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THE SIMPLEST PSU

by Harry Lythall - SM0VPO

One of the most useful circuits for the home constructor is
the Power Supply Unit. A transformer with tapped secondaries
can give a range of output voltages for testing and operating
home-made equipment.

This is a PSU in its simplest state. It is an ideal project
for the beginner to homebrew. This circuit will deliver up to
3 amperes at 15 volts. The voltage may be reduced to 11.8 (ish)
by selecting the 9v tapping. The voltage will of course fall
a little when a heavy load is placed upon it. NO OUTPUT CURRENT
LIMITING or voltage regulation has been provided. Shorting the
output can become quite nasty so take care. The four diodes D1
- D4 may be replaced with a single bridge rectifier component.

The transformer tappings may be selected for any 3 volt
increments from 3 volts (4.4v DC out approx) to 24 volts
(34v DC out approx). Connect the bridge rectifier to the
9 and 12 volt tranformer taps for 3vDC output. 9 - 15 taps
will give about 6.5 volts. 0 - 9 = 11.8 etc. The output
voltage is equal to (Vt x 1.4) minus 1.4 volts, where Vt =
the transformer tappings used.

Here the circuit has been modified to include a very simple
voltage regulator. This will give an output voltage between
0 volts and 15 volts. The power output transistor must be
fitted to a heatsink. Again, NO OUTPUT CURRENT LIMITING is
provided so care is still needed. All the component values
not marked are the same as in the previous circuit.

The size of the heatsink can be quite complex but
here is a guide.

The addition of a voltmeter accross the output terminals
will be a distinct advantage. The DRAIN of TR1 is the
correct point to insert an ammeter. The power FET chosen
normally has the terminals marked (d, s + g).

Here the circuit has been modified to provide current limiting
to reduce the output voltage if too much current is demanded.
This will reduct the possibility of damaging equipment by drawing
excessive current. All component values not marked are the same
as in the previous circuits. R1 = 0.7 / Amperes.

- R1 = 10 ohms for 60 mA
- R1 = 7 ohms for 100 mA
- R1 = 2.5 ohms for 300 mA
- R1 = 1 ohm for 700 mA
- R1 = 0.7 ohms for 1 A
- R1 = 0.25 ohms for 3 A

Have fun with this circuit. Best regards Harry.

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