Dirty big variac

[Daemon]

Does anyone have any knowledge of variacs?

I was wondering if they are stable enough to use to drive my 24VAC lighting.

I can get a big sucker 240v input(which we have here in New Zealand) and 0-270v output. 1.6KVA

Could I use this set o 24v as a transformer?

Apart from the big inrush current when you turn it on are there any other downsides to them?

[prof]

Two major drawbacks.

As the variac is not isolated the lights will be at mains potential and need to be treated accordingly.

Variacs do not like to be run at very low percentage settings. You will not be able to get 1.6kVA of load at 24V out of it. (At full load, 1600VA / 240V = 6.6A drawn by the variac). As the variac is just a single winding you basically can only get about the same current out as you put in. i.e. 24V x 6.6A = 160VA. And not the 66A you think you should. The variac will try to supply this load but will only end up letting the magic smoke out.

Prof.

[Daemon]

Thanks Prof

I knew there had to be a good reason they weren't being used.

Many thanks for the reply
Steve

[piesrule58]

I have 2 big variacs that I used last year without a problem. 240v input - 0v to 270v output. I had one set to 24v and one set to 36v. Ran approx 10,000 lights from each one. Worked great.

[mrpackethead]

I have 2 big variacs that I used last year without a problem. 240v input - 0v to 270v output. I had one set to 24v and one set to 36v. Ran approx 10,000 lights from each one. Worked great.

Sure it works, but you need to treat the lights as if they are at mains potential, because between the lights and Earth ( ie you ) they are at mains potential!

[piesrule58]

Sure it works, but you need to treat the lights as if they are at mains potential, because between the lights and Earth ( ie you ) they are at mains potential!

Sorry, I am not an electrician and I do not understand what you mean by mains potential.
BTW, my son is an electrician. He got the variacs for me and set them up. So I assume that they are safe to use.

[mrpackethead]

Sorry, I am not an electrician and I do not understand what you mean by mains potential.
BTW, my son is an electrician. He got the variacs for me and set them up. So I assume that they are safe to use.

THe voltage across teh load ( the lamps ) may be 24V, but the voltage between phase and earth could be at 240V ( mains potential ).. So if you were to come into contact with one of the wires, you may be in for a shocking suprize!

A variac only has one winding, with multiple taps. Your electrican son should know better than to do this. Are you sure you've got a variac?

[sjwilson122]

Wouldn't an isolated variac eliminate this problem? I used one for years when I was an electronics repair tech. I think they changed the name now to variable transformer but years ago the were called isolated variacs.

[ErnieHorning]

VARIAC is a trade name. It originally stood for ‘Vary AC’. ‘Variable Transformer’ is just another name for the same thing.

There are isolated VARIAC’s but they’re not very common. You could use a 1:1 isolation transformer on the input or output but they’re just as expensive as the VARIAC.

It is possible to use two transformers back to back, i.e. tie the secondaries together and use the primaries as input and output. The higher the secondary voltage, the higher the current will be for a given size. This will at least provide isolation.

Personally when I use a VARIAC while testing, I just know not to be touching anything that I wouldn’t if it was plugged in directly.

[mrpackethead]

VARIAC is a trade name. It originally stood for ‘Vary AC’. ‘Variable Transformer’ is just another name for the same thing.

Check out "autotransformer" at wikipedia for a good explaination.