Wiring your own... (mumble)... transformers

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Well, this is the situation: i need a 18-0-18v 400 (at least) VA transformer for my dual '96 JLH. This is not a "standart" voltage, so it has to be custom built. After some averiaguations, it's arround $40-$50 for an E-I. Double that for a toroidal..

Now, my dad asked "100 bucks? Why don't you just build your own? We made our own transformers all the time in University"... good question :rolleyes: The maths behind the turns are not complicated, but, in practice, has anyone tried this out? I'd like a toroidal; it's a bit messy to work with, but since we're talking a few hundred turns i can live with that. E-I is out of the question; eddy currents are not easy to be made small enough on homemade transformers... or not?

So... Comments? Links? Prices? Thanks in advice!
So get your Dad to wind them...
Okay, maybe not.
There have been several threads on this. Do a search and see if you can scratch up some basic info. I think there were links to sites with info. However, I think your basic problem is going to be finding cores, whether EI or toroid. Once upon a time, they were more available...nowadays, they're scarce.
One option is to take a transformer of known VA, strip the secondary (leaving the primary) and rewind an entirely new secondary. Keep a count of the turns you remove so as to be able to calculate the turns you'll need to get the right voltage. The current rating, of course, will depend on the gauge of the wire you use. I've done this. It works well, but is tedious.
Another possibility is to take surplus transformers that have a slightly higher voltage (say, 20V) and simply remove a couple of turns.

GRollins said:
So get your Dad to wind them...
Okay, maybe not.


He'd be happy to supply me with literature on the subject, but i beleive that's about it :D

Today i'll storm the local transformer winding shops and see what i can find... the idea on the old transformer is good, specially if the VA ratings are met. I'll see what i come up with... my idea was to spend $20-30 for a ring core and the wire.

So, besides doing the maths well, picking a suitable wire & core, keep the turns tidy and covering as much as the core as posible, is there anything i should be aware of?
I've just rewound a 500VA toroid. This was
originally 25-0-25V, but I wanted 35-0-35V plus a
low-current 45-0-45V winding.

It's a perfectly straightforward job, but it does
take forever to get twenty metres of 1.6mm wire
off the core and then wind nearly thirty metres
back on (and then 35 metres of the thinner stuff
on top). It's also a real pain to get the turns
tight and tidy, especially with the thicker wire.

cp642, what does the Ruff tool do?

GRollins said:
Local transformer winding shops?
Wish I had a few of those.
Nah, sounds like you've pretty much got it covered. Just take it slowly and do a good, clean job. You'll be fine.

Well... yes. I just picked up the phonebook and got several adresses and emails. You give them the ratings, they make it: from small signal transformers to mounstrous industriall stuff. Scary :D

Anyway, best quotes i had were $40 for an E-I and $80 for toroidal. Still had no luck finding cores and separate parts... BUT, i do have an old bulky E-I with the secondary shorted. I'll take some measurments too see if the core can take the VA rating and perhaps i'll work with that one.

Again, thanks a lot for the feedback!
Last time I ordered from Plintron I bought a custom 750 VA dual 45V secondaries with integra thermal switch (I hate smoke). Delivered, this was around $75 US. Plintron is in Canada by the way, not USA.

The 400 VA transformers are a lot cheaper than the 750 VA.


Hi friends,
I had a toroidal transformer of 18-0-18 and i rewinded it to 25-0-25 successfully.Now i wanna rewind a 0-55/8Ampere transformer to get 30-0-30. The secondary of the transformer has got two wires running in parallel instead of a single wire. What should i do to get 30-0-30 .
hope someone would help me,
If you want a toroid that is not a "standard" voltage out, simply buy one that is rated for the next lower, "standard", voltage out and add a few turns of wire to the secondary. There's no point in unwinding what's already there when all you have to do is add a few more turns.

The same technique can be used to lower the voltage of an existing transformer- just wind the extra turns in the opposite direction. The voltage across the "new" winding will subtract from the original winding voltage. Use wire of a gauge that is appropriate for the output current.

If you need an extra winding to get some power for some other circuits, or a cooling fan, or whatever, just add another winding yourself. Low voltage windings don't require much wire or time. Don't know the turns/volt ratio of the transformer? Wind 10 turns around the core, measure the voltage, and divide by 10.

It's easy, cheap (because you can use a surplus transformer), and it works. What more could you ask for?

When you buy surplus toroids, be sure the center is not potted!

OK, we have a general idea of what it would take to wind a Toroid, but what about details..............

*If you do get a toroid core, what gauge would be recommended for primary and secondary for 400VA, or 800VA

Also, where could you get empty, large toroid cores and be sure they can be wound for 60hz 120V operation? Any websites sell empty toroids?
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