Deconstructing a "E-I" Transformer

Thread Starter

André Ferrato

Joined Apr 5, 2015
Hi, i am working on project that needs to belong to the lightweight division, so i was wondering if i could dismantle the E-I Transformer i have, with that i say, take off the laminations and mount it on the PCB with just the plastic that the copper wire is envolved, by doing that it almost weights nothing. My question is, if i take the laminations off, what would this cause to transformer and circuit?

Another question, i want to match two transformers in parallel operation, to do that i am thinking of ensuring the same voltage at no load, this will also match the impedance right ? Because, if the voltages are the same, there will be no circulating induced current on the secondary circuit and the if the impedance is the same, the load they will share will also be the same, right ? Or there are other parameters to consider, like the size of the copper wire that each transformer has, or going even further, the voltage is produced with turn ratios, so they could be producing both 15.7 but with different impedances.

So i have a transformer with 17.8VAC at no load and another one with 15.7VAC at no load, they are both from the same manufacturer, they are almost the same thing, both rated for 15VAC( I know, i know, 17.8VAC rated for 15? State-of-the-art manufacturers). To easier way is to reduce the 17.8VAC to 15.7VAC as i see, them i just need to remove secondary turns on the 17.8 transformer ?


Joined Feb 24, 2006
If you remove the laminations it pretty much stops being a transformer. It is just two air coupled inductors, and the losses would huuuuge.

Identical transformers in parallel will not have identical voltages and you need to account for that. Most people don't do this kind of messing around, they size a single transformer for their requirements.

If you want to rework your transformer by removing copper, knock yourself out.


Joined Mar 14, 2008
If you need to reduce the size of the transformer in a power application then you could go to a switching power supply, which operates at a frequency much higher than the mains and thus allows a much smaller transformer for a given power (size of the magnetics is roughly inversely proportional to the frequency for a given power).


Joined Jun 22, 2012
No the lamintion strips are there to make it work, if size is what you need, go for a switchmode psu, like in phone chargers, plenty on the Internet or car boot sales.


Joined Dec 2, 2013
I see a very serious problem with this type of questions and you are not the only one.

First of all do never play with things without a study; like what is it how does it work what are the risks. Etc. Arguments like I can’t find information are rubbish it’s all there on the internet.

Failing to do so puts you in an extreme stupid position it’s like ramming your head in an Oliphant ass hoping for air and light to look around.

Yes, I do like to help people whom get stuck but refuse to do the work for the lazy ones.


Joined Feb 5, 2010
Research toroidal tranformers. They can be about 50% lighter for the same output rating.

They are much more intolerant of DC voltage offset than EI transformers are, but offer low magnetic field leakage and a slightly smaller size.
I know about the switchmode psu, but i dont want to head there yet. What i asked here, has been done, i noticed there are a lot of parameters to put two transformers in parallel, and i managed to match the voltages of both transformers, but the impedances remain diferent. The first one is receiving a much higher load, so its still heating. Its pretty hard to match two and i gave up on that. But its not impossible. The toroidals are a good chance, but for now, i'll deal with the heat of using one. Thanks for all the answers, the main point of the thread, was to see different opinions on the parallel operation.


Joined Nov 4, 2013
Paralleling identical transformers is not a problem it's the trying to do it with mismatched ones that typically does not work so well.


Joined May 19, 2014
There is a technique to parallel variable transformers mounted on the same shaft. A center tapped saturable inductor,(saturable reactor) is used to combine the two outputs. When the common load is low the inductor will have a high reactance, reducing the amount the transformers fight one another. When the load current is high, the inductor will saturate, its reactance drops to nil, allowing max current to the load. Under high load current, the internal resistance of the transformers now balances the power.
This technique is very old, first used when the U.S. actually knew how to build specialty transformers and inductors.

Edit: The common name for the above inductor is: Paralleling Choke

Also notice in the schematic above they (paralleling choke) are drawn as transformers. This contributes to the balancing action at light loads.


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Lestraveled, this is an awesome thing, for real, that was a solution, but i already gave up on paralleling the transformers. Anyway thanks for the interest and time to dig this knowledge for me, i'll keep that in mind the next time i deal with this issue. I'll definitely look on to this.