Problems with grounding of variac - Page 1
Forum section: electronics / projects

author: mn1247
author_rank: Newbie
timestamp: June 18, 2014, 02:43:08 pm
content: I have a question about a variac. I just bough a new variac, with isolation, to do testing on a switched-mode power supply. The variac isolation seems okay, but the output connections are not what I had expected. It appears that the output "hot" is tied to the brush (no problem), and the output neutral is tied to one end of the output coil (so far so good). However, the ground is tied to the other end of the output coil (as well as to input ground). So, in operation, there is 115VAC between output neutral and the common ground!Is it supposed to be this way? It's making it difficult to test my curcuit with scope referenced to ground. I feel like the second end of the coil should have been left unconnected, rather than tied to ground.Thoughts on this?ThanksEric

author: nctnico
author_rank: Super Contributor
timestamp: June 18, 2014, 02:54:03 pm
content: Use differential probes. Way better than an isolation transformer. As soon as you clip the ground of a regular probe somewhere in the circuit, the circuit is grounded again. If you don't have differential probes then use the add/substract mode on your scope.

author: retrolefty
author_rank: Super Contributor
timestamp: June 18, 2014, 03:33:40 pm
content: What is the 'input hot' wired to? Maybe a schematic would make what you have more clear.

author: mn1247
author_rank: Newbie
timestamp: June 18, 2014, 04:23:19 pm
content: Sorry for the lack of schematic - I don't have a schematic for the variac itself. The variac is an enclosed, commercial unit that plugs into mains. So, the inputs are just my home hot, neutral, and ground (from a standard wall recepticle). The input and output ground connections are tied together within the variac (I checked with a multimeter). The hot and neutral inputs and outputs are isolated internally, presumably using an independent coil on the core.This is a link to the unit...http://www.amazon.com/gp/product/B006N2ZGUS/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1

author: What_NZ
author_rank: Regular Contributor
timestamp: June 19, 2014, 11:03:09 am
content: Quote from: mn1247 on June 18, 2014, 02:43:08 pmFrom what you are describing it doesn't sound right. You can think of it as a Variac connected to the output of an Isolating transformer, yes the Grounds will be common (safety) The output Phase and Neutral of the Variac will be totally isolated from the Phase and Neutral inputs on the isolating transformer and ground. so are you sure it is a real 115AC and not leakage through some filter capacitor. If you have one you could always put another isolating transformer on the Variac output. Just watch the total load and losses are not exceeded.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 19, 2014, 11:15:27 am
content: QuoteNo that is wrong, how is that isolated? You're right to be worried.What did you measure? the voltage when it is turned on or the resistance with everything disconnected? Only measuring the resistance will tell you if they are isolated.A mega is best for this.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2014, 09:18:16 pm
content: Quote from: nctnico on June 18, 2014, 02:54:03 pmWith off-line circuits if I use differential probing, then I use my isolation transformer anyway for fear of a ground lead slipping. Of course that will not save anything if there are two ground leads and just one slips. Using isolation *and* attaching the ground leads usually yields more accurate high frequency measurements.

author: C
author_rank: Super Contributor
timestamp: June 22, 2014, 12:49:56 am
content: Here is a schematic and description of an variac, with isolationhttp://www.allaboutcircuits.com/vol_2/chpt_9/5.html

author: What_NZ
author_rank: Regular Contributor
timestamp: June 22, 2014, 02:08:03 am
content: Yeah that is all it should be. I think his AC voltage is coming from some sort of Filter.To the OP. I see your thread appears to be only related to SMPS design?Just a side note for SMPS repair - although there was one available I never used a variac much for SMPS repair and never had or needed a differential probe. Didn't use the Light Bulb trick either. An Isolating Transformer, Variable DC power supply, a Multimeter (or two) and an Oscilloscope were a must though.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 22, 2014, 03:15:55 am
content: Quote from: mn1247 on June 18, 2014, 02:43:08 pmYou are correct that makes ABSOLUTELY no sense at all. It is so incredibly WRONG that I sincerely wonder whether you traced the circuit out properly.If you were to plug that into almost anything it would either electrocute the user or blow the breaker or both.It seems incredible (in the original sense of the word!) that it could have been wired like. that!It isn't clear how "isolating" the circuit from green-wire safety ground is to your advantage? No matter WHAT you are working on!

author: Orpheus
author_rank: Regular Contributor
timestamp: June 22, 2014, 05:41:29 pm
content: Do you have a isolated variable transformer or an isolated autotransformer?Technically, "Variacs" are autotransformers, but the term is used generically for both

author: What_NZ
author_rank: Regular Contributor
timestamp: June 22, 2014, 10:35:23 pm
content: An Autotransformer will just have a fixed tap(s) hence why technically Variacs are Variable Autotransformers.The 'isolation' will either be as a part of the Variable Autotransformer as shown in C's post #7 or a separate isolation transformer connected before the variac.Either way they can be considered as 'functionally' the same i.e total isolation between the Phase In / Phase Out and Neutral In / Neutral Out, the Phase/Neutral Outputs will also be isolated from Earth. All Earth's In and Out will be connected together and to the Chassis of the Variac.Considering the price and size of the Isolated Variac shown in the link the OP provided you would have to believe it is of the type shown in C's #7 post.

author: saturation
author_rank: Super Contributor
timestamp: June 23, 2014, 04:51:17 pm
content: These Chinese variacs are copies or variations of the Staco design but with added isolation. For electrical use, I'd be very cautious and skeptical about the quality, assembly and materials to actually use it safely without a tear down and check of all connections, regardless what the manufacturer says. After its been determined its indeed isolated, I personally would derate the electrical specs 50%.Consider its rated at 1500VA, at 100Vac. But the US power line is 117-120Vac, so it really should be specified and fused to 12 or 13A, not 15A. Variacs output up to 130-140Vac from 120Vac line, so the rating on the unit is misleading: it must be rated to the maximum output of the device, so its really about 11A. So derating it at 50% for a fuse at 8A is reasonable since one cannot know how well the windings and wiper arm will hold up at 1500VA.A Staco non-isolated version cost about 2x more, near $800, 4x more if isolated, but in return you are guaranteed the performance its specified for. Even if it failed, its CSA rated for safety. Note its 2.1KVA rated for the same amperage as the Chinese version. True Staco models will last a lifetime. For isolation, I'd get a used medical grade isolation transformer; most for sale on eBay tend to be in very good shape given the requirements of hospitals.