darlington high-power voltage regulation circuit. RPl, RP2, RP3, R4, R5, R6, VDl2, VD13, VDl4 and other components form an output voltage sampling circuit. The op amp IC1 amplifies the output voltage change, and shunts the base current of the Darlington tube from the output terminal ⑥ through VD10 to achieve the purpose of voltage regulation. Among them, RP1 and RP2 are the output voltage coarse and fine adjustment potentiometers on the panel, and RP3 is the maximum output voltage setting adjustable resistor inside the circuit board.
The operational amplifier IC2 and the peripheral circuits form a steady current regulation circuit. RP4, RP5, RP6, R7, R8, R9, R10, VD15, etc. sample the current on R3 and amplify it through IC2. The base current of the Darlington tube is shunted by VD9 from the output terminal ⑥ of the IC2 operational amplifier to achieve the purpose of steady current adjustment. Among them, RP4 and RP5 are the output current coarse and fine adjustment potentiometers on the panel, and RP6 is the internal maximum output current setting adjustable resistor: VD9 and VD10 (i.e., the voltage and current control circuit) form an "AND gate" working mode. When the output current does not reach the set current value, VD9 is cut off and has no effect on the output voltage. When the output current reaches the set current value, VD9 starts to conduct, and the driving current of the voltage regulation circuit is 10 flowing, so that the output voltage drops, ensuring that the output current is at the set current value, that is, the so-called steady current regulation and the voltage and current stabilization work is automatically converted, which can effectively prevent damage to the repaired electrical appliances and the power supply itself.
The LEDs LED1 and LED2 are the voltage and current stabilization working indication circuits. When the current stabilization IC2 is not working, its ⑥ pin is high level, VD11 and DZ2 are turned on, making the transistor V8 saturated, and the green LED1 lights up, indicating that the power supply is working in the voltage stabilization state; when the current stabilization IC2 is working, the voltage of its ⑥ pin drops. VD11 and DZ2 are cut off, V9 is saturated and turned on, and the red LED2 lights up, indicating that the power supply is working in the current stabilization state (at this time, the output voltage drops and is lower than the set working voltage).
Relay JK1, V6, V7 and other circuits form an AC input voltage automatic switching circuit, which is controlled by the output voltage. When the output voltage is below 12V, the normally closed contact of JK1 is connected, and the input voltage works at the AC 15V winding; when the output voltage is adjusted to above DC 13.5V, the relay JK1 is energized, the normally open contact is connected, and the AC input voltage works at the 30V winding 7. To ensure that the input and output voltages are within the normal voltage difference range, so as to reduce the power consumption of the adjustment tube. High-end power supplies are generally designed with three sets of windings. Further reduce the useless power consumption of the voltage regulator. Among them, the resistor R11* is an adjustment resistor, which is appropriately adjusted to the appropriate output voltage switching point. Because the circuit has a hysteresis characteristic, it will not cause the relay to be repeatedly attracted and released at the switching voltage point to produce oscillation instability. Relays JK2 and K2 are output voltage control switches. When adjusting the output voltage, if the output voltage needs to be turned off, K2 can be pressed without removing the output voltage terminal, which is convenient for operation. The output voltage indicator is directly connected to the output voltage terminal, and the ammeter is connected to the current sampling resistor R3 (0.18Ω/SW). A 3.5-digit digital meter head is used to accurately display the current and voltage. The
power transformer T1 has a power of 120VA, a secondary main winding of 0.151V-30v current, and the current is required to reach more than 3A. The auxiliary winding is AC 20V and the current is 1A. A shielding layer is wound between the primary and secondary, and grounded to ensure the effect of reducing interference. Among them, the 20V auxiliary winding, IC3 (7812), IC4 (μA431), DZt (1N4735), V5 (D880) and other components constitute a complete machine working power supply with excellent voltage stabilization performance. The circuit's stability, ripple characteristics, temperature drift, etc. are all guaranteed by this power supply. All resistors are metal oxide film resistors.
When using the power supply, you can set the working current first, and then adjust the working voltage.

When setting the working current, the output voltage can be adjusted to about 1V (or below 1V), then short-circuit the output terminals U+ and U-, adjust RP4 and RP5 so that the current value displayed on the ammeter is at the predetermined current, and then open the output terminal, adjust RP1 and RP2 so that the output voltage reaches the predetermined voltage value, and then it can be put into use (if the output voltage is adjusted too high to 20-30V. Directly short-circuiting the output voltage terminal, although the power supply has a current stabilization effect, it will cause short-circuit sparks and excessive impact current, affecting the life of the internal components of the power supply).
When making it, the transformer power must be sufficient, the high-power adjustment tubes V1 and V2 must be installed on a large radiator, and the main power supply rectifier bridge must use more than 6A, preferably installed on a heat sink. If the voltage adjustment knob and current adjustment knob use multi-turn potentiometers, the fine-tuning knob can be omitted, the performance is better, and the adjustment operation is more convenient.