Simple Bench Power Supply Circuit

This circuit is a simple but effective adjustable voltage and current regulated bench power supply.

What is a Bench Power Supply:

A bench power supply is a versatile electronic device used in laboratories, workshops and electronics testing environments to provide a stable and adjustable source of electrical power.

It is designed to supply a controllable and well regulated voltage and current to electronic circuits and devices during development, testing or repair.

Circuit Working:

Parts List:

A 24V, 5A transformer is used as the power source.

The transformer output is rectified using a bridge rectifier to convert AC to DC.

A filter capacitor is connected across the rectified output to smooth out the DC voltage.

The filtered DC output from the bridge rectifier provides a 30V input to the bench power supply circuit.

The TIP35 is configured as an emitter follower power transistor.

Its base is connected directly to the 30V input.

A feedback loop is established using a 2N2222 transistor and a potentiometer.

The emitter output of the TIP35 is connected to the base of the 2N2222 transistor.

The potentiometer allows for adjustable feedback controlling the output voltage.

A zener diode is connected between the emitter and ground of the 2N2222 transistor to stabilize the voltage across the transistor.

A current limiting resistor is placed in the emitter output line of the TIP35 transistor.

A BC547 transistor is connected in conjunction with this resistor and the base of the TIP35 transistor.

If the output load current exceeds a set value the BC547 transistor turns on and disables the TIP35 transistor effectively shutting off the output current to the load.

The 2N2222 transistor and the potentiometer control the output voltage by adjusting the feedback to the TIP35 transistor.

The BC547 transistor controls the output current by acting as a current limiter.

Formulas and Calculations:

In this design, the voltage drop across R1 mostly caused by the current pulled by T2, controls the base voltage of T1 and, consequently the output voltage.

T2 is cut off when the slider arm of the pot VR1 reaches the extreme end of the ground because its base is now grounded, allowing the base current of T1 to create the only voltage drop across R1.

The output voltage at the T1 emitter in this case will be nearly equal to the collector voltage and is as follows:

VE = Vin – 0.7

where,

• 0.7 is the normal forward voltage drop value for BJT T1 base/emitter leads and
• VE is the emitter side voltage of T1.

Thus, the following can be anticipated as the output if the input supply is 15V:

VE = 15 – 0.7 = 14.3V

T2 will now be able to access the entire emitter side voltage of T1 when the pot VR1 slider arm is moved to the higher positive end, which will result in very strong conductivity.

Through this action, the zener diode D1 and R1 will be directly connected. In other words, the zener voltage Vz and the base voltage VB of the T1 will now be exactly equal.

Thus, the result will be:

VE = Vz – 0.7

Thus, the output voltage that may be anticipated if the D1 value is 6 V is just:

VE = 6 – 0.7 = 5.3V

Therefore, when the pot is spun to its lowest setting, the zener voltage determines the minimal output voltage that could be obtained from this series pass power supply.

The aforementioned method for creating a bench power supply is simple and efficient, but it has a significant drawback in that it is not short circuit proof.

That means that the T1 will quickly heat up and burn if the circuits output terminals are unintentionally shorted or if an over load current is supplied.

This circuit might be easily improved to prevent this scenario by including a current control feature, as detailed in the section that follows.

How to Build:

Building the described adjustable voltage and current regulated bench power supply circuit involves assembling the components.

Power Supply Section:

• Connect the secondary terminals of the transformer to the input of the bridge rectifier.
• Connect the output of the bridge rectifier to the input of the filter capacitor.

Transistor TIP35:

• Connect the emitter of the TIP35 transistor to the positive supply.
• Connect the collector to the load.
• Connect the base directly to the 30V input.

Feedback Loop:

• Connect the emitter of the 2N2222 transistor to the output of the TIP35.
• Connect the base of the 2N2222 transistor to the junction of a potentiometer and a resistor.
• Connect the other end of the resistor to the emitter of the TIP35.
• Connect the collector of the 2N2222 transistor to the positive supply.
• Connect a zener diode between the emitter and ground of the 2N2222 transistor.

Current Limiting Section:

• Connect a current limiting resistor in series with the emitter of the TIP35.
• Connect the base of the BC547 transistor to the junction of the current limiting resistor and the emitter of the TIP35.
• Connect the collector of the BC547 transistor to the positive supply.
• Connect the emitter of the BC547 transistor to the base of the TIP35.

Control:

• Connect the wiper of the potentiometer to the base of the 2N2222 transistor.
• Connect one end of the potentiometer to the positive supply.
• Adjust the potentiometer to control the feedback and regulate the output voltage.

Load Connection:

• Connect the load e.g. a resistor or electronic circuit to the collector of the TIP35 transistor.

Heat Sinks:

• Attach heat sinks to the TIP35 transistor to dissipate heat generated during operation.

Additional Considerations:

• Double check all connections and ensure proper polarity.
• Use appropriate resistor values to set the desired current limit and feedback.
• Consider adding protection diodes and capacitors for stability.
• Connect the ground of the circuit to the ground of the power supply.

Note:

• Once the circuit is assembled, you can test it by gradually adjusting the potentiometer to vary the output voltage and observing the current limit operation with different load conditions.
• Ensure that the components do not exceed their specified voltage and current ratings.

Conclusion:

To conclude, the potentiometer and 2N2222 transistor control the voltage while the BC547 transistor acts as a current limiter to prevent excessive current through the circuit.

This setup allows for a regulated and adjustable bench power supply circuit with both voltage and current control.

Adjusting the potentiometer alters the feedback to the TIP35 regulating the output voltage and the current limit is enforced by the BC547 transistor.

References

Regulated power supply

bench power supply