Parallel power transformers?

paulb

Member
2001-06-01 4:53 pm
Calgary
In another thread, the topic came up of trying to combine the power from two differently-sized transformers (same nominal voltage). "downhere" was asking about it. I originally said to use two separate bridge rectifiers and combine the outputs at the capacitors, but after thinking about it I'm not so sure. I think there's a good chance that one would grab most/all of the load. Perhaps low-valued resistors to help share the load would be a good idea.
Anyone out there have more thoughts on this?
 

paulb

Member
2001-06-01 4:53 pm
Calgary
Okay, I thought it through some more. It should work. Anyone have comments on the following:
A transformer's voltage rating is at its full load (at least the Hammond ones are). If a transformer can be considered a linear voltage source, it can be modelled as a voltage source and a source resistance.
Higher power transformers will obviously have a lower source resistance, because they can supply more current. This means their voltage drops less under load as compared to a lower power one. And this, in turn, means they will also have a lower "no-load" voltage (Hammond used to spec this, but they don't seem to any more).
So if two transformers are run through separate bridge rectifiers to a common set of caps, the higher voltage source will supply most of the current. At light loads, this will be the smaller transformer; however, because it droops more rapidly than the bigger transformer at some point the latter will start to supply current as well. The rectifiers plus the internal resistance of the transformers will balance the loads.
So it should work fine, but it NEEDS to have separate rectifier bridges to work properly. Sound correct?
 
Well it's not really going to work. The two transformer must have equal voltage at anytime in any load. If voltage is deifferent the one one with the highest voltage will suply current to the lowest. All this power is lost and the total power output must be subsracted from this current between the two transformer.

Also the smallest one is going to fail at full power.
 
I've done experiments with a variable power supply and a battery that are coupled together with diodes. If their voltages are very close, but not quite identical, both will supply power to the load - both diodes are on the knee of their conduction curve. My thinking was that the source resistances of the transformers would even this out.
But your opinion intrigues me enough I'm going to try simulating it.
The higher voltage transformer cannot supply current to the lowest, because they are isolated by the diodes.
 

paulb

Member
2001-06-01 4:53 pm
Calgary
I think you can ONLY if the two transformers have almost identical voltage outputs at full load. If they are part of the same line from the same manufacturer, this is probably true. If they are different, it probably won't work, one will supply almost all the power.
downhere, I suggest you just use the larger transformer and just put up with a little less output power. Much simpler.