Wiring up a variac.

[diif]

I have bought a variac and have wired it up as per instructions given but I'm only getting 90v output wherever the wiper is. Clearly I've wired it up wrong, but I can't see how. Can someone point out what I'm missing please. Thank you.

I was instructed power comes in at the top and out at the bottom, but looking at the cover i'm not so sure this is correct.

[R_J]

Re: Wiring up a variac.

Look at the diagram, you have nothing connected to the wiper screw (3) You don't have the primary connected to screw (1) either. LOOK AT THE DIAGRAM
INPUT: across 2 and 5
OUTPUT: across 2 and 3
Why did you connect to 6 and 5 ?

Wired like this and with 240v input you might get close to 270v at the output if you turn it right up
Also remember this is an auto transformer, not an isolation transformer
The way you had it connected it was acting as a step down auto transformer, 240 in 90 out

[vinceroger69]

Re: Wiring up a variac.

this ebay advert shows one with wiring connected i don't know if its any help
https://www.ebay.co.uk/itm/Variac-3A...p2047675.l2557

[diif]

Re: Wiring up a variac.

Thanks RJ, once you point it out, it's obvious, thank you, re-reading the provided instructions I see where my confusion about the centre pin occurred.
The vendor mentioned it wasn't an isolation transformer also, but I have one of those.

That was the buyer I bought from vinceroger69, it confused me a little as the instructions didn't align with what I was seeing. looking at it now I understand what's going on, it was wired up for 270v before.

[vinceroger69]

Re: Wiring up a variac.

The main thing is you have it figured it out now and can use it

[diif]

Re: Wiring up a variac.

Yeah thanks guys.

[eccerr0r]

Re: Wiring up a variac.

At least you didn't accidentally hook up input power directly to the wiper. That could have some bad consequences depending on where the wiper was rotated to...

[redwire]

Re: Wiring up a variac.

I'd suggest a fuse or circuit breaker for any variac, on the input side. They are worth it.
A 3A fuse would clear at 5A which is better than the mains breaker, to protect the variac.

[eccerr0r]

Re: Wiring up a variac.

Actually variacs should have fuses on the *output* side; though of course this won't help against miswires...

Having one both sides would be best at double the cost

[diif]

Re: Wiring up a variac.

3A fuse in the plug on the input and then any device will have a fuse in its plug, plugged into the socket wired to the variac. Plus it's plugged into an isolation transformer that has a resettable breaker in it. The dedicated ring as all of them do has an RCD fitted.

[redwire]

Re: Wiring up a variac.

I added a $1 AC voltmeter to my variac box. Great to know output voltage without tying up a multimeter. Thanks to stj for the heads-up search eBay 22mm AC Voltmeter

You have to bring out the Vsense wire and connect to variac output, and power the voltmeter from mains. So it's modified to a 3-wire affair.

Big Clive did a good review and teardown:
(Big Clive youtube video) Inside a 22mm digital panel indicator/voltmeter
(Big Clive youtube video) Voltmeter/indicator schematic

[eccerr0r]

Re: Wiring up a variac.

Now I'd need to mod one in order to use it for my puny 0-140V variac...or is there a version for lower than 110V...

[vinceroger69]

Re: Wiring up a variac.

This looked a good mod
https://www.youtube.com/watch?v=2mrlha-uzro