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

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.