Building A Power Supply
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	The heart of a good power supply is: THE TRANSFORMER. These
beauties can be found at most surplus stores or even at you local HAM
SWAP MEETS. The key to finding the one for your needs is very simple, if,
you follow these simple rules. Look for one that is about as big as both 
of your fists put together. Try to determine visually what the seconday
voltage is supposed to be. Some are marked and some are not. If you have
access to test equipment, and KNOW which is the primary and which is the
secondary, you can hook up a temporary lead to it and measure the voltage.
18 volts is IDEAL. Up to 22 is ok, but. You can run into a heat problem
in the pass transistors later with more than 18 volts.
Then you want to make sure to look at the size of the wire on the seconday.
The larger the wire size, the more amps you can draw. Let's say, for example,
10ga wire is on the seconday winding.
If you go to the ARRL Handbook, you will find, in the back somewhere, a
scale of current per size of wire. 10ga wire is usually good for 25 to 30
amp surges. If you are still not sure of the rating, you could hook up some
high current diodes to the seconday and with some high wattage resistors
that have been previously calculated for the right load, put a load on the
secondary while monitoring the secondary voltage drop. If it is a 20 volt
transformer and it drops 3 volts only, the load that you have on it is the
maximum that you will be able to safely draw at the output of the finished
power supply. You should always try to keep at least a 5 volt buffer for
good regulation. You can use either a CENTER TAPPED secondary or a conventional
secondary. If you measure from the outside wire to the center wire, you
should get half the full secondary voltage. If the centertapped wire shows you
18 volts or so, that is fine. the two outside wires of the seconday should
show you about twice that or 36 volts. This is ok cause you will put one high
current diode on each of the secondary main wires and you will conntect the
center tap to the negative of the supply or the negative of the main FILTER
CAPACITOR.
Next is the choice of DIODES for the conversion from AC to DC voltage. You
can use a FULL WAVE BRIDGE configuration or two stud mount type diodes.
Just make sure that the rating on them is more than you plan on drawing
at the output of the power supply.

    I will say again: Surplus. Look for electronic surplus stores in your
area. Parts can be had a lot cheaper, there, than trying to buy them new.

    FILTER CAPS are very important. Here is how to tell what you need for
your application. The hard core stead fast rule, especially for and FM rigs,
is to have at least 3,000 mf of capacitance for every AMP of current that you
want the out put to totally handle. If you are going for a 20 amp jobber,
then you would need a minimum of 60,000 mf of capacitance. The voltage value is
also very important. I always try to have a tall overhead in the values of
CAPS. If your seconday voltage is 20 volts, go for a CAP that is rated at
30 volts minimum. There is a problem with these high capacitance filters.
SIZE, they can get rather large. Any good COMPUTER grade CAP, with the
right values will work. If you find some 50 volt CAPS that are rated at
60,000, they could very well be as big as a 12 oz. pop can. You can use
several lower in capacitance rating and just parallel them to add up the
capacitance value. The voltage rating will stay the same in a paralleled
configuration. I don't like to connect CAPS in series for low voltage
applications. It just gets to cumbersome. High voltage supplies demand this.
Our low voltage supplies do not. When you series CAPS, the voltage can be
increased, but ! The capcitance drops by half value. So let's stick with one
or two in parallel if need be. Once you have a suitable transformer and diode
configuation, and you are sure they are all connected up right, you can plug
the transformer into a variac or variable transformer, and bring the primary
of the transformer slowly up to the maximum of the variac. If you cannot use
a variable type transformer, you are at risk by just plugging it into the wall !
Please be careful ! Many hams have died by electrocution ! Put a volt meter
on the AC scale and measure the rectified voltage. It will be somewhat lower
than what the actual transformer secondary was, because of loss due to the
diodes. Then, make sure to unplug everything and connect up your FILTER CAPS.
Taking great care in observing the POLARITY of the CAPS. The CAPS should be
marked in some fashion. Now, reapply the voltage to the primary of your
supplies transformer. Carefully measure the voltage at the positive and negative
of the FILTER CAPS. It will be a great deal higher than you have seen so far.
Not to worry if you have started with less that 22 volts on the bare secondary.
I call this fictious voltage cause it kinda goes away just at the time there is
a small load put on the output. I don't use BLEEDER resistors. I use a DC
light on the output to bleed off the COMPLETED power supply. NOW ! After you
have determined that the voltage is about right, disconnect the power source
and DRAIN the CAPS ! CAPS that are not drained off are very dangerous !
They can and will store DC electricity for a very long time. I suggest you
find a rather large resistor of a value of about 1k to use to bleed or drain
the CAPS for the next stage. Put a volt meter back on the CAPS to make sure
that they are in fact drained !

OKay ! You have the Transformer, Cabnet, Diode configuration, and the
Filter Caps. that you want to use. Right ?
Also pick up the following:
1 AC indicator light. 105 to 120 volts.
1 DC indicator light, 12 to 15 volts.
1 On/Off switch rated at 10 to 15 amps.
 (old phenolic plated toggles switches work well)
2 fuse holders (one for AC and one for the DC)
1 AC power cord with ground lug.
Large enough HEATSINK to cover the back of your cabnet.
Drilled and machines for as many pass transistors as possible.

I use 2n3771 NPN PASS TRANSISTORS. They seem to be the most durable.
They are rated for a maximum current handeling of about 30 amps each.
I do not recomend pushing these to the maximum, tho. They will run cooler
and last longer if you paralell them and let them SHARE the current flow.
You can use 2n3055's. They are rather plentiful on the surplus market.
If you go and buy 2n3771's new, try to pick all the same manufacturer and
lot numbers. If they are surplus, don't worry about it. If you are going to
try for a 20 amp jobber, plan on rounding up 5 PASS TRANSISTORS. One to
DRIVE the other four. This will let the regulator run very cool cause
it does not have to work much at all. You will drive the DRIVE transistor
with the regulator. The drive transistor will drive the pass transistors.
Let's keep this first supply real simple and basic. Let's use a 7812
voltage regulator. Connect the filtered DC source from the filter caps
to the Vin of the 7812 voltage regulator. Connect the Vout of the voltage
regulator to the BASE of the drive transistor. Connect the EMITTER of the
Drive transistor to the BASE of each and every PASS transistor. Now !
On the EMITTERS of the PASS transistors you need to use a resistor to
cause Current sharing of the PASS transistors. This prevents one transistor
from current hogging. And therefore self destructing over time. These
pass transistors usually shut down when over heated or over worked and
you need to let them cool down for a while before getting them to function
for you again. You will hook all the COLLECTORS of both the DRIVE transistor
and the PASS transistors together to the main DC sourse at the filter caps.
The negative of the filter caps goes to the GROUND pin of the 7812 regulator
and to the Negative output terminal of the supply. Connect all of the out puts
of the resistors that are on the EMITTERS of the PASS transistors to the
possitive out put terminal of the supply. You can experiment with a variable
resistor connected between the gound pin of the 7812 voltage regulator and
negative of the filter caps. to vary the final output voltage of the supply.