Tweaking a toroidal transformer

[Max N]

Hi everyone
I've bought a couple of toroidal transformers. Despite trying to spec them correctly, the output voltage is too high. They are nominally 230v in, 9v out. Even using a variac to drop the input down to 230, I am getting 9.6 out. These are feeding some of Andrew's VCCS heater supplies, and I'm trying to keep the heat dissipated in the supplies from being unnecessarily high.

I've unwound some turns on the secondaries, and the output is now where I want it. But there is now a gap between the secondary wires. I guess this means I am not exploiting all of the flux (because each secondary is only spanning about 330 degrees). Is this an issue? I think that to re-distribute the secondaries I would have to pretty much unwind them and rewind them - is it worth it?

My other question is, where can I get the adhesive wrap to re-wrap the transformers? Is it mylar? I don't think it's a safety issue because the primary is still wrapped, but I want to end up with a tidy job. I can re-use the majority of the tape, because it's non-adhesive, but I could do with some more of the adhesive part

Thanks

Max

[Max N]

To answer my own post, I don't think it matters if I cover the whole 360 degrees of the core - the flux is what the flux is. Think of a twin-bobbin EI, or a Lundahl - large parts of the core do not have any turns around them.....
Unless there is something advantageous with toroidal transformers having windings covering the whole core which I am missing - which is entirely possible.
Less stray flux or something?

[pre65]

My usual "fix" is a couple of 1R 5/7W resistors.

[Nick]

Doubt it will be a problem. Your reference to a EI is a good one. Remember for the future that you can also reduce the voltage by adding extra turns and wiring them out of phase.

[Max N]

Hi Phil
The intention for this amp, if it ends up sounding good enough, is to have it on my desk, in a box, so I can listen to music while working.
Currently it's spread over the bench, just breadboard(s), and it's throwing out too much heat. So I'm looking at where I can reduce the dissipation. probably needs to be a bit here and there. Trying to avoid dropper resistors, heat in transformer windings, dissipation in the heater supplies....

[Max N]

Doubt it will be a problem. Your reference to a EI is a good one. Remember for the future that you can also reduce the voltage by adding extra turns and wiring them out of phase.

Cheers Nick. Yes I forgot about windings out of phase. I'll try that on the other transformer. Should be much easier!

[Nick]

Trying to avoid dropper resistors, heat in transformer windings, dissipation in the heater supplies

And I don't know about you, but wasting energy like that goes against what little engineering instinct I have left.

[Max N]

Yep, and I also feel the same way about gain. I think gain always comes with some added distortion, which is still there after you throw the unneeded gain away in a volume control

[Mike H]

I suggest they're overwound a few percent to compensate for losses, at full load. Did you measure the Voltage at full load?

[Nick]

AFAIK, the rated voltage is at the rated load, so will always be higher at lower loads. How high is given by the regulation figure. And with the stated primary voltage and frequency.

[Max N]

I suggest they're overwound a few percent to compensate for losses, at full load. Did you measure the Voltage at full load?

AFAIK, the rated voltage is at the rated load, so will always be higher at lower loads. How high is given by the regulation figure. And with the stated primary voltage and frequency.

Thanks both. I think I know where I went wrong. I checked out the datasheet for the 30VA range, then ordered the 50VA ones. I think they are pretty close to spec actually. I was slightly disappointed that they didn’t include mounting hardware, but again that’s my fault for not checking.

Anyway, all good now.