Toroidal Transformer Interference
 

Hi all,

I'm currently working on a project that requires the mounting of two toroids close together (one 500VA 35-0-35 toroid and another 20VA 15-15 toroid). How much care should I take to make sure they aren't too close together? How great a problem would mounting one on top of another be?

Steve.

50/60Hz toroids won't interact no matter how you mount them. The only thing to avoid is to create a shorted turn through the center hole with the mounting screw and the case. That screw should not touch the top cover (at least in a conductive metal case).

Thanks for the response!

I was just curious as I remember reading an article about the influence mounting crossover inductors close together can have on their values/effects. Think it was something Troels wrote?

Most crossover inductors work in a different way because they are either air cored or ferrite-drum cored, and thus their magnetic path is open. This involves producing strong magnetic fields around the inductor, picking up any external field produced by anything close, and interacting with ferromagnetic or conductive surfaces if they are close enough.

50/60Hz toroids are closed magnetic circuits, magnetic leakage and pickup is very low.

Thanks again!

That makes sense, considering their different structure and function.

I really wich I'd continued my physics study. In retrospect, it's much more interesting than physiology and biochem. :P

Toroids do have a field. You probably don't need to worry about coupling between your transformers. But if you have circuitry with gain nearby--especially circuitry that has any significant "loop area" (i.e. traces running around a PC board)--you can get some AC line freqency noise in your audio circuits.

National Semi had to shield the toroids in their reference amp using the LM4702 to get the THD+N of the amp down to the true levels of the circuit. And Douglas Self discusses it often being a problem with several designs he's worked on. Self suggests leaving extra lead length (at least temporarily) so you can rotate the transformer for minimum hum. I would expect the field from a toroid to be fairly uniform but apparently you can find "nulls" by rotating it. I personally haven't tried it as there's usually a fair distance between the toroids and gain circuitry in my amps.

It is; leakage flux is however larger at the points where the leads come out of the toroid.

Have fun, Hannes

Re: Toroidal Transformer Interference
 

Use only one trafo with the two windings. one of 500VA other of 20VA...

If i could find one or have one wound cheaply, believe me I would! Hehe.

I do not know how in australia, but generally companies that manufacture Trafo have versions(audio) with additional winding 12V+12V.