parallel transformers

[Aunkst]

I have three 60VA iron core transformers out put is 20v-0-20v. can the three xfmrs be put in parallel to get 180VA?

should there be a voltage matching resistor on the outputs before tying them together?
like using a very small, high wattage resistor (.1 ohm 20watt) in series with each out put . that would take care of any voltage differences, while letting the current add together. right?

[Centauri]

Whilst they could be straight paralleled (observing phase!), if you are going to rectify them for DC supply, better might be to use separate rectifiers for each transformer, paralleling +ve and -ve outputs from rectifiers.

Cheers

[Sch3mat1c]

Same problem. You have low impedance outputs, one WILL tend to source all the current drawn. Whether at the AC output or after the bridge. Consider that the slightly higher voltage transformer will charge the filter capacitor to a voltage greater than the other transformers, their diodes won't even switch on!

Current equalizing resistors will work, but you lose regulation. Up to you.

Tim

[Brian Donaldson]

As you load the supply, the higher voltage trans will sag and the others will kick in.

[Aunkst]

OK. so are you saying it is safe? should I use any components between the seperate transformers OR

just run them parallel and keep careful track of the phase.

[quasi]

If the transformers are identical then go ahead and parallel the primaries (watch your phasing).

You can then series or parallel your secondaries to get the voltages you need (watch the phasing).

There will be some mis-match between the transformers but they will sort themselves out (might run a little warmer than normal)

I have never run 3 in parallel but have run 2 often.

I'd be inclined to do this before the rectifier because I'd rather waste a bit of power in the transformer than in the rectifier.

Good Luck

[Brian Donaldson]

Use separate rectifiers and the transformers can't backfeed each other and heat up. You'll loose the same voltage as one rectifier, but the current will be spread ofer 3 bridges instead of 1, but you'll have roughly the same total loss.

[Sch3mat1c]

OMG! I just EXPLAINED why that won't work ... and they go on recommending it.

Tim

[quasi]

If the transformers are identical i.e. if they have exactly the same core and the same number of turns on the primary and secondary then the AC voltages on the secondary will be the same. I doubt more than 1% difference.

In my experience commercially manufactured transformers that are the same probably share current better than transistors and diodes that are the same. In percentage terms I have seen larger variances between diode bridges than I have seen from identical transformers.

[Aunkst]

The transformers are identical. and after thinking about what I know about rectifiers I will tend to agree with Sch3mat1c. also it is done and i intend to run them with a load tonight. I'll let you know what happens.