Hello, I've had this variac floating around my lab for a while and I'd like to get it working again, but I need some help getting it wired up.
General Radio type V20HM 8 amp variac
- How do I test it to make sure it works?
- How do I wire it up so that it works again?
Here is the link to the album of images for this variac:
http://imgur.com/a/0fl8iAny links to a guide on how to test variacs and/or how to wire them would be most helpful.
I plan on using it with US 120V input.
I am assuming here that the variac is fed with a 220V input.
What makes this complicated is that we are not sure which is the 'cold' end on the variac where there is no output.
So...................
1) Wire the incoming neutral to 4 and the incoming live to 1. Neutral out is 4 and live out is 3
Test the variac. If you get 0V with the dial at 0 and 220V with the dial at 220 then we are good to go. If the dial appears to work the wrong way around proceed to step 2.
2) Wire the incoming neutral to 2 and the incoming live to 5. Neutral out is 2 and live out is 3.
Finally, please remember that it's a bad idea to work on mains powered devices on your own.
There is more than enough information (i.e. the diagrams on the terminal plate) in the photographs to be able to configure this thing. If you cannot interpret the information it may be better to leave it alone - confidence is not the same as competence, and this is not an area to be guessing in - if in doubt, DON'T
The device is symmetrical so it can be wired for either clockwise or anticlockwise operation. If you get the direction wrong you can either flip the wiring, or (usually) flip the dial. Pin 3 is always the output.
You can check (no mains power) for basic continuity with an ohm meter, and check for insulation integrity to ground - but none of this guarantees safe operation when on line or under load.
Figures 6 and 9 show the connection for 115V operation.
http://jamminpower.com/PDF/GR%20V20%20Variac.pdfBe safe.
There really is no cold end on a variac IMO. It's a single winding with a tap at each end and usually one about about 80% of the way from one of those to the other. The fourth terminal is attached to a wiper that runs the entire length of the coil. Apply power at the two end terminals and pick off power between terminal 1 and the wiper terminal. Usually terminal 1 connected to neutral so it could be considered cold but since variacs almost almost always have a single winding, everything beyond terminal one has voltage on it. In this configuration the output will vary between 0 and 100% of what ever the input voltage is. But you can also connect the line side to terminal 2 instead and that will boast the output to between 0 and (input/80%) volts or about 130 VAC in the US (with 115 VAC input).
This one is the same except it has multiple taps. However connect neutral to terminal 2 and line voltage to terminal 4 and then take the output from terminal 2 and the wiper (terminal 3) and you will be able to vary the output from 0 to 100% of the input. Note this variac is made for up to 270 volts across the entire coil but you can certainly run it at a lower, safer voltage. If you want to experiment with it. Run it off of a isolation transformer with a lower voltage AC output such as 12 VAC. The voltages involved will be a lot safer to work with. This one has other taps but I wouldn't bother with them until you understand variacs better.
The biggest fault with a variac is that it is a single winding and basically, everything is connected to the AC line. Never forget that! If your neutral terminal or the winding were to suddenly open you could have full line voltage at the output regardless of the wiper setting. That's why I suggest powering it from an isolation transformer while tinkering with it.
PS ignore the numbers of the dial. Those only agree with the output when it's wired in one specific manner and with a specific voltage input. I much prefer to to think of the output in terms of percentage of the input voltage and as a function of the percentage of wiper rotation. But even that can be altered by connecting it differently. Variacs are amazingly versatile if you understand them!