Using a not fully loaded PT to your advantage?

[pjd3]

Hello, I have a silly little question but, it wouldn't be silly if it worked.

Say for instance, you were powering a couple of 6V6's with a power transformer that had two 6.3vac coils one is rated for 3A and the other at 2A. (the HV is 290-0-55-290. With SS rect you should see about 405vdc plate voltage.

But, if you were not using the 6.3vac/3A coil at all, and only using the 2A, you would likely see a fair increase in the voltage of the other coil since it is not loaded anywhere near its full rated current draw.

If by chance this scenario rose both the B+ and the 6.3vac 2A used coil to a still acceptable voltage value, do you think its still wise to use a power transformer that is not fully loaded due to any other unforseen issue? Like say, far worse voltage regulation. I guess thats the thing - I don't know how a power transformer behaves when its not fully loaded, especially when 3A of its rated/expected current is not being drawn.

Thank you for any insight.
Best,
Phil D

[R.G.]

Using a PT at less than its rated output load is perfectly fine (except for it being bigger, heavier, and more expensive than a perfect-rated one would be); it will get less hot and have less voltage drop losses to its outputs because it's made with bigger wires inside.

Transformers can be understood at a simple level by imagining that there is a perfect, ideal transformer hidden inside the physical transformer, and there are resistors and inductors "hiding" the perfect transformer. The resistors have a value equal to the wire's DC resistance, the inductors are the primary inductor in parallel with the primary wires of the "perfect" transformer (but hidden inside the wire resistances) and the leakage inductances in series with the wire resistors.

For PTs, we usually ignore the inductances. They matter in the models of audio signal transformers to a much greater degree.

The perfect transformer can be sniffed out by measuring the no-load (that is, zero current) voltage on the windings. With all of the windings unloaded, apply voltage to the primary, and measure the voltage across each winding, including the primary. The voltage ratios of the hidden inside perfect transformer are very very nearly the ratios of the voltages.

When you load a secondary, current flows in the secondary and in the primary windings. Those currents flow through the primary winding resistance and the secondary winding resistance. So the voltage that the inside perfect transformer primary gets is lowered by the voltage that is lost to the V=I*R loss in the primary wire resistance. This drops the voltage to each perfect-tranformer secondary. Now the voltage and current from each perfect secondary has to go out to the real world through its wire resistance, and that secondary's output voltage is lowered by the V=I*R loss in its resistance. Other secondaries with no load on them have their no-load voltage lowered by the primary resistance voltage loss, but not to the V=I*R loss on their particular wires, because they have no current in them.

Does that help, or did I muddy it up?

[wpaulvogel]

It’s perfectly fine to have unused windings in a transformer. Hammond transformers offers some of their power transformers with both 115 and 125 volt primary. Unfortunately if the input voltage from the power company is really close to 120, this is going to end up high or low voltage output. I’ve taken the unused 5 volt secondary winding and series connected it to the 115 primary winding and corrected the input voltage mismatch. Yes the 5 volt winding is labeled secondary and it’s also really thick wire compared to the primary but it’s wraps of wire in the core. Each coil is independently insulated from each other and can be used/unused as long as you don’t exceed its insulation capacity. I was skeptical about using the winding this way because it’s generally wrapped around the far outside of the coil and the true primary is the first coil but everything works perfectly. After learning more about winding geometry and output transformer interleaving, I concur that the windings are all useful. It’s even possible to hook windings out of phase so to speak and cancel. An example would be to hook the 125 and 5 out of phase to produce the same 120 volt primary.