can I run 2 torroid in parallel operation?

[ericpeters]

I want to increase, actually double, the size of the transformer in one of my DIY amps.
Can I add another identical transformer and run both parallel.
if so does 2* 500 VA have the same characteristics as one 1000VA?

[quasi]

Provided the transformers are identical then it should be ok. Watch the phasing though.

As far as characteristics go, the 1000va would probably have slightly better regulation at full power than 2 x 500va.

If your amp is stereo, then another (better) alternative would be to run 2 discreet supplies for each channel. This means another bridge and more capacitors but this would be a better setup.

Cheers

[Nelson Pass]

Giving each its own rectifier bridge will minimize
mismatch difficulties.

[dulel]

Do not parallel transformers. Slightly voltage mismatch will cause large currents to flow through transformer windings, due to low winding resistance. This will happen without any transformer load.

[quasi]

I have run identical transformers in parallel a few times before without any problem.

The DC resistance of the secondary is not as relevant as the AC impedance of the secondary. A slight (and I mean slight) voltage difference across the impedance of the other secondary does not result in a large current draw, if of course the phasing is correct. You can try this by putting a small 50hz (60 hz) voltage across the secondary of an unconnected mains transformer and noting the current drawn. You will find it to be very little.

One way to check it would be to measure the current drawn by each primary before and after the paralleling. If the current drawn by each primary stays the same (or negligible change) then no problem.

I doubt there would be more than a few watts wasted. At the end of the day I guess it's all a question of economics.

Cheers & Beers

[jacco vermeulen]

Originally posted by dulel
Do not parallel transformers.

Maybe Sphinx did a bad thing with its Project Sixteen hybrid Class A amplifier.

Rated at 100 watts Class A at any load from 2-4-6-8 ohm.
With 4 toroidal tranformers, relay-switcheable parallel or series secundaries.

The only of their series with a switcheable powersupply, for some the only one that sounded really good.

[dulel]

[QUOTE]Originally posted by quasi
[The DC resistance of the secondary is not as relevant as the AC impedance of the secondary. A slight (and I mean slight) voltage difference across the impedance of the other secondary does not result in a large current draw, if of course the phasing is correct. You can try this by putting a small 50hz (60 hz) voltage across the secondary of an unconnected mains transformer and noting the current drawn. You will find it to be very little.

Sorry but your example is not valid.
If you disconnect primary winding, then you can apply anything up to nominal voltage to its secondary and current will be low.

term "AC impedance" you probably use for impedance of magnetizing inductance. it is usefull for calculating magnetizing current which is only a small part of nominal current, and can be ignored almost always.

once magnetizing current is ignored transformer can be modelled as a pure ressistive element, thus secondary current will be voltage difference divided by the sum of both windings resistancies (DC), and can be large as I said before.

In my opinion only bifilar windings of one transformer can be paralleled safely.

Greetings,

[quasi]

Yes Dulel I agree with you. My analysis was incorrect because as you say it only dealt with an unloaded impedance.

In the real world (well my world anyway) paralleling identical transformers has not caused me any problems. If there are any, they are so small that I have not noticed any performance compromises. I.e. I have not noticed any abnormal temperature rise or execessive primary current being drawn to indicate a loading by one of the secondaries.

Cheers

[paulb]

It seems fine to parallel, I'm doing it now for a Class-A amp I'm building and haven't had any problems. Others in this forum have reported similar success. I am using low-valued resistors (0.47 ohm) in series with each secondary winding to help them share the load.
If you're worried, follow Nelson's advice and use a bridge rectifier per transformer and parallel their outputs. As long as the transformers are the same, they should provide roughly equal currents, but the rectifiers will prevent any reverse current from flowing back into one of the transformers.

[jacco vermeulen]

Originally posted by paulb
If you're worried, follow Nelson's advice

Some are even more worried, and use a rectifier for each secondary winding !