This circuit presents a simple bench power supply that is easy yet efficient with adjustable voltage and current monitoring.
What is a Bench Power Supply:
A multipurpose electronic tool called a bench power supply provides a constant and manageable electrical power source for use in workshops, labs and electronics testing scenarios.
Its main purpose is to supply a controlled voltage and current to electronic circuits and devices during development, testing and repair.
Circuit Working:
Parts List:
| Category | Description | Quantity |
|---|---|---|
| Resistors | 100Ω 2W | 1 |
| 1k | 1 | |
| R2 (calculated as 0.6 / max output current) | 1 | |
| Potentiometer | 10k | 1 |
| Capacitor | Electrolytic 1000µF 25V | 1 |
| Semiconductors | Transistors 2N2222, TIP35, BC547 | 1 each |
| Bridge diodes 6A4 | 4 | |
| Transformer 0-24V 5 Amps | 1 |
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 formed 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 maintain 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.
The output current to the load is effectively shut off by the BC547 transistor which acts when the output load current exceeds a certain threshold.
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:
This design manages the base voltage of transistor T1 and in turn the output voltage by adjusting the voltage drop across the resistor R1 which is mainly carried on by the current formed by transistor T2.
Since the base of the pot VR1 is grounded the base current of T1 may generate the only voltage drop across R1 turning off T2 when the slider arm reaches the very end of the ground.
The following is the output voltage at the T1 emitter which will be very similar to the collector voltage:
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.
Therefore if the input supply is 15V the following output is expected as:
VE = 15 – 0.7 = 14.3V
T2 will 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 resistor R1 will be directly connected.
Lets say the zener voltage Vz and the base voltage VB of the transistor T1 will 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
Thus the zener voltage sets the lowest output voltage that gets achieved from this series pass power supply when the pot turns to its lowest position.
Despite being simple and effective the earlier mentioned technique to build a bench power supply includes a major drawback which is not short circuit proof.
This means that if the circuits output terminals are accidentally shorted or if an excessive current is supplied than the T1 transistor will quickly heat up and burn.
As described in the next part this circuit might be easily improved to avoid this situation by adding a current control feature.
How to Build:
To build a Simple Bench Power Supply Circuit follow the below steps:
Power Supply Section:
- Connect the secondary terminals of the transformer to the input of the bridge rectifier.
- And 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.
- The collector of the transistor should be connected 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.
- The base of the 2N2222 transistor connect to the junction of a potentiometer and a resistor.
- Connect the other end of the resistor to the emitter of the TIP35.
- The collector of the 2N2222 transistor connect to the positive supply.
- Connect a zener diode between the emitter and ground of the 2N2222 transistor.
Current Limiting Section:
- A current limiting resistor in series should be connected with the emitter of the TIP35.
- And the base of the BC547 transistor connect to the junction of the current limiting resistor and the emitter of the TIP35.
- The collector of the BC547 transistor should be connected to the positive supply.
- And the emitter of the BC547 transistor connect to the base of the TIP35.
Control:
- The wiper of the potentiometer should be connected to the base of the 2N2222 transistor.
- The other end of the potentiometer connect to the positive supply.
- To control the feedback and regulate the output voltage adjust the potentiometer
Load Connection:
- Add the load to the TIP35 transistors collector such as a resistor or electronic circuit.
Heat Sinks:
- To dissipate heat generated during operation add heat sinks to the TIP35 transistor
Additional Considerations:
- Verify all connections twice to make sure the polarity is correct.
- To provide the desired current limit and feedback, use the proper resistor values.
- For stability think about adding capacitors and protection diodes.
- The circuits ground should be connected to the power supplys ground.
Note:
- After the circuit is put together you can test it by carefully adjusting the output voltage with the potentiometer and seeing how the current limit functions under various load situations.
- Make sure the components not exceed over their stated current and voltage ratings.
Conclusion:
Overall the BC547 transistor serves as a current limiter to stop excessive current from flowing through the circuit while the potentiometer and 2N2222 transistor regulate the voltage.
This configuration enables a controlled and adjustable circuit for a bench power supply that has both voltage and current control.
The BC547 transistor controls the current restriction and the potentiometers adjustment modifies the feedback to the TIP35 controlling the output voltage.