Diagrams g4wpw

Transformer         18 volt 20 Amp
Bridge rectifier    25 Amp
C.                  1000uF For each Amp required.ie 20,000 uF For 20 Amp
IC.                 7812 Regulator.
Diodes.             Any 1Amp diode i.e.2N5408 ,BY100,BY133 etc
Q1,Q2,              2SC3281
R1,R2,              0.1 Ohm 7Watt wire wound.

The above is a simple but efficient 20 Amp power supply using minimum components
it can be reduced to a 10 Amp supply by using only one pass transistor ( Q1,Q2 )
or increased to a   30 Amp supply  by adding another pass transistor and 
current sharing resistor of the same type and value, of course the Transformer 
and bridge rectifier would have to be upgraded for increased current demand.

The diodes in the diagram are to increase the output voltage of the 12volt regulator
from 12 volts to 13.8 volts each extra diode would increase the output by 0.6 volts.

A fuse may be added in series with the mains input to the Transformer.the value can
be calculated as follows....
(power in the secondary of the transformer = power in the primary of the transformer)
Therefore for 13.8v at 20 Amps the secondary power  = 13.8V * 20Amp = 276 watts
Calculating from that for a mains supply of 240 volts = 276/240  = 1.15 amp fuse.
I use a 2 Amp fuse in my supply this gives some provision for the large capacitor
charging current ( inrush current )during switch on

The above simple ( slow start circuit ) may be incorperated into the power supply .
The time taken in switching  from position 1 through 2 to 3 is enough to reduce the 
inrush ( capacitor charging ) current.



Switch            3 Way rotary.
Fuse.             2 Amp.( calculate as above )
Resistor          20 ohm,50 ohm or 100 ohm ( value not critical ) 25 Watt.wire wound.