Variable voltage power supply is essential for any electronic hobbyist to begin their electronics lessons. For different applications you may need different voltages. So it becomes impossible to use multiple fixed power supplies all the time.Also this variable power supply circuit ensures the protection of other circuit parts from voltage fluctuations. So in this article we will teach you how to build a variable power supply on your own. Why to buy when you can make one of your own easily using very little components?
Assemble this circuit on a PCB (Printed Circuit Board) and start your electronics projects. You can connect the output to a multimeter for displaying the voltage.
How to make a variable power supply?
This supply unit uses a LM3 17 voltage regulator IC. The rectified output from the full wave bridge rectifier is fed to a LM317 regulator IC. By changing the value of potentiometer used in this circuit, the output voltage can be controlled easily.
Note: For safety, use this circuit to get a maximum of 30 volts.
Variable DC power supply schematic
Click on the image for enlarged view
- LM317 Regulator IC
- Step down transformer (230V to 30V)
- Resistor (1KΩ; 220Ω)
- Potentio meter (5KΩ)
- Capacitor (1µF; 35V)
- Capacitor (2200µF, 0.1µF)
- Bridge rectifier
- 1N4007 Diode
Working of Adjustable Power supply circuit
- The first section is a full wave diode bridge rectifier which we have already posted previously.
- LM 317 provides output voltage that can be varied from 1.2V to 35V maximum by adjusting the potentiometer.
- Another striking thing is that lm317 has inbuilt current limiting and thermal shutdown features.
- A reference voltage of 1.25 V is maintained at the time of operation across the resistor R2.
- The 1uF capacitor across the output helps in getting better transient response.
- You can refer the basic configuration figure shown below for understanding the above said points.
- The voltage at the output is given by the relation:
- Diode D1 is a protection diode that prevents the C3 from discharging into the regulator IC caused by short circuit of input to ground.
See below how I have integrated this circuit to a multimeter to show instantaneous reading. Let us know through the comment box below if you have any trouble in setting up a similar one.