Pathetically dumb question here, how do I wire up this variac?

[Mr Boochie]

Someone correct me if I'm wrong, but if one's line voltage drops well below 120VAC, as mine does (sometimes as low as 113VAC in the summer) wouldn't one *want* the wider range to output voltages great than the input voltage?

Need to remember that you give up some available current to gain voltage. Nothing is free, even with electricity.

 

[zackthedog]

Need to remember that you give up some available current to gain voltage. Nothing is free, even with electricity.

Good point!

 

[gadget73]

You can use it to bump voltage, useful if your mains are abnormally low. Mine is wired to do 0-100% of mains because i don't want to have an oops moment by winding the thing full open and giving something 140 volts instead of 120. I just don't trust myself to not do stupid things, mostly because I find myself having done something stupid way too often.

 

[Watthour]

As was eluded to previously, operating with a DBT of a given wattage at a given load will typically result in a lower device voltage (downstream of the DBT). To compensate, and still have sufficient current limiting for safety, a higher voltage to the lamp is about the only solution.

 

[gadget73]

yeah if you're using it with a dim bulb it probably makes more sense. I don't use mine with a DBT so if I wired it to give 140, I risk it going full stupid if I bump the control.

 

[sddave]

I'd also like to add small panel mount digital volt meter and digital ammeter. Anybody want to spec those units out for me?

Here is one that will work:

https://a.co/d/0dODEq4L

Even though I prefer analog meters here so I can look at the charge deflection contour of the power supply. 7.5A is a lot. But what I would do is run a 4A isolation transformer in front of it and then a 3.5A circuit breaker on the output. Dim bulb option isn't bad, but I would reserve that for troubleshooting and have a way to switch it out. I would also put a 0-200mA meter from neutral on the mains power cord side with a banana jack to stick a probe to and a push button switch to reference the output of the variac temporarily to the mains neutral so I would have an AC leakage tester.

 

[ngoyette]

I use one of these on my variac to monitor the voltage, about $12

LED Digital Voltage Meter DM55-1 AC 110-220V US Plug Voltage Tester Voltmeter​

 

[whoaru99]

Need to remember that you give up some available current to gain voltage. Nothing is free, even with electricity.

It's rated 7.5A output. I'd think it wouldn't really matter to the point of 120V or 140V as long as you don't exceed 7.5A (at least not more than briefly).

As side note 140V @ 7.5A would be more watts than 120V @ 7.5A.