Variable Power Supply Using Fixed Regulator

Description

This is a similar power supply that I used to power my FM Transmitter. After suffering long problems with mains hum, this design using a pi filtered C-L-C approach. This circuit offers excellent ripple rejection. 

Parts List:

• T1 Transformer 10:1 Secondary 24V @ 2A
• BR1 Bridge Rectifier 50V PIV 2A rating
• C1 4700u (35V)
• C2 0.001u
• C3 2200u (35V)
• C4 0.001u
• C5 4.7u (25)
• C6 0.01u
• R1 10k potentiometer
• L1 see text
• U1 7805 N.B. This may be changed for different output voltages e.g. 7812 for higher output voltage
• ZD1 15V zener @ 1.3W

Notes

Looking at the above schematic, the specific inductance of the ferrite (core)is important. 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.

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. I used MR751 which is rated 6 amps, but another good choice is 1N5400. 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, 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. All the parts should be available at local electronic shops. 

Formula for calculating Ripple Factor in Filters:

Where:

• Φ = Ripple Factor 
• F = frequency of supply in Hz
• L = inductance of L1 in uH
• C1 = capacitance of C1 in uF
• C3 = capacitance of C3 in uF
• RL = load resistance in ohms