Adjustable Power Supply

Description: A variable power supply is required for repairs on CB and audio equipment, featuring excellent current limiting and protection. The designed circuit delivers a current from zero to approximately 4.25A at a voltage range of zero to about 23V. The short circuit protection is set at 400 mA, adjustable to lower values to light an LED without damage. The circuit, which is straightforward to build, is claimed to outperform many commercial alternatives and has proven reliable over more than a year of use. The first operational amplifier (IC1) functions as a comparator, assessing the voltage drop across R10 against the voltage set by the wiper of VR1, derived from Zener diodes ZD1 and ZD2. Two Zener diodes are used in series to stabilize the reference voltage, as Zener diodes below 5V exhibit a negative temperature coefficient, while ZD2 has a positive coefficient. Current adjustment is possible through VR1. The output from IC1 is fed into VR2, which reacts to current variations through R10, with ZD3 providing a reference voltage for VR2. The second operational amplifier (IC2) compares the voltage at the junction of R14 and R15 with the voltage set by the wiper of VR2, allowing for output voltage adjustments. Diodes D1 and D2, along with R5, implement "foldback" current limiting, restricting output current to about 400 mA during a short circuit. If a short circuit occurs, the voltage at IC2's non-inverting input drops, limiting base drive to Tr2. A significant current through R10 will trigger IC1 to reduce the reference voltage, stabilizing the circuit and minimizing current flow through R10. If a heavy load is applied, Tr2 and R10 may overheat unless quickly controlled. The presence of D2 allows rapid discharging of C4, facilitating quicker shutdown by IC2. Tr1 should have a gain of at least 250, with a BC182LB or equivalent transistor being suitable. Tr2 must be a high-power Darlington transistor, ideally a T03 type with an adequate heat sink, such as the MJ1001 or MJ3001.

The circuit design incorporates essential components to ensure reliable operation and safety during use. The variable power supply is structured around two operational amplifiers, which provide feedback control for both current and voltage outputs. The first op-amp (IC1) is crucial for maintaining current limits, utilizing the voltage drop across a sense resistor (R10) to monitor the output current. This feedback mechanism is essential in preventing damage to both the circuit and connected components during operation.

The use of Zener diodes (ZD1 and ZD2) for voltage reference is a strategic choice, as it mitigates the temperature drift often associated with single Zener diodes, ensuring stable operation across varying temperatures. The inclusion of a second op-amp (IC2) allows for fine-tuning of the output voltage, enhancing the versatility of the power supply.

The implementation of foldback current limiting through diodes D1 and D2, along with resistor R5, is a critical safety feature. This mechanism ensures that in the event of a short circuit, the current is limited to a safe level, preventing potential damage to the power supply and connected devices. The design also accounts for rapid discharge of the output capacitor (C4) through D2, which is vital for quick response times during fault conditions.

Transistor selection is another important aspect of the design. Tr1, with its required gain, ensures sufficient control over the output stage, while Tr2, as a high-power Darlington transistor, is capable of handling the significant currents involved without overheating, provided it is adequately heat-sinked.

Overall, the design presents a comprehensive solution for a variable power supply that is both user-friendly and robust, suitable for a variety of electronic repair applications.As I carry out a lot of repairs on C. B. and audio equipment, I need a variable power supply with very good current limiting and protection. The circuit I designed for this purpose is shown in the diagram below. This power unit will deliver a current from zero to about 4. 25A at a voltage from zero to about 23V. With the component values shown, the short circuit protection is 400 mA, but it can be set below this figure; for instance, an LED can be connected directly to the power supply and the current adjusted to light it without destroying it. The unit is not difficult to build, and in my opinion it outperforms many commercial power supplies. Mine has been in use for more than a year, and has given no trouble at all in that time. The first op-amp (IC1) acts as a comparator, comparing the voltage drop across R10 with that set by the wiper of VR1, derived from the Zener diodes ZD1 and ZD2.

Two diodes are used in series instead of a single one because Zener diodes below 5V have a negative temperature coefficient. ZD2 has a positive temperature coefficient, so the two tend to cancel out and the reference voltage is more stable.

The current available from the unit can be increased or decreased by adjusting VR1. The output from IC1 is applied to VR2, and because it is constantly reacting to variations in the current through R10, ZD3 is placed in the circuit to provide a reference voltage for VR2. The second op-amp (IC2) compares the voltage at the junction of R14 and R15 with the voltage set by the wiper of VR2, and by varying this setting the output voltage can be increased or decreased.

Diodes Dl and D2 together with R5 provide "foldback" current limiting and in the event of a short circuit the current output is limited to about 400mA. If the output is short circuited, the voltage at the non-inverting input of IC2 would fall almost to zero and this would limit the base drive to Tr2.

If the current through R10 rises to a point at which the voltage across it is about 70% of the reference voltage applied to the non-inverting input of IC1, the output of IC1 will swing below the voltage at the junction of R3 and R4. Dl then conducts through R5 and IC1; this reduces the reference voltage on its non-inverting input which in turn takes the output more negative until it is at ground potential, the circuit is stabilised and practically no current flows through R10.

If a large load were applied to the output, a very large current would flow through Tr2 and R10, which would burn out Tr2 if not quickly checked. If, for example, the voltage at the slider of VR2 was 1OV, the voltage across C4 would be equal to this.

If a large current was flowing, IC2 would try to reduce the base drive to Tr2; it would also try to discharge C4, but could only do this through VR2, R7 and R9. This would take too long, and Tr2 would be destroyed. The presence of D2 allows C4 to discharge rapidly, so reducing the time for IC2 to shut down. Trl should have a gain of at least 250. A BC182LB was used in the circuit, but any equivalent transistor of similar gain could be used. Tr2 must be a high power Darlington, preferably a T03 type mounted on an adequate heat sink. Either MJ1001 or MJ3001 would be suitable.