Unwinding a toroidal, what tape?

[Greolt]

I need to take a few turns off one of the secondary windings on a toroidal transformer to drop the voltage.

I hope that the winding I am interested in is on the outside.

My question is about the clear tape that is the final covering and will need to be replaced.

What should I use? Is it something special? Can it be got?

And are there any other gotchas that I may not have thought of?

[Madclicker]

When I rewound the microwave transformer for my project I just used fiberglass reinforced packing tape.

Is the voltage drop you need small enough to use a few power diodes? Would be a shame to screw up a nice transformer.

[Al_The_Man]

Originally Posted by Greolt I need to take a few turns off one of the secondary windings on a toroidal transformer to drop the voltage. I hope that the winding I am interested in is on the outside. My question is about the clear tape that is the final covering and will need to be replaced. What should I use? Is it something special? Can it be got?

The secondaries are almost always wound last, if it is a dual winding secondary, it is normally wound together (bifilar) so you should take both turns off if using both windings.
The tape can be reused if removed carefully, otherwise any motor winding shop will probabally sell you a few feet of winding tape.
Al.

[Greolt]

Just to clarify Al, I must take the bifilar secondary off as one.

I can not take one off till I get one voltage and take the other till a lesser voltage.

Is this correct?

The toroid I am asking about has a secondary of 30v plus 30v and in addition to this has a minor secondary of 15v plus 15v.

It is this minor winding I would like to reduce to 9v and 6v.

Can I do this ?

[Al_The_Man]

I assume you are going to use the 15v windings as separate supplies?
You can treat them as completely separate, but HOW you use them depends alot on whether they are 15-0-15 i.e. centre tapped windings or two completely separate windings intended to be either series'd or paralleled up.
If the latter, then you have maximum flexibility.
Al.

[Greolt]

Yes this toroid has a main secondary of 30v and 30v. This runs my drivers.

Then it has a minor secondary which is 15v and 15v at much less amps.

These I have connected to a board I made with 5v regulator for logic and separate 12 regulator for cooling fans.

EDIT: Two separate circuits each off it's own winding :END EDIT

Problem is the regulators are disipating too much power and getting very hot. Yes they do have heat sinks.

So I want to reduce one winding to approx 9v for the fans (approx 12v DC) and do away with the reg.

The the other to about 5 or 6v to supply the 5v reg.

Hope that all makes sense.

[Al_The_Man]

You should have no problem, you will probabally find they are wound at about 1.5~2 turns/volt.
Al.

[Greolt]

Thanks for your help Al.

I'll let you know how I get on.

[Greolt]

Well wasn't that fun. NOT!!

Unwound the outer layer of tape only to discover that the major secondary winding (30v) was now exposed.

I would have had to take all that winding off. Heavy copper.

Then take the next level of tape off.

Then change the winding I wanted to alter.

Re wrap the tape.

Rewind the major secondary.

Then at last rewrap the outer tape.

NOT LIKELY!!!!!

It was fiddly enough just rewinding the outer layer of tape. Took me ages.

Plan "B" Fit a small 9v-9v toroid and leave the main one just powering the drives.

[Al_The_Man]

You could also look at the possibility of adding a couple of small gauge windings on top, if you have the room, it would not constitute many turns.
Al.

[Greolt]

That could be a good way to go Al.

Say I needed to wind twenty turns on, do they need to be evenly spaced around the full
diameter of the core. Or wouldn't it matter if they were only on say one quarter.

[Al_The_Man]

For that low current you need it should not make much difference, I often wind them on around a full turn in order for the ends to come out together.
Al.