Can I buy a transformer without secondary windings

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
Hello; I need a new power transformer for a new personal project; maybe roughly 50VA. It would be best if I could wrap my own secondary windings. One of the windings would be 3-5 volts, but doesn't need much current (to activate a relay). But then I also want at-least one winding with only up to 1.5 volts, but very high current. The primary side would be connected to 100-120v 60hz North American mains power.

I don't think I could get a premade transformer that would work great for this project. But then, getting the parts together and building one from scratch appears to be quite complicated; Finding somewhere to buy the core and determining if it's good for 60hz AC, and making a primary winding that wouldn't short-out the AC power in my house.

It would be great if there was anyone selling unfinished transformers built for North American AC power, but that doesn't come with any secondary windings. I want to custom make the secondary windings, but I'm not paying for a pre-built custom transformer. I would prefer a toroid core (they're more efficient, aren't they?), but it's no big deal.
 

MrChips

Joined Oct 2, 2009
20,884
Tell us your location so we can find a you a toroidal transformer.
Search for a 240V to 3V toroidal transformer. Then you can add the 3-5V winding.
 

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
Tell us your location so we can find a you a toroidal transformer.
Search for a 240V to 3V toroidal transformer. Then you can add the 3-5V winding.
I live in Vancouver, BC, Canada.
If I added extra windings to an existing transformer, would I have to find extra space on the core to wrap them around, or can they just be wound on-top of the existing windings (without any efficiency drawback)?
 

MrChips

Joined Oct 2, 2009
20,884
If you get a dual primary 120VAC to 6VAC toroidal transformer, you can wire the primaries in series as if meant for 240VAC input. This will give you 3VAC output with 120VAC input.

Then you can unwind half the number of turns on the secondary to give 1.5VAC. Don't cut the wire without testing first to confirm that you get the correct voltage under load.

Then add the turns needed to give the 3-5VAC.

Always remember that AC voltages are quoted as RMS values.
If you rectify the AC voltage, remember to multiply by 1.4 and subtract the diode voltage drop (one or two diodes?)

Have you dug out from under the snow from Storm Caly yet?
 

RichardO

Joined May 4, 2013
2,271
It would be a lot easier to just use two transformers. You will have some extra wiring, though. The relay transformer should be small so not much more space will be required.
 
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