Toroidal transformer modification

[sureshot]

I was thinking of asking this in an existing thread, but thought it might be useful for other applications if it's a doable option. So I won't go it to the other thread details, other than I need a separate power source of at least 20mA to power a panel meter. My thoughts have turned to the idea of adding some suitable wire windings to my toroidal mains transformer. Then winding teflon tape over these windings. I haven't tried this yet, but think it might be a possibility for the 4.5 - 6 Volts I need. At around 100mA. Then current limit that with a 100mA fuse for safety. So does this sound a viable option for additional source of power for this low current application ? I would have asked this question in the panel meter thread I started. But feel it would be lost in there, as if it's viable, it could be useful to others that might need a separate low power source.
Thanks for reading, any thoughts appreciated.

[capt bullshot]

Yes, that'll do the job.

[Gyro]

Yes, perfectly doable. The primary winding is almost universally buried under the existing secondary, so there are no particular safety isolation issues. I've done it several times for auxiliary windings.

Make sure you use normal insulated transformer wire, never bare uninsulated (obvious I know, but some have tried - and failed!). You'll want to do a test winding to establish the turns per volt and then wind accordingly, spreading turns evenly around to core. Teflon / Kapton / Polyester tape should be fine.

P.S. Try to find a little bobbin for the wire, small enough to fit through the center hole, trying to thread long lengths of un-tethered wire through the hole is a pain.

[JS]

Yes you could, wibd a few turns, let's say 10, and measure the voltage. Do the math to find out how many turns you need.

The caveat here is the original VA rating of the transformer should be remain unchanged, so from the load you had available you need to take this new out. You are only talking about 0.6W, you could be using about 1VA for that. So if you had a 100VA transformer and using 50VA from it you are totally fine, but if you had a 10VA transformer already tight to the requirements you can have some problems, the first one to appear would be lower voltage than expected in all windings I guess.

JS

[sureshot]

Thank you for your replys. That's great, it means I don't have to add a separate small transformer. The toroidal transformer is 300 VA at 15 Volts twin secondary windings, the primarys are 2 x 115 Volts windings. Save a lot of hassle not needing a separate power source.the bobbin idea is great, saves threading and dragging wire through the core.
Thanks again much appreciated.

[SeanB]

Probably you will need 20 turns to get 6VAC, which will be fine to drive a 7106 clone chip and meter.

[Zero999]

Yes, it will work. Note that the voltage will drop, when loaded, so when you conduct your experiment, do so with the desired load current and scale the numbers accordingly. The leakage inductance is likely to be higher, than the factory wound secondary, because there will be more layers of insulation and windings between the primary and the new secondary winding.

[T3sl4co1l]

All the above.  Just FYI: definitely not Teflon(R) tape.  As in, what... plumbing thread seal tape?  I don't remember there being an adhesive kind.  It's extremely thin, and porous -- no good for electrical use!  PVC electrical tape would actually be better, but much better still to simply get the right stuff: polyester or polyimide.

Tim

[sureshot]

All noted thanks. I will probably use kapton tape, only thought teflon as it's a good insulation. Awesome Matt uses it in his transformer MOT winding. This only needs to be from 4.5 - 6 Volts to power the meter, specs says it draws around 20mA current. So 100mA at that voltage range will hopefully be fine. I will fuse it as well.
Thanks again for all your replys. 

[ArthurDent]

I've wound secondary windings on toroids by first winding at least the length of wire I need on a long thin board, even stiff cardboard, or a small ruler, and feed that through the donut hole of the toroid.

The primary is probably the inside winding but check and make sure. If it isn't, make sure you have enough insulating tape that won't deform like regular electrical plastic tape to insulate your new winding.

[Brumby]

As per all the above.

I've done it myself.  Worked a treat.

[james_s]

Polyester tape is what's normally used, it's very slightly stretchy so you can wind it nice and tight and it ends up almost like shrink wrap.

[sureshot]

Ever find one solution leads to another problem...
One of the units I'm doing didn't account for extra winding size, added windings to this particular unit will foul the 10 turn potentiometer on the front panel. Lol. Totally unrealated to this topic, forward thinking that's whats needed in future. Allow some tolerances around inside a project enclosure.

[eblc1388]

Are you saying the base of the 10 turn potentiometer is currently touching the transformer surface?

If so, why can't you  just avoid that portion of the core and wind the new coil evenly on the remaining area?

[sureshot]

Yes i could do that, mark it with a sharpie and avoid that small area, it is only tiny after all.  Thanks for the idea.