"Jim, N2VX" wrote in message
...
On 27 Oct 2003 19:02:07 -0800, (clc) wrote:

Start scrounging hamfests, etc. for electrolytic capacitors. You
can't have too much filtering. In this case more is better.

For a regulator use a 7815 IC to drive several 2N3055's. ARRL
handbooks have a good example of 2N3055's in parallel. 5 2N3055's on
a decent heatsink will run nice and cool. Scrounge for heatsinks,
too. They tend to be cheap at hamfests because most people are
appliance operators.

And if you want to make life easier for the rectifiers add a soft
start cicruit using a relay. The handbook has an example of it.

Good luck and 73,
Jim


Here is the article (schematic link at bottom)
http://www.seits.org/features/pwrsup.htm

KBT Info page on linear supplies
http://www.kbt-dc-supplies.com/index.php

and Astron schematics are already on the Internet
http://www.repeater-builder.com/rbtip/astron-index.html

Greg
w9gb

Paralleling DC supplies can be problematic if they are regulated supplies and
not designed for paralleling. Another solution to consider is using a deep
cycle battery and a charger. Works fine, even during a power outage...

On 27 Oct 2003 19:02:07 -0800, (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 ?

I did it with 2 of them and it worked OK. As you described use a
separate rectifier on each and combine the rectifier outputs.

Start scrounging hamfests, etc. for electrolytic capacitors. You
can't have too much filtering. In this case more is better.

For a regulator use a 7815 IC to drive several 2N3055's. ARRL
handbooks have a good example of 2N3055's in parallel. 5 2N3055's on
a decent heatsink will run nice and cool. Scrounge for heatsinks,
too. They tend to be cheap at hamfests because most people are
appliance operators.

And if you want to make life easier for the rectifiers add a soft
start cicruit using a relay. The handbook has an example of it.

Good luck and 73,
Jim

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.

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.