Variac won't drop below line voltage - Page 1
Forum section: electronics / repair

author: myeeth22
author_rank: Newbie
timestamp: February 28, 2021, 07:18:25 pm
content: Hello,I have a cheap 30A variac that looks like this :https://i.ebayimg.com/images/g/uxYAAOSw6hNgD8d-/s-l1600.jpgUnfortunately, I can't seem to get it working. It has a pretty weird failure mode where when I turn the knob below about line (120V) voltage, the varic rapidly draws more current (10-15A) and eventually flips the breaker. Confusingly, it's stable and fine turned all the way up ~ 130V, where it draws a few mA.I have hot connected to the red input terminal, neutral to black and ground to a chassis screw, with no output load. This is per the instructions and some for a similar model I saw at circuit specialists.Internally, hot is connected to the variac wiper and neutral is connected to the transformer windings and output terminal. There's a switch and a small panel meter attached as well. It's quite clean, and the carbon brush is in good shape. I've also verified there is no extraneous copper between windings.Overall, I'm pretty confused, and I'm guessing I've miss wired it or it came miswired. Either way, I would love some troubleshooting advice!EDIT:Might be the dumbest person in the history of the world. I have it hooked up backwards.

author: gmilliorn
author_rank: Contributor
timestamp: February 28, 2021, 07:28:06 pm
content: "Internally, hot is connected to the variac wiper""Hot" as in the input? If so as you crank it down it will apply more and more power against fewerwindings, which seems to match your description of blowing the circuit breaker. So, it sounds backwards -the wiper is the output.That might also explain why it's blowing the breaker and not the fuse.

author: Stray Electron
author_rank: Super Contributor
timestamp: February 28, 2021, 07:37:49 pm
content: It's not a bug! It's a feature!My guess is that they connected the input wires to the wrong terminals and that they connectedthe power inputto the wiper and to one end of the coil and that when you move the wiper near that end of the coil you are decreasing the number of turns in that circuit until you could possible reach zero turns, i.e. a dead short and that that is why you are seeing a huge current increase.The wiper should be connected to the output and the power input should be across the full length of the coil or possibly at a tap that is usually at about 80% of the full coil.Look up some of the schematics of how to hook up a variac and it should be easy to understand. One of the largest manufacturers of variacs is/was GR (General Radio) and I know that some of their catalogs are online but I'm not sure they contain a schematic or not.

author: CaptDon
author_rank: Super Contributor
timestamp: February 28, 2021, 07:38:09 pm
content: Cheap-assed chinese shit is wired wrong internally!!!!! The wiper is the OUTPUT, the low end of the variac goes to both BLACK terminal posts and the INPUT post should to to the far fixed end away from the low side. I never saw 30 amps go through posts like those!!! 10 amps maybe? Chinese shit guaranteed to burn your house down!!! I have a real 30 amp variac, weighs around 25 pounds!!!

author: gmilliorn
author_rank: Contributor
timestamp: February 28, 2021, 07:50:02 pm
content: Not to mention the use of bare-wire binding posts conveniently placed near power switch!

author: Ian.M
author_rank: Super Contributor
timestamp: February 28, 2021, 07:59:11 pm
content: +1That's unsafe. There is absolutely no provision to ground the metal casing, and the use of binding posts to connect live mains in close proximity to the on/off switch is horrifying.*IF* there are no other major design faults (IMHO there will be), at the very least it would need the switch relocated, a second fuse fitted so both input and output are fused, ground terminals fitted with a bonding strap to the main body, and to the rotor shaft, and a securely fastened insulating cover over the terminals to make them and any normally insulated wires connected to them touch-proof. It will probably also need derating considerably.If you open it up I bet you'll find other dangerous design faults inside it. If in doubt, post photos here and ask!Hopefully its better than the 500VA 'red peril' variacs - When 'tkamiya' tore one down to investigate it:https://www.eevblog.com/forum/beginners/shopping-for-a-variac-shopping/msg3066512/#msg3066512he eventually concluded it was impractical to make it safe and scrapped it:https://www.eevblog.com/forum/beginners/shopping-for-a-variac-shopping/msg3068574/#msg3068574

author: Gyro
author_rank: Super Contributor
timestamp: February 28, 2021, 08:33:27 pm
content: I haven't seen one with terminals on theinputbefore!(output is bad enough).Ditch and blank off all of the terminals, fit a proper mains input lead and an output flying lead socket, and make it 'safe' internally with correct wiring and earthing. If it's beyond your capabilities and comfort zone then you'll need to either get someone experienced to do it or find a way return it for refund or safely recycle it.