variable power supply 78xx regulator

Variable Power Supply with 78XX regulator

This variable power supply is using 7805, 7809, 7812 or 7815 voltage regulators, where the last 2 digits represents the maximum output voltage of the IC.
This circuit offers excellent ripple rejection, eliminates mains hum, and has a design using a pi filtered C-L-C.

A core should be chosen to work within the specific frequency as stated by the manufacturer. L1 is a powder core and has 32 turns of 0.75mm wire.

Variable Power Supply Circuit Diagram

variable power supply 78xx regulator

The transformer has a 240V primary and has a secondary rated 24V at 2A. The bridge rectifier contains 4 diodes, their current rating needs to be high with respect to the transformers output current; if not the current may damage the diodes. C1 is the mainfiltering capacitor, the supply is further smoothed by the combination of L1 and C3. C2 and C4 are decoupling capacitors; their action further reduce ripple factor.

The regulator 78xxr, U1 utilizes the action of zener diode ZD1 which is in parallel with the potentiometer, R1. The tuning action of R1 produces a variable regulator output. The output voltage is variable from the regulator output to the regulator output plus the zener voltage. E.G. A 7805 regulator and 10V zener give an output adjustable from 5 to 15 Volts. The regulator may be changed to provide different output voltages as may the zener. the zener should be rated a minimum of 1.3 Watts.

T1 Transformer 10:1 Secondary 24V @ 2A
BR1 Bridge Rectifier 50V PIV 2A rating
U1 7805 N.B. This may be changed for different output voltages e.g. 7812 for higher output voltage
ZD1 15V zener @ 1.3W



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  • breakthedawn

    Desgin flaw! There is no reverse diode connected to the 7805 which can cause damage to it.

  • Louis Bertrand

    Why use a Zener? The regulator has much tighter tolerance on the voltage reference. If you put a 510 ohm resistor between the 7805 output and the common, you get a 10mA current source. Now take that current and put it through a potentiometer connected as a variable resistor between common pin and ground. The current will produce a voltage proportional to the resistance of the variable resistor. For example, a 1k resistor will produce 10V, therefore the overall output is 10V+5V=15V.

  • Jim Keith

    The inductor here is very non-critical. In my opinion, it is not needed at all.

  • Kuberkoos

    The diagram shows a inductor with 32 turns of 0.75mm wire and a powdered core. What would the impedance of this inductor be? The one I have available has plus minus 36 turns and an impedance of 1400 uH. Is that close enough???

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