Why Are 3 or 4 Secondary Winding Toroidal Transformers Hard To Find?

[Alley_Cat_Jack]

I am trying to design my first linear variable power supply, but I am running into a bit of a problem. I am having trouble finding toroidal transformers with more than 2 secondary windings. The idea being of course to use relays to connect additional windings as voltage is increased.

Does anyone know where to find these transformers. Or is there another way to do this that I have not realised yet?

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[amspire]

I am trying to design my first linear variable power supply, but I am running into a bit of a problem. I am having trouble finding toroidal transformers with more than 2 secondary windings. The idea being of course to use relays to connect additional windings as voltage is increased.

Does anyone know where to find these transformers. Or is there another way to do this that I have not realised yet?

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If there is room to add more windings through the middle of the toroid, you can add your own extra windings. Also I would have thought that two centre-tapped windings are probably common. That gives you 4 different points to tap off which is usually enough. Add an extra low power winding or two to power the control circuit/logic and you are set.

Just as an example, there is something like this:

http://www.digikey.com/product-detail/en/triad-magnetics/VPM36-2780/237-2041-ND/5721760
Two 18V centre tapped windings. That lets you tap off 9v, 18v, 27V and 36V at 2.78A AC.

It looks like this transformer has plenty of room to add a small extra winding to power the regulator circuit. An extra low powered winding doesn't have to go all the way around the transformer - you can just wind it over over 5 or 10 percent of the core, and it will still be fine. Add tape underneath and above to make sure it is well insulated. Finding out the number of turns is easy. Add 10 turns of any wire. Connect the primary to mains and measure the voltage across the 10 turns. Divide the voltage by 10 and you have the volts/turn.

[Alley_Cat_Jack]

Yeah, I forgot to mention I was having difficulty finding toroidal transformers with additional taps as well. Clearly I wasn't looking hard enough, as I was looking on digikey. Either that or they don't exist at the specs I was looking for.

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[Alley_Cat_Jack]

 Also, that transformer you linked to has the center tap on the primary, not the secondary.

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[uncle_bob]

Hi

Some of the transformer outfits sell "extras" at some amazingly low prices on eBay. It's the real commercial transformer, from the real deal manufacturer. They might have an odd label on them from the (likely canceled) contract they were built for. Stacking a few 25VA / $5 transformers is a perfectly acceptable thing to do. It will take up a bit more room than a single 100VA device (that costs $56).

All the usual disclaimers apply ... US dollars not Canadian. Shipping to anywhere not included on any of it.

Bob

[Alley_Cat_Jack]

Yeah, I was wondering if I should go with a pair of transformers and if there is anything to worry about when doing so.

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[Brumby]

For series connection - just the phase.  For parallel connection, there is the phase once again (get that wrong and you'll short out both windings!) but voltages should match or be pretty close.

[amspire]

Also, that transformer you linked to has the center tap on the primary, not the secondary.

You are right. I assumed they meant the secondary but I was wrong.

[Alley_Cat_Jack]

Just found this one.

http://www.antekinc.com/an-5415-500va-15v-transformer/

This has 4 windings on the secondary. It is actually more power than I want, so I'm sure there are smaller ones out there. I was hoping to find 7.5V on each winding at 5A.

[edpalmer42]

Once I added a center tap to the secondary of a toroidal transformer.  It was surprisingly easy.  The secondary was one layer of wire loosely wound on the outside of the transformer.  All I had to do was unwind the plastic strip that insulated the outside of the transformer, scrape off the enamel from the wire and solder on an extra lead.  That might be something you could consider.  Decide what voltage levels you want, buy a transformer that has your maximum voltage, and add your own taps.  I suspect that the secondary would typically be on the outside to reduce the insulation requirements on the outside of the transformer.

[tautech]

Yeah, I was wondering if I should go with a pair of transformers and if there is anything to worry about when doing so.

My first real PSU was built using 2 old 12VAC halogen transformers got for free, series'd or paralleled depending on output levels required and configured as Brumby has mentioned with correct phasing.
LM338K to get 5A capability....easy build.

[uncle_bob]

Yeah, I was wondering if I should go with a pair of transformers and if there is anything to worry about when doing so.

Hi

Quick and simple way to phase check the transformers:

1) Hook up the primary of each of the two transformers in parallel.
2) Hook up the secondary(s) in series.
3) Put a volt meter on the secondary series combo.

You will either get:

A) Secondary A + Secondary B on the multimeter

-or-

B) Secondary A - Secondary B on the meter

As long as you are careful staying away from the line voltage, there isn't much risk in doing this. Once you know the phase, a couple pieces of colored tape is about all you need to preserve that information.

The same process also works for "lining up" the phase on multiple secondary transformers.

One caution:

On multi primary transformers with unknown phase, start with only one primary connected. Be careful with the other primary since it will have line voltage on it. Generally the safest way to sort out unmarked primaries is to drive a secondary with (very) low voltage AC. You then sort them out just like the secondaries above.

Bob

[bill8n95]

I am trying to design my first linear variable power supply, but I am running into a bit of a problem. I am having trouble finding toroidal transformers with more than 2 secondary windings. The idea being of course to use relays to connect additional windings as voltage is increased.

Does anyone know where to find these transformers. Or is there another way to do this that I have not realised yet?

Sent from my HTC One_M8 using Tapatalk

If there is room to add more windings through the middle of the toroid, you can add your own extra windings. Also I would have thought that two centre-tapped windings are probably common. That gives you 4 different points to tap off which is usually enough. Add an extra low power winding or two to power the control circuit/logic and you are set.

Just as an example, there is something like this:

http://www.digikey.com/product-detail/en/triad-magnetics/VPM36-2780/237-2041-ND/5721760
Two 18V centre tapped windings. That lets you tap off 9v, 18v, 27V and 36V at 2.78A AC.

It looks like this transformer has plenty of room to add a small extra winding to power the regulator circuit. An extra low powered winding doesn't have to go all the way around the transformer - you can just wind it over over 5 or 10 percent of the core, and it will still be fine. Add tape underneath and above to make sure it is well insulated. Finding out the number of turns is easy. Add 10 turns of any wire. Connect the primary to mains and measure the voltage across the 10 turns. Divide the voltage by 10 and you have the volts/turn.

I really don't understand how can someone tap to 9V (or 27V) on this transformer. I think that a transformer like this with 2x18V taps (4 wires) or 18V- 0V- 18V configuration (3 wires) , just lets you output 18V , or 36V . (All voltages in RMS)