Parallel transformers and bridge

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I'm about to finish an amp for my XLS's, and want to use two toroids of 500VA each that I have already. There are two XLS per channel, and there will be one power supply for both stereo channels.
Since this is for a sub amp, used only up to 110Hz, not only the capacitors value is important but the transformer size. So I'm thinking about paralleling the two little monsters and this is what I came up with.

Has anyone build a similar thing?
 

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Ummm...

Why on Earth, Mars or Uranus would you need sixteen diodes? I'm highly confused on your reasoning here.

Aside from that, paralleling transformers is a bad idea, even windings inside a single transformer. The slightest mismatch in voltage will cause a mild short-circuit current to flow between the windings themselves. If nothing else, the guaranteed different winding resistance will cause uneven current sharing.

Constant-voltage elements do not like to parallel because they each have their own idea of what that voltage is. Now series, perfect.

Tim
 
If your trafos are identical, ie same manufacturer and model/batch I would try to wire them in parallei. You can easily check how well they work together by wiring them in parallei and checking primary current without any load. If it is much more than 2 times higher than single trafo prim current then they are not good match.

Then there is also current sharing problem, but if they are identical models winding resistances are probably better than 10% match. Not so big deal if another runs at 95% load and another 105%.
 
mzzj said:
If your trafos are identical, ie same manufacturer and model/batch I would try to wire them in parallei. You can easily check how well they work together by wiring them in parallei and checking primary current without any load. If it is much more than 2 times higher than single trafo prim current then they are not good match.
I'll agree here but using individual bridges is 100% fool proof, still if the transformers are the same type they most likely have identical windings when it comes to number of turns.
 
Thanks for your reply

The toroids I have are identical, at least they are the same RS code and were bought at the same time. So they will be more identical than different.

If I understood correctly from you all, this will not get fire out of the pcb, but I still have the last minute question... would be more the advantage of the of the lower output impedance of the transformers in parallel, than the possible problems I will have to solve (grounding loops) in the active system I'm finishing (because of the single PS)?

Does anyone has a different suggestion?
 
Toroids are almost allways bifilar wound, so close matching between secondaries may be assumed. Otherwise, they would tend to saturate even when operated in the usual way.

For the sake of simplicity, I would just connect in paralell the secondaries of each toroid and use each transformer to supply one rail through a diode bridge.

Also, there is a trick to further reduce PSU voltage drop and ripple in stereo LF arrangements. This is achieved by inverting the input signal of one of the channels and reversing the polarity of the load. As a conequence of that, both channels will seldom be drawing current from the same supply rail at the same time and PSU efficiency will be improved.
 
Re: Thanks for your reply

Raka said:
The toroids I have are identical, at least they are the same RS code and were bought at the same time. So they will be more identical than different.


Does anyone has a different suggestion?


Watch out! toroids may look be identical....but could be made by different vendors using same RS label. Check off load secondary winding voltages first. If the primary has taps, DON'T parallel them. Why you might ask ? A manufacturer might round up the taps to next integer turn and with thinner wire can overheat if connected in parallel with another tranny.
There's nothing wrong in parallel operation so long the voltages are identical.

Hang on.......Mains toroid transformers are NOT wound Bifiliar.....no interwinding screen ...eh ? Manufacturer wouldn't get isolation and creepage approval. Some are wound so badly that the leakage inductance is way worse than EI types.

richj
 
Yes, mine are from Nuvotem, and the reference from RS is 223-8897. Will be easy now to check if for this application is best to use one single PS (with 500+500VA) or to use two separate PS (500VA each).
The amplifier power is not going to be very high, as the voltage of the transformers is just 25V, but I'm more concerned about the current delivery needed for the paralleled XLS10.
 
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