Variac having problems at 0 voltage? - Page 1
Forum section: electronics / beginners

author: primegaps
author_rank: Contributor
timestamp: December 05, 2017, 02:47:01 pm
content: I've got a few variacs that i picked up. Some are in very rough shape and have the main cord cut, so presumably there is something wrong with these ones, and someone is trying to prevent them from being plugged in again.But i also have a few that work fine. One seems to work fine over the full range, but when I turn it all the way to zero, it suddenly jumps up to around 40VAC. It's range is 0-140VAC, and it has the full range. If im really careful about where I set the dial to, i get get it to around 0, all the way up to around 145VAC, but if i turn it all the way down, it suddenly jumps up to around 40VAC.What might be the problem? I took the casing off to have a look at the coil, but i cannot see anything wrong with it. When it is turned all the way down towards the zero side, the brush actually comes off the coil, and shouldnt be making any contact at all. I dont see why there should be any voltage here, it looks to me like its totally off the coil. So when i have it all the way down, it sits at around 40VAC, and i think it is in this position. I can't actually tell, since the case isnt off when i operate it, but it has the same feel, i can feel the brush click off the coil, off that little "step" and it suddenly jumps up to ~40VAC.This is the brush off the coil, it seems to sit at ~40VAC in this position:But when I adjust it very carefully to the very start of the coil,(i think it would be in this position, again, i cant tell because the housing is on when i operate it) i can get it to read a very low voltage, pretty much 0VAC, if im careful, and watching my meter, and go really slow as to not let it click off the coil. Maybe somewhat similar to this position, JUST at the start of the coil.What might be the problem here? How is it even getting voltage when it appears the brush is not making contact with the coil at all? I thought I could solve this just by adding a mechanical stop such that the brush can never come off the coil, but that seems like a pretty dodgy fix. Any advice or insight is greatly appreciated!Thank you for reading!

author: Andy Watson
author_rank: Super Contributor
timestamp: December 05, 2017, 02:56:10 pm
content: Are you loading the output with anything other than the meter?

author: mikerj
author_rank: Super Contributor
timestamp: December 05, 2017, 02:58:23 pm
content: Do you have a load on the output of the variac? If not then the ~40v on the output when the wiper falls off the end of the track is just from capacitive coupling, and nothing to worry about. Try adding a resistive load (e.g. an incandescent lamp) and then measure the voltage.

author: primegaps
author_rank: Contributor
timestamp: December 05, 2017, 03:19:10 pm
content: There is indeed no load asides from the meter on the variac.Could you please explain the nature of this capacitive coupling in the context of this transformer? I thought maybe it was charge stored in the coil or something, but the voltage does not dissipate, and is present even upon just plugging in the unit and turning it on, when the knob is all the way down, off the coil.None of my other 4 working variacs seem to do this, they go from near 0 to 145VAC, and dont seem to have this capacitive jump.Very interesting! I will place a resistive load across the transformer and try this test again.I find it very odd that none of my other units do this. So if i were to attach a load and turn it all the way down, where it reads ~40VAC with no load asides from the meter, there should be no power going to the load? Sometimes it reads even as high as ~60VAC there.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 05, 2017, 03:35:14 pm
content: Quote from: primegaps on December 05, 2017, 03:19:10 pmDo your other variacs have the same "off" position? All my variacs maintain contact with the coil winding at 0V output.The other thing you might want to check is that the 0V end of the variac winding is common to the neutral or low pole of your mains wiring. It is possible for the variac to function with the live/hot wire as common - this is possibly a dangerous condition since you would expect the output to be dead when the control is at 0V - although I would never touch a circuit relying on just the variac for isolation.

author: Vtile
author_rank: Super Contributor
timestamp: December 05, 2017, 03:49:32 pm
content: Take your DMM put it to the AC voltage settings as it is used. Attach the another lead to any known grounded object ie. metal case of grounded PSU etc. and now touch the another lead of the meter, should show some voltage, maybe only 10 to 15 volts. That is capacitive coupling in action. It is also known as ghost voltage/current in electrician lingo and only seen with high impedance meters (not old passive analog meters), the current flowing in this way istypicallysome micro amps max. This is the same phenomenon which is used in the old type glimmer light mains test pens and Non-Contact Voltage Testers. This is also the reason some (ie. Daves Brymen) do have the low impedance mode, with low impedance you do notseethis ghost voltage (while the small power flow is still there).

author: eKretz
author_rank: Frequent Contributor
timestamp: December 05, 2017, 04:08:40 pm
content: The others are correct, this is what most would consider "phantom voltage." If you place any kind of load across the output it will fall back to zero. Most Variacs I've encountered don't let the brush come off the low side of the coil. There may be a way to adjust the resting position alignment if you poke around a bit.

author: primegaps
author_rank: Contributor
timestamp: December 05, 2017, 07:48:08 pm
content: I have resolved the issue. When I first took apart the variac in question, i didn't notice anything fall out, but later, i saw a small aluminum ring, with one side countersunk, and wondered where it had come from. I have all sorts of projects and parts going on, so it didnt occur to me right away that it had anything to do with the variac. So I threw it away and didnt really think anything of it.So i opened on one of my variacs i know to be in working order, for a point of comparison. I noticed that on the troublesome variac, the thing that limited the rotation of the knob was just a small button head screw (8-32), but on the working variac, there was a small aluminum ring with a countersunk screw holding it down. A-ha! i went to my garbage bin to find this mystery ring I had tossed earlier. Someone in the past had taken out this ring along with the fastener, and replaced it with a simple button head screw for some unknown reason, but had left the aluminum ring just flapping around in the housing. That seems horrible, probably could have shorted something. So on the working variac, the ring limited the angle of rotation just slightly more, enough to not let the brush come off the coil.I had to cut an 8-32 counter sunk brass screw (bolt) i had to length, and i was able to fasten down the missing ring. Now it behaves like normal, and does not have this phantom voltage.

author: james_s
author_rank: Super Contributor
timestamp: December 05, 2017, 08:08:04 pm
content: Any time you have two conductors near each other you have a capacitor. Capacitors pass AC voltage so if you have one metal object connected to the AC line and a nearby metal object electrically floating then there will be some degree of coupling and a sensitive meter will register a voltage between that floating element and ground. That's roughly what's going on here when the brush moves onto the insulated pad it is floating and picks up voltage from the other windings. The available current is negligible though as the value of the capacitor it forms is only a few picofarads.

author: primegaps
author_rank: Contributor
timestamp: December 06, 2017, 05:45:52 am
content: Thank you for that explanation, that really cleared it up for me.