# Using identical coils on the same transformer?

Discussion in 'The Projects Forum' started by Flamin_D, May 3, 2010.

1. ### Flamin_D Thread Starter New Member

May 3, 2010
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0
Hey, For a project im working on im using an old transformer i have to hand, which takes 240Vac and has two identical coils giving 12Vac. I only have one load to connect, and want to utilise both coils for maximum power - i thought it would be as simple as 'paralleling' the coils together, but that seemed to make the transformer sizzle a bit. Is is possible to use both on the same load or do they have to be separate?

Sam

2. ### mik3 Senior Member

Feb 4, 2008
4,846
69
You can use the center wire and one of the other two wires to get 12V AC.

Also, you can use only two wires, except the middle one, and get 24v AC.

3. ### retched AAC Fanatic!

Dec 5, 2009
5,200
316
The amount of current will be determined by the thickness (gauge) of the wire used.

So you may have "boosted" the current above the rating of the wire used on the transformer.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
When you connect the secondaries in parallel you have to be careful to connect them with the correct polarity - otherwise you may have the two secondaries working against each other and producing an effective shorted condition - "sizzling" being the likely result. What voltage do you see across the secondaries when connected in the parallel arrangement you applied? If the voltage is zero or very low (i.e. not about 12V) you need to reverse one of the windings in the parallel arrangement. In the parallel arrangement you should be able to supply a load current of twice the current rating of a single secondary winding.

5. ### Markd77 Senior Member

Sep 7, 2009
2,796
595
If you only have access to the centre tap then your choice of rectifier circuit could give you either 12 or 24 V
http://en.wikipedia.org/wiki/Rectifier
A 2 diode setup will give 12V and a 4 diode will give 24V

6. ### Flamin_D Thread Starter New Member

May 3, 2010
18
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I have not tested the voltage of the two in parallel as i switched it off immidiately due to the sound it was making! Im sure the polarity was correct, i wired red to red / blue to blue. But you may be right in saying reverse one of them, never actually tried it.

The application is a high current 12 - 14.5V DC power supply, i have three 35A rectifiers in parallel, two 14v sealed acid batteries in parallel, a 1.6F cap; next thing im working on is a regulator as currently the voltage soars to around 20v on 0 load. Probably going to use 5 or 6 2N3055s with a 7812 plus caps/resistors.

7. ### davebee Well-Known Member

Oct 22, 2008
539
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It should be easy to test this without actually short-circuiting the transformer.

Disconnect one pair of the connected secondary winding wires and measure the voltage across them.

Then disconnect the other pair of secondary wires, reconnect the two secondaries in the other phase and measure the voltage across that configuration.

You should find that one connection configuration gives you twice the voltage of each secondary and the other connection gives you just about zero volts.

The zero-volts connctions are what you'd use for parallelling the secondaries.

If you measure anything other than zero volts, then this is probably not the right transformer for this project.

8. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,189
421

I believe your transformer sizzling is caused by wiring the two secondaries OUT OF PHASE, which will give the equivalent of a dead short. If they are identical, you should be able to connect them in parallel just fine.

Eric

9. ### Bychon Member

Mar 12, 2010
469
41
On the transformers I have met, the 2 red wires are a winding and the 2 blue wires are a winding. Connect red to red and you've shorted a winding. Connect blue to blue and you've shorted a winding.

Start with a volt meter and map out the windings or you will only cause sizzling.

10. ### JoeJester AAC Fanatic!

Apr 26, 2005
4,018
1,772
A picture is worth 1000 words.

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