# Transformers in series...

#### MikeA

Joined Jan 20, 2013
257
I found a pair of identical transformers, with 1:16 windings, bolted down to a plate. I've connected a variac to the input on one of them and ran it from 0 to 140v to find out the 1:16 ratio.

The interesting part is that the outputs are connected in series. What does that do? Double the voltage when both transformers are energized on the primary?

#### strantor

Joined Oct 3, 2010
5,588
Yes

Joined Feb 20, 2016
3,882
Yes. It is quite ok to series the secondaries like that.
Another trick that is used sometimes, if for instance you have 110V mains but need 100V, a 110V to 10V transformer can be used with the 10V secondary wired in series with the mains but antiphase to drop the mains output supply by 10V to give 100V.

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#### MikeA

Joined Jan 20, 2013
257
So if transformers are identical and in series, current is equal to one of them, and voltage is double?

What about if one is rated 1A and one is 2A and 6v output. Two of those in series would be 12v output at 1A?

Joined Feb 20, 2016
3,882
So if transformers are identical and in series, current is equal to one of them, and voltage is double?

What about if one is rated 1A and one is 2A and 6v output. Two of those in series would be 12v output at 1A?
Yes. You are limited by the lowest current. Both transformers must be powered and the phasing must be correct.
Multiple transformers and/or windings can be used to change the voltage out. And of course, identical secondaries can be wired in parallel to double the current.

#### #12

Joined Nov 30, 2010
18,222
10V secondary wired in series with the mains but antiphase to drop the mains output supply bu 10V to give 100V.
That's what I do with my 115V outdoor lights because my local voltage is 125V. It makes a lot of difference in the life of the bulbs.

#### MikeA

Joined Jan 20, 2013
257
And of course, identical secondaries can be wired in parallel to double the current.
I didn't think this would be the case.

My transformers have a 30A fuse on the primary. Is there way to guess what their power rating is?

Joined Feb 20, 2016
3,882
You cannot go by the fuse size. There are calculations available to do that. It depends on the cross sectional area of the core through the coil.
I'll try to find the info. This is the first one I found...

From the Radiotron Designer's Handbook 4th Edition page 235:
core sized needed (for 60 Hz.)
cross-sectional area in inches = (V*A)1/2 / 5.58
So, solving for Volt*Amps, knowing core cross-sectional area:
(5.58 x core area)2 = Volt*Amps capacity

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#### MikeA

Joined Jan 20, 2013
257
I measured the core area through the coil at 9.3 square inches.

(5.58 x 9.3)^2 = 120v * 22.44

So 22.44 amps @ 120v?