Where to buy 200VA toroidal transformer

[NoSpam]

I'm going to need a toroidal mains transformer, about 200VA with 20-0-20 output. CPC & RS have got 18-0-18 and 22-0-22 which will probably do but is there anywhere else I should look?

Dave

[pete]

Another option for 0 - 18 *2.

number 88-3828

[andrew]

ebay?

[Andy Wade]

20-0-20 suggests that it might be to feed a rectifier and, perhaps, that you were thinking of using the 2-diodes and centre-tap ('bi-phase') configuration.

If that's the case may I suggest that you consider using a bridge, since it makes significantly more efficient use of the transformer and gives you a higher DC output current rating, or cooler running of the tranny for the same load current as well as better output voltage regulation. True you have the cost of two more diodes (not usually very much) and the o/p voltage is slightly lower, but the better transformer utilisation usually wins out.

Most of the stock transformers come with two identical secondaries. Using a bridge gives you the choice of two 20 V secs in parallel or two

10 V ones in series.

[Dave Plowman (News)]

These are one of the major makers.

[Tabby]

You don't say why you want it to be toroidal. Unpotted toroids exist, and those are easy to add a few extra turns on to ge them up to 20v, but you cant do that on the more common resin filled hole ones. Another maybe is a small second toroidal wound to give a pair of 2v 8A windings. Now I'm wondering why you'd need 20 and not 18/22, and whether there might be easier ways to achieve the same end.

NT

[NoSpam]

[Bob Minchin]

An easy way to adjust the voltage is to work out how many turns per volt have been used to wind the transformer and wind on 24 volts worth of turns using ordinary hook up wire such as 1/0.6 or 7/0.2.

The connect this in series with the primary according to which way you connect it, the output voltage will be increased or decreased by 10% (24v/240v)

The big advantage of this method is that the current flowing through the extra winding is much lower than if you did the same trick with the secondary and you would only need one winding instead of two to reduce a centre tapped secondary voltage.

Bob

[Tabby]

The downside is it takes 100 turns instead of 8, and you'd need t add a new insulation layer over the existing outer low voltage windings. And that will reduce the power handling of the tf slightly.

NT

[Bob Minchin]

I'd certainly prefer to fit 100 turns of light gauge wire than 2 x 8 turns of wire capable of carrying 5 amps. Everyone to their own

Bob

[NoSpam]

It feeds a bridge and the centre tap goes to ground.

[NoSpam]

Toroidal to minimise the leakage fields; I may also need to add a mumetal hat.

I think you have the answer: an un-potted 22-0-22 and remove a few turns

-I should have thought of that. Why is it always so easy to see the solution with hindsight ;-)

[NoSpam]

I think it may be easier to go for a 22-0-22 and remove a few turns from the secondary, but buying the easily available part and modifying is the way to go. Thanks Bob (and everyone elese)

[pete]

Since mains voltage can vary, I would worry about getting the secondary voltage bang on 20V, so either 18V or 22V would be good enough. If the output voltage needs to e exact, there are better ways of doing it.

[Tabby]

Sounds good. But good luck finding one unpotted :) And I trust you're aware of the issue with fixing unpotted ones.

NT

[Tabby]

Have you ever wound transformers? I have, and I'd pick the 8 turns and no glass fibre tape every time.

NT

[NoSpam]

It's running into a high power class D amp so the voltage isn't too critical, I just wanted to get the voltage as high as possible once the tolerances for traffo and mains were applied.

[NoSpam]

During my early post-grad career I was tasked with designing the electronics for a paper tape printer that was going to be in low volume production - I spec'd a special toroidal traffo and learned the hard way about mounting them.

[jkn]

High power? Pah! ;-)

I'm building an amp based on the Tripath TD2500 module at the moment (2x 500W), with a nice 1000VA transformer from Antrim ...

J^n

[Tabby]

Making an amp transformerless is a fun challenge.

NT