Parallel transformers and bridge

[Raka]

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?

[Sch3mat1c]

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

[Raka]

How would you parallel two power supply transformers?

PD : I'm replying from the moon...

[Sch3mat1c]

Simulpost will ya? While editing no less

Tim

[Raka]

Wouldn't the bridges stop the currents to other secundaries?

[Sch3mat1c]

Yes, but then one will supply all the current and the other might as well not exist.

Plus at worst you only need two FWBs following the "prevent backflow" logic.

Tim

[mzzj]

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%.

[Sch3mat1c]

If empirical testing shows they aren't antisocial (i.e. they play well together ) then you can go ahead.

Make sure you match the same windings. If not bifilar (simultaneously) wound, one will be "higher" in the wind and as such longer, having more resistance. Voltage matching is your first priority though.

Tim

[peranders]

Originally posted by mzzj
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.

[Raka]

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?