multiple toroids


2004-01-04 8:41 pm
I need + and - 30V for my amp. The transformer must be 300VA rated.

This requires 22Vx2 before rectification which is unavailable domestically short of a custom winding.

I can find 185VA 2x22V toroids and think it should be possible to use two of these with a single rectifier to give my required +/-30V.
But I am unsure how to wire this up.
Stupid question time........

What happens if it comes out higher, say 33 V?????

If were in the postion of having to use 2 toroids, I would use one for one rail.......the other one for the other rail. Use separate bridges for each supply, tie the commons (from the caps!) together at a star ground. I would advise that over one bridge any day, in any configuration. Unless you can not tolerate the extra voltage drop.

Should not be a problem, since most toroids that you are likley to buy have dual secondaries not tied together.

Yeah, but the only way you can get constant voltage sources to behave is to connect them in series (and BTW, still only one FWB is needed). That's twice the voltage he needs.

If, between the four windings you have, two pair of them match (most likely pairs on the same toroid, but in case those aren't well matched, try against the other toroid's windings as well), you can connect them in parallel. The voltages need to be very well matched, otherwise 1. the higher voltage winding will force a current through the lower winding (the voltages subtract, putting the difference across the winding resistance), effectively a shorted turn of some magnitude; 2. the higher voltage winding will also source more current than the other, since to draw any positive current from the lower voltage winding, you must first subtract the current being forced *into* it by the higher winding. At this point, the higher winding has approx. double the idle current flowing, while the lower winding has null current!

For loads several times greater than the shorting current, current draw is effectively shared. There will always be some shorting current consumed whenever you parallel the best matched transformers; the matter is determining if the magnitude is large enough to be of concern.

This is something that has been already discussed. The secondary windings of toroids are bifilar wound so their turn count and wire lenght is always matched. There is nothing bad in connecting them in paralell, with the right polarity of course, they are also intended to be used this way.

On the other hand, in SMPS it's a common practice to use multifilar windings and connect all the magnet wires in paralell, even when their lengths are not exactly the same. It works fine as long as all the wires have the same turn count and similar leakage inductance, length mismatching only makes current sharing between wires a bit worse, but heating tends to compensate for it anyway.

What might not work always properly is to paralell windings of different transformers since turn counts are not guaranteed to be exactly the same.