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Variac input voltage?

Posted: Mon May 31, 2021 10:02 pm
by mpward
I'm in the market for a budget variable transformer for my bench.

All of the chinese variacs you find on eBay or Amazon are rated at 110v input, but typically mains power is 120v. Does matching the mains voltage to the input voltage spec matter all that much?

My gut tells me it does somewhat, but that I can easily compensate for it: since we're dealing with transformer windings, I'm pretty sure that 10v difference will affect the output voltage I get, at a minimum creating a difference between the dial setting and the actual voltage out. Can I use a Kill A Watt or something similar to monitor the output voltage to get what I really want instead of relying on the dial settings of the variac?

Is there something else my newbie mind is missing?



Re: Variac input voltage?

Posted: Wed Jun 02, 2021 12:34 am
by audiofanman
My take on this is the 110V version give you the ability to dial past 120.
Useful when using load limiting (Not that a few volts less probably matters anyway)

My setup is main house -> Vairac -> Isolation transformer than the load limiter if needed.
When dialing the variac up, I watch the output voltage of whatever the device under test is plugged into...
Because of the 175w bulb I'm using (Couldn't find anything small that's a higher wattage) I noticed more than a few volts missing. Since the variac can get me well past 120V (135-140v if I remember correctly)
120 is easy to reach...

I would guess (at least for the unit I have) the only downside is at its lowest setting I get a small voltage. If I remember correctly, some where around 10V. I believe my signal generator goes to 12v, so if I needed say 6v, I can switch over to it.

Re: Variac input voltage?

Posted: Wed Jun 02, 2021 9:56 am
by Hugo
If it is rated at 110VAC and you give it 120VAC, you will get to 120VAC out of it (OK it is Chinesium so in real life you can expect everything).
My guess is that the 110VAC rating is a cheeky way to get out of all warranty claims.

PS, Some varicacs get beyond the input voltage. My vintage Philips variac is rated at 220/240VAC input and goes to 270VAC output.

Re: Variac input voltage?

Posted: Wed Jun 02, 2021 2:25 pm
by mpward
Thanks to both of you for the info, that helps. I'll keep an eye out for a quality variac at a reasonable price, and when the time comes to pull the trigger we'll see what I end up with.


Re: Variac input voltage?

Posted: Wed Jun 02, 2021 8:00 pm
by Tables_and_Tubes
I purchased the little 5A blue variac from Circuit Specialists. The digital voltage readout is fairly close (although I monitor the voltage separately) and the unit performs well. The price is right and if you are a first time buyer, there is a discount. ... splay.html