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

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


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.


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.

The Vbe for the 2N3055 is quite high, it might be well over 1 V at
some larger current. Thus, the output voltage would be under 14 V and
with some emitter resistors to share the load between the transistors
even less than that.

Nominally, the 7815 can only supply 1 A and assuming worst case hFE
for the 2N3055 of 20 at 4 A, the output current would only be 20 A, so
five 2N3055 devices would nicely give 20 A. If more current is needed,
more series pass transistors are needed, but the 7815 is not capable
of supplying the required amount of base current, so a driver
transistor is required between the 7815 and the series pass 2N3055
transistors. With two Vbe drops, the output voltage would be 13 V or
less.

Paul OH3LWR

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


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.


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.

The Vbe for the 2N3055 is quite high, it might be well over 1 V at
some larger current. Thus, the output voltage would be under 14 V and
with some emitter resistors to share the load between the transistors
even less than that.

Nominally, the 7815 can only supply 1 A and assuming worst case hFE
for the 2N3055 of 20 at 4 A, the output current would only be 20 A, so
five 2N3055 devices would nicely give 20 A. If more current is needed,
more series pass transistors are needed, but the 7815 is not capable
of supplying the required amount of base current, so a driver
transistor is required between the 7815 and the series pass 2N3055
transistors. With two Vbe drops, the output voltage would be 13 V or
less.

Paul OH3LWR

"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

"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

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.
===========
Question : With a load having 2 connections , with the centre tap connected
to 1 connection of the load ,where to connect the other connection of the
load ?

Frank GM0CSZ / KN6WH

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.
===========
Question : With a load having 2 connections , with the centre tap connected
to 1 connection of the load ,where to connect the other connection of the
load ?

Frank GM0CSZ / KN6WH

On Thu, 30 Oct 2003 11:31:22 -0000, "Frank Dinger"
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.
===========
Question : With a load having 2 connections , with the centre tap connected
to 1 connection of the load ,where to connect the other connection of the
load ?

To the "cold" end of both main transformers, which are connected
together. You have to connect the primaries in such a way that the
"hot" ends of the secondary of each transformer is in the same phase,
thus the voltage of the hot end of each secondary is nearly equal and
the centre tapped choke will take care of the rest.

In three phase systems, connect the star points of the transformers
together, which is also used as a return for single phase loads. A
balancing CT choke is needed for each phase and also in this case you
have to observe the primary polarity when connecting the primaries.

Paul OH3LWR

On Thu, 30 Oct 2003 11:31:22 -0000, "Frank Dinger"
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.
===========
Question : With a load having 2 connections , with the centre tap connected
to 1 connection of the load ,where to connect the other connection of the
load ?

To the "cold" end of both main transformers, which are connected
together. You have to connect the primaries in such a way that the
"hot" ends of the secondary of each transformer is in the same phase,
thus the voltage of the hot end of each secondary is nearly equal and
the centre tapped choke will take care of the rest.

In three phase systems, connect the star points of the transformers
together, which is also used as a return for single phase loads. A
balancing CT choke is needed for each phase and also in this case you
have to observe the primary polarity when connecting the primaries.

Paul OH3LWR

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.
===========
Question : With a load having 2 connections , with the centre tap
connected
to 1 connection of the load ,where to connect the other connection of the
load ?

To the "cold" end of both main transformers, which are connected
together. You have to connect the primaries in such a way that the
"hot" ends of the secondary of each transformer is in the same phase,
thus the voltage of the hot end of each secondary is nearly equal and
the centre tapped choke will take care of the rest.

In three phase systems, connect the star points of the transformers
together, which is also used as a return for single phase loads. A
balancing CT choke is needed for each phase and also in this case you
have to observe the primary polarity when connecting the primaries.
========================
Tnx Paul , for the very useful info ,which I have filed for future
reference.

Frank GM0CSZ / KN6WH