Hans wrote:

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 ?


Why don't you connect the transformers in parallel and rectify /
stabilize the total AC current?

Hans

There is a "chance" that the voltages won't be exactly the same. As a
result you can
have circulating currents in the transformers from that difference. The
diodes solve
that potential problem.

Bill K7NOM

just paralleling up the power supplies will probably not result in any
power exceeding the power supplied by the transformer with the largest
swing.
let me describe this for you:

your rectifiers act as switches, whenever the voltage across them
exceeds 0.6 volts (for silicon diodes), they will conduct. now you
connect the anodes of two rectifiers that are driven from the other
side by a voltage generators. If you imagine that one of the two
diodes are driven by a voltage source that is higher than the other by
0.6 volts, then the diode will never switch on as the other diode will
reverse bias this diode. so, simply paralleling up the dc outputs may
not work. A 0.6 volts difference between two transformers is almost a
given.

my recommendation to you would be to wire up the primary windings in
parallel and the secondary (18v) windings in series. That way, if the
phase is proper you will get 18-0-18v (36 v) across the two
secondaries. A bridge will get you 18V.

Something like this :

----+-3|
| 3|C--------------||---+
| 3|C |
| 3|C------+ |
-+--|-3| | |
| | | +-------+----O 18 DC +v
| | |---- Gnd | |
-|--+-3| | | ===
| 3|C------+ | === filter cap.
| 3|C | |
| 3|C--------------||---+ Gnd
-+----3|

this uses just two of the four transformers. besure that the voltages
of the secondary are in phase. (you will not get more than a volt or
so if the are not). there is probably a way to also utilise four
transformers, it is in the corner of my eye, but the above given is a
cookbook recipe.
However, if you require 36v, then it is a simple matter to extend this
by using two supplies with secondaries in series on each side of the
rectifier bridge.

- farhan

just paralleling up the power supplies will probably not result in any
power exceeding the power supplied by the transformer with the largest
swing.
let me describe this for you:

your rectifiers act as switches, whenever the voltage across them
exceeds 0.6 volts (for silicon diodes), they will conduct. now you
connect the anodes of two rectifiers that are driven from the other
side by a voltage generators. If you imagine that one of the two
diodes are driven by a voltage source that is higher than the other by
0.6 volts, then the diode will never switch on as the other diode will
reverse bias this diode. so, simply paralleling up the dc outputs may
not work. A 0.6 volts difference between two transformers is almost a
given.

my recommendation to you would be to wire up the primary windings in
parallel and the secondary (18v) windings in series. That way, if the
phase is proper you will get 18-0-18v (36 v) across the two
secondaries. A bridge will get you 18V.

Something like this :

----+-3|
| 3|C--------------||---+
| 3|C |
| 3|C------+ |
-+--|-3| | |
| | | +-------+----O 18 DC +v
| | |---- Gnd | |
-|--+-3| | | ===
| 3|C------+ | === filter cap.
| 3|C | |
| 3|C--------------||---+ Gnd
-+----3|

this uses just two of the four transformers. besure that the voltages
of the secondary are in phase. (you will not get more than a volt or
so if the are not). there is probably a way to also utilise four
transformers, it is in the corner of my eye, but the above given is a
cookbook recipe.
However, if you require 36v, then it is a simple matter to extend this
by using two supplies with secondaries in series on each side of the
rectifier bridge.

- farhan

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

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

On Tue, 28 Oct 2003 23:45:42 GMT, (J M Noeding)
wrote:

Connecting transformers in parallel is sometimes a risky business,

Some big power distribution transformers are sometimes connecting in
parallel with some external inductance to reduce the imbalance.

Take a separate smallish transformer with a centre tapped low voltage
secondary winding. Connect the centre tap to the load and the two end
of the small transformer to the respective hot terminals on the main
transformers. The primary of the smallish transformer is not
connected, but can contain quite voltages.

A similar approach is used when hot switching between different taps
on a large power transformer. Normally the two ends of the CT coils
are connected to the same tap on the main transformer. Then one end is
moved (break before make) to the adjacent tap and after a while the
other end is also moved. In this way, the power distribution is not
interrupted when changing main transformer taps and it does not cause
momentary short circuits on the main transformer.

Paul OH3LWR

On Tue, 28 Oct 2003 23:45:42 GMT, (J M Noeding)
wrote:

Connecting transformers in parallel is sometimes a risky business,

Some big power distribution transformers are sometimes connecting in
parallel with some external inductance to reduce the imbalance.

Take a separate smallish transformer with a centre tapped low voltage
secondary winding. Connect the centre tap to the load and the two end
of the small transformer to the respective hot terminals on the main
transformers. The primary of the smallish transformer is not
connected, but can contain quite voltages.

A similar approach is used when hot switching between different taps
on a large power transformer. Normally the two ends of the CT coils
are connected to the same tap on the main transformer. Then one end is
moved (break before make) to the adjacent tap and after a while the
other end is also moved. In this way, the power distribution is not
interrupted when changing main transformer taps and it does not cause
momentary short circuits on the main transformer.

Paul OH3LWR

"Jim, N2VX" wrote in message . ..
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

A 7815 would give 15 volts, wouldnt that be alittle high? A 7812 would be a
little low, so i thought of a 723 ang just adjust the voltage to 13.8.

"Jim, N2VX" wrote in message . ..
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

A 7815 would give 15 volts, wouldnt that be alittle high? A 7812 would be a
little low, so i thought of a 723 ang just adjust the voltage to 13.8.

On 29 Oct 2003 10:16:56 -0800, (clc) wrote:

"Jim, N2VX" wrote in message . ..
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

A 7815 would give 15 volts, wouldnt that be alittle high? A 7812 would be a
little low, so i thought of a 723 ang just adjust the voltage to 13.8.

You get 15 - (transistor drop + balancing emitter resistor drop). I
got 14 volts out of it. That's in the range of car charging systems
and I've measured that much on some cars with the engine running.

There's another setup where you get IC regulator voltage + transistor
drop + resistor drop, using PNP transistors. Check the handbook for
details. Power PNP's are less common and it's hard to beat the
venerable 2N3055's price and availability.

How about a LM317? As I recall they are good for more current than
723's and easy to use. Regulator current at the IC is
(output current) / (transistor gain)
Then you can adjust to get 13.8 on the output.

73,
Jim