This is 0-30V Transistor Based Variable Power Supply Circuit which gives adjustable output voltage and maximum current is around 2A.
The main power transistor is 2N3055 and it works as series pass transistor in the circuit.
Since, it is linear type regulator the output is smooth and stable.
Therefore, it is good for lab testing and also, it is useful for electronics hobby work which can power small amplifier, Arduino board, motors and other DC projects.
Moreover, this circuit is easy to build and all components are easily available in market.
Circuit Working:
Parts List:
| Components | Value | Quantity |
|---|---|---|
| Resistors | 220Ω 1/4 watt, 3.3k 1/4 watt, 0.33Ω 5 watt | 1 each |
| Preset 10k | 1 | |
| Capacitors | Electrolytic 2200uF 25V, 220uF 25V | 1 each |
| Semiconductors | Transistor BC547 | 2 |
| Power Transistor 2N3055 | 1 | |
| Zener Diode 30V 1W | 1 | |
| 5mm LED any color | 1 | |
| Rectifier Diode 1N5408 | 2 | |
| Transformer primary 220V AC 50Hz, secondary 24V-0V-24V | 1 | |
| SPST ON/OFF Switch | 1 | |
| Fuse 0.5A | 1 |
At start, 220V AC mains supply is given to transformer and then voltage is stepped down to 24V AC.
Next, diodes D1 and D2 change AC voltage into pulsating DC.
After, that the capacitor C1 removes ripple and because of this the DC becomes smooth.
Now unregulated DC voltage is about:
Vdc = Vac × 1.414
Vdc = 24 × 1.414
Vdc = 33.9V
However, due to diode drop and ripple under load the practical DC voltage is around 32V.
This unregulated DC is given to collector of 2N3055 transistor.
Meanwhile, the control section starts working and the 30V Zener diode gives stable reference voltage.
Preset VR1 is used to adjust base voltage of driver transistor and when this VR1 is rotated the base voltage of Q1 BC547 changes and so emitter voltage of Q1 also changes.
This emitter voltage drives base of Q2 2N3055 and because of this the base current of 2N3055 is controlled.
When base current changes the output voltage also changes and therefore, output voltage can adjust from 0V up to nearly 30V.
Formulas with Calculation:
Output Voltage Formula:
Output voltage mainly depends on base-emitter drop and preset setting.
Vout = Vz − Vbe
where,
- Vz is the Zener voltage
- Vbe is 0.7V
So:
Vout = 30 − 0.7
Vout = 29.3V maximum
Practically, maximum Output Voltage is around 28-29V.
Current Calculation:
Emitter resistor R3 = 0.33 ohm
Maximum safe current approx:
I = 0.7 / R
I = 0.7 / 0.33
I = 2.12A
Therefore, the maximum safe current is near 2A.
How to Build:
To build a 0-30V Transistor Based Variable Power Supply Circuit, follow the below connection steps:
- First, collect all required components.
- Then start with transistor Q1 BC547 and connect its collector to unregulated DC.
- Next, connect base of Q1 to junction of VR1 center pin and positive of capacitor C2.
- After that connect emitter of Q1 to base of Q2.
- Now take transistor Q2 2N3055 and first connect its base to emitter of Q1.
- Then connect collector of Q2 to unregulated DC.
- After that connect emitter of Q2 to base of Q3 and one end of resistor R3.
- Next, take transistor Q3 BC547 and connect its emitter to other end of R3 and to Vout (0–30V).
- Then connect collector of Q3 to junction of VR1 center pin, capacitor C2 positive and base of Q1.
- Now connect capacitor C1 between unregulated DC and Ground.
- After that connect LED anode to unregulated DC and then connect LED cathode to one end of R1 and other end of R1 to Ground.
- Next, connect resistor R2 from unregulated DC and connect other end to upper pin of VR1 and cathode of Zener diode.
- Then connect Zener diode anode to Ground and also connect lower pin of VR1 to Ground.
- Now do transformer connection.
- First, connect one primary wire to ON/OFF switch and then connect other primary wire through fuse to mains supply.
- After that connect first 24V secondary wire to anode of diode D1.
- Then connect third 24V secondary wire to anode of diode D2.
- Finally, connect 0V center tap wire directly to circuit Ground.
Conclusion:
This 0-30V Transistor Based Variable Power Supply Circuit is simple and with strong design.
It gives adjustable output from 0 to 30V and also its current is up to 2A is possible.
However, efficiency is low because it is linear type supply, but output voltage is clean and stable.
The circuit is very good for bench testing and hobby electronics work.
Also if proper big heat sink is used and good quality transformer is connected then the circuit works reliable for long time.
So it is good project for beginners and at the same time it is also useful for workshop and repair work.