# How to connect transformer secondaries in parallel.

Discussion in 'General Electronics Chat' started by Subterranean, Jan 6, 2011.

1. ### Subterranean Thread Starter New Member

Jul 27, 2010
13
0
I have a torroidal transformer with 2 seconaries, both 0-26v (measure at ~30Vac with no load). Since this is all the voltage I will need, I'd like to parallel the outputs to allow as much current as possible. I have red, orange and 2 black wires on the secondary side. The black ones are definitely the middle taps, and measuring voltage from red to orange gives ~60V as expected.

To connect in parallel do I just connect the red and orange wires together, and use this with the black wires to get the 30v? What about phase since the center tap wires must be connected inside the transformer somewhere to be able to measure 60V across the entire secondary?

This may be a really simple question but I'd hate to get it wrong and melt something!

Thanks for the help.

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,852
968
Without un-wrapping and finding the wire connections, you will have to take a 50/50 guess and connect one black wire with the red and one with orange.

Hook your meter up to them and watch it as you BRIEFLY energize the transformer. You will either get 0 volts or 30 volts.

I'm guessing you know what to do from there.

3. ### soda Active Member

Dec 7, 2008
174
13
Hi

Connect the 0ne winding 26v output to the next winding 0v output. This wil give you one output of 52V- 0V. You can also connect the 2, 0volts together which will give you a 26-0-26v output and this will then add up to 52v. Do not connect the orange and red wires together because this will cause one hell of a short.

On the torroidal transformer must be a label that shows which color belong to which voltage.

Last edited: Jan 6, 2011
4. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
No matter what you do, the following is the proper way to parallel two secondary windings.

To parallel two secondary windings, you have to make two connections. Don't make both of them yet.

Make the first connection and connect the transformer primary to AC mains. Then use an AC meter set to 100V range(more than double the secondary voltage), measures the voltage across the two points which you would intended to connect together later.

If there is 60V or so, your first connection polarity is wrong and you would need to select the alternative point for the first connection. Power down and redo the connection.

If there is no voltage shown on meter, reduce meter range and see if there is some voltage present. It is normal for the two windings to differs by a fraction of a volt so I would say anything below 0.5V would be OK.

Power off and make the final connection.

5. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
It is seldom for transformer to have connection between two windings made internally but leads out separate wires for this common connection. If this is so, it would be clearly marked on the transformer label. Mostly the connection is left to the user to have more flexibility on using series or parallel configuration. I can only say it must be for current rating purposes.

6. ### Subterranean Thread Starter New Member

Jul 27, 2010
13
0
Yes, I'm pretty sure the two centre wires are already connected internally. The transformer itself is already mounted on an aluminium 'housing' consisting of a bottom and two sides, which is fitted with fuses for each secondary winding and a mains kettle socket so it wouldnt surprise me if tehre was some other specific connections made.

Given that I only need the ~26V of a single secondary and that I'd still like to use this transformer since it was very cheap for its ratings and I already have it, what would be the best way to proceed?

I can currently think of 3 possible actions:
1. Use just one of the seconaries and forget about the other one.
2. Use one output to power some things, while using the other to power others, trying to balance power usage for each.
3. Given that I will be rectifying it into DC anyway, use seperate rectifiers for each output, then connect them both to the smoothing capacitor.

Number 3 seems the best to me but are there any problems with doing this other than having to use 2 rectifiers rather than just 1? I already have two of the common 1kV 50A rectifiers you've probably seen around, and since the rectifiers are made of diodes anyway, I can't see a problem with reverse bias issues. Plus it splits the load between the 2 which may help with heat dissipation.

If anyones wondering, I managed to find a 56,000uF 60V capacitor to use for smoothing which should give good ripple reduction even with high current draw.

Last edited: Jan 9, 2011
7. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
All your three proposals are workable and commonly used.

For number three, there is no problem at all. The configuration is known as "fulllwave rectification" and requires one rectifier for each winding. As a single rectifier will conduct only half of the time, the other rectifier can 'fill in' the gap because the polarity of the voltage on the two windings is 180° apart so the two rectifier conducts alternatively.

Big smoothing capacitor could well be your next challenge.

An uncharged capacitor of this large value will present itself as a short circuit to the rectifier and transformer on power up. The high inrush current would cause the protection fuse to blow instantly or after a couples of on/off operations.

Possible solutions often involve SCRs, thermistor or series resistor in current path which is removed once the capacitor has charged up.

8. ### soda Active Member

Dec 7, 2008
174
13
Hi

You can use only one rectifier with two or one smoothing cap but a 56,000 is too high. Rather go for between 10,000 to 15,000uF which will be ok.

If i were you i would use 2 times 4700uF in parallel. 1500uF for each amp the transformer can supply is usually ok.