View Full Version : transformer ????

04-21-2012, 11:09 PM
My "bad" transformer was 110/220/240v input with 24/110 output and rated at 100VA.

I've seen some that are 110v input, 24v output rated at 5 amps. Would this be equivalent to 120VA?

Also some are rated in watts. What's the formula for converting?

Would there be any problem with installing a transformer that was rated at more than 100VA? As much as 2-300VA? Lower than 100VA may not have the power to activate/hold all of the relays?


04-22-2012, 12:48 AM
VA=Volts x Amps = Watts. (more or less and I'm sure someone will pick nits)
Ok to goto a higher VA but if you go lower it will burn up.

04-22-2012, 01:25 AM
VA is a measure of "power" uncorrected for power factor. It is V x A without regard to phase. With power factors close to unity, VA would be pretty close to Power, with VA being the more conservative.

Larger VA rating is OK.

04-22-2012, 08:36 AM
Thanks for the replies,

J Tiers
04-22-2012, 09:38 AM
What was the "110V" output used for?

You have two possibilities, depending.

1) If the "110V" was simply used to run a standard type motor or light, etc, and the transformer was to adapt it to 240v etc, you can likely run it on 120V direct. Must be a pretty small motor.

2) if that output was for some other electrical controls, or if it runs a motor through electronic controls, it may actually need to be isolated.

Finding a transformer with that combination of voltage will not be as easy, although they probably exist. (they used to, anyway)

04-22-2012, 11:28 AM
My "bad" transformer was 110/220/240v input with 24/110 output and rated at 100VA.

I've seen some that are 110v input, 24v output rated at 5 amps. Would this be equivalent to 120VA?

100va is not that much, if you don't need the 110 secondary, just the 24v for relays, try Home Depot for bell or furnace transformers, I just picked a 24v up from there.

04-22-2012, 11:52 AM
This sounds as if it was a specialty transformer made specifically for this application. With the exception of having both a 24 volt and a 110 volt output this appears to be a job for a standard control transformer. These are commonly used in industrial control systems to match control system voltage requirements to the available equipment supply voltages. Typically reducing 440/220 volts down to lower voltages. Various sizes are usually available at industrial electrial supply companys - definitely not the same as typical home supply stores. Ask your local electricans if you don't know where one is.

In your case you may need to replace your transformer with 2 separate transformers. one supplying 24 volts and one for 110 volts. (I asume that the 110 volt output is required for isolation purposes, otherwise you could just feed the related components directly with 110 volts.)

For your purposes V x A = watts. Just not quite true if you are taking/teaching an EE course or designing/building large or high tech equipment.

Note also that transformers work by ratio. For example a 440 volt to 110 volt transformer will happily reduce 110 volts to 24 volts (nominal values), just don't feed the higher voltage into a lower rated input or you will let the majic smoke out..

04-22-2012, 12:56 PM
Sorry, I should have been more specific.

In my case the transformer was only used on the 24v output to supply power for the relays. There was no wiring to the 110v output.

Maybe a 40 VA for AC/Furnace application will be plenty for 3 small relays and possibly a light.

No need in spending $50+ if a $10 will serve the purpose.