Control transformer in reverse

[Sierrasparky]

I was approached to use a doorbell type 120v to 24v transformer in reverse to power a 120 volt trouble light fed from a 24v control circuit.
I said it was hokey and not proper listed use of the transformer.

 

[drcampbell]

There's nothing inherently wrong with using a transformer in reverse, although derating is sometimes required.

Three questions come to mind:
Most doorbell transformers are only capable of ~20 VA. Will that be sufficient?
Does the 24-volt circuit have enough spare capacity?
How is it that you have a 24-volt AC circuit available, but not a 120-volt circuit?

 

[Sierrasparky]

There's nothing inherently wrong with using a transformer in reverse, although derating is sometimes required.

Three questions come to mind:
Most doorbell transformers are only capable of ~20 VA. Will that be sufficient?
Does the 24-volt circuit have enough spare capacity?
How is it that you have a 24-volt AC circuit available, but not a 120-volt circuit?

Yes, runs only .05ma.

Reason for request : Trouble light wire was CL2 Lo VO wire run in same conduit as other Power limited cable. The suggestion is to be able to re-use the status/strobe light.

 

[sameguy]

Look at Acme xformers web site they show you how to size machine tool xfmrs in reverse 1kv and less I believe.

 

[Little Bill]

Look at Acme xformers web site they show you how to size machine tool xfmrs in reverse 1kv and less I believe.

"Acme", ahh the company Wil-E Coyote uses!

 

[sameguy]

"Acme", ahh the company Wil-E Coyote uses!

Well yes, they patented "The Acme Hole" maybe you have seen or used them!

 

[Besoeker]

Yes, runs only .05ma.

That's just 0.006VA at 120V
Is that really enough for your application?

 

[Sierrasparky]

That's just 0.006VA at 120V
Is that really enough for your application?

.5ma is the device at 120v

 

[Besoeker]

.5ma is the device at 120v

That's still only 0.06VA

 

[Sierrasparky]

That's still only 0.06VA

The typical trans is rated at 24v 20va

 

[gar]

180907-1650 EDT

Is 6 milliwatts of light enough to be useful?

.

 

[LarryFine]

An HVAC around 50-60 va transformer would be a better choice.

 

[Besoeker]

180907-1650 EDT

Is 6 milliwatts of light enough to be useful?

.

It's 0.06 VA based on his revised current.
But I'm with you. Is that useful for a light of any sort?

 

[GoldDigger]

It's 0.06 VA based on his revised current.
But I'm with you. Is that useful for a light of any sort?

Here in the US we use "trouble light" in two different senses:
1. A temporary with a cage shield and/or reflector light on a long cord used as a portable worklight, or
2. An indicator light showing that there is a problem.
.06VA would not do much for 1, but might be fine for 2.
Also, .5A instead of .5mA gives a much more believable result for 1.

Sent from my XT1585 using Tapatalk

 

[Sierrasparky]

How are you calculating the available current in a reverse transformer? I don't get what you folks get.

 

[Besoeker]

The typical trans is rated at 24v 20va

My question is what can you really do with 0.06VA?

 

[Sierrasparky]

My question is what can you really do with 0.06VA?

I can get a larger 24v transformer.
I don't see how you get .06VA from a 20va trans,

 

[Besoeker]

I can get a larger 24v transformer.
I don't see how you get .06VA from a 20va trans,

Simple.
Your quoted

.5ma is the device at 120v

 

[Sierrasparky]

I get .16 from a 20 va.

 

[LarryFine]

Correctamundo! a = va/v = 20/120 = 0.166 @ 120v = 0.833a @ 24v. (presuming 100% efficiency).

If you think about it, all we acare about is the power; a 20va transformer will just light up a 20w bulb.

I found several 24v/75va HVAC transformers in the $35 range. Should obviously give you about 75w.