clc wrote:

Im still trying to come up with a high current DC supply for my mobile HF rig.
I have 4 - transformers that are all the same part number, there output is 4amp
at 18.5 vac. I want to hook a bridge to each & then connect the DC outputs of
each in parallel to increase current to my regulator circut.
I should end up with 17 - 18 VDC @ 16 amps. Does this sound like it would
work ok, Im not sure about paralleling DC supplies ?

It should work fine, but your DC output voltage will end
up higher with the addition of the required filter cap.
Voltage across the cap will be about 26 volts, no load.
Parallel the output of the bridges, install a big filter
cap, and run the resultant voltage to a voltage regulator.
Do not regulate prior to paralleling the supplies!


Your regulator can be an LM317 or 7812, with pass
transistors in a "collector wraparound" configuration.
I'll describe such configuration below, using a 7812.
You can parallel a number of pass transistors - I'd use 4
2N3055's (2N3055's because I have a lot of them) with
a .1 ohm 5W resistor in each emitter leg. You can use a
7812 with 2 or 3 diodes in the ground leg between the leg
and ground, connect the output of the 7812 to the parallel
xsistors collectors. The base of the transistor goes to
the input pin of the 7812. From there, a 2 ohm 1 watt
resistor connects to the + side of the cap where the emitter
resistors also connect. If you look at the 7812 datasheet
application notes, you will find an example of the circuit
there with a single pass transistor (and thus no emitter
resistors) and no diodes in the ground leg.
http://www.national.com/ds/LM/LM340.pdf

The pass transistors have to handle about 205 watts
(12.8 volts * 16 amps) so you need a good heatsink
for them. You also need to heatsink the 7812.

The LM317 circuit is essentially the same. Look at
its datasheet. You won't need the 2 diodes in the
ground leg, but you will need to add a couple of
resistors.