SOLVED: Building a Variac+isolation transformer Combo - Page 1
Forum section: electronics / beginners

author: Axel1973
author_rank: Regular Contributor
timestamp: February 02, 2012, 06:16:14 pm
content: Hi Experts,soon i want to start experiments with transformers, high voltage n stuff. so ill need some beefy AC source that would be adjustable and more safe than working with mains.Therefor i searched around and found two main things that i would need for sure:- Variac / adjustable Transformer- Two chamber isolation transformer for galvanic decoupling- some optional parts like U/I Meters, Fuse n all those stuff of course.Now.. while searching around for parts AND/OR ready made Devices it seems there are at least TWO MAJOR DESIGNS around.Design:1.Mains > insulator transformer > variac > outlet2.Mains > variac > insulation transformer > outletIve found TWO german Companies that build such Devices on their own, all tested, certified, CE, TÜV and what not. So both should be considered "safe designs". Ratings/Specs of both Devices been 500VA, 230V, 2A. But each company uses another design as mentioned above.My Question to you is:Does it realy not matter what comes first?I would have been expecting the safty-transformer (insulator transformer) comeing first. but what do i know....u know.What do you know about it?best regardsAxel

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 02, 2012, 06:26:42 pm
content: I'd put the isolator first, as variacs can typically go to 110% of the line voltage, which may cause saturation of the isolator if put on the output.You also want to be careful with floating supplies if working with HV, as you don't want your supply to be floating at a high voltage if there's a fault. e.g. connect your supply to a microwave transformer, forget to ground the core and make the output arc to earth and you suddenly have 2KV on your variac windings, which could make it arc over internally and at the very least damage the insulation.

author: Axel1973
author_rank: Regular Contributor
timestamp: February 02, 2012, 07:30:00 pm
content: Quote from: mikeselectricstuff on February 02, 2012, 06:26:42 pmIs that "saturation" thing not just a result of frequency? does realy the 110-120% AV-in make the difference on saturations of transformers?beside that, thanks for the advices!73Axel

author: saturation
author_rank: Super Contributor
timestamp: February 02, 2012, 09:38:49 pm
content: I'm not sure if feeding the primary line voltage above 10-20% will cause a marked malfunction from saturation or near saturation [ the isolator may have an operational safety margin built in because, at least in the USA, the worst case acceptable power line fluctuation is +/- 10%, so the maker may exceed that level even more], but its avoidable by having the isolator before the variac.Saturation is both a function of applied voltage [ for a specific design of transformer] and its operating frequency [lower is worse].http://www.opamp-electronics.com/tutorials/core_saturation_2_09_11.htmIf operated above the rated voltage, the transformer could overheat without a load, induce harmonics in the secondary windings and potential cause a failure in the insulation windings. These effects maybe reduced if the draw in secondary windings is high, but this effect cannot be relied on to 'desaturate' the transformer.Quote from: Axel1973 on February 02, 2012, 07:30:00 pm

author: Axel1973
author_rank: Regular Contributor
timestamp: February 03, 2012, 06:48:37 am
content: Quote from: saturation on February 02, 2012, 09:38:49 pmAHHH!! Thanks a lot! That is quiet a good article there! again learned something new.now i understand why more Uin will not allways lead to more Uout because the transformers core "may" not be capable to take on "more magnetic flux" through it, which would just distort the output waveform since once "saturated" (flux to the max of the core) flux will no longer "change", thus there will be no longer induction to the secondary coil that leads to some "breakdown of voltage".well..thats my awefull explaination with my awefull english. but i think i got the point.Thanks again! That was a realy usefull link n tip!73, Axel

author: saturation
author_rank: Super Contributor
timestamp: February 03, 2012, 05:47:19 pm
content: You are welcome, glad it helps. I was in the market for such and after waiting over 1 year for a 'good price' on a premade one, decided to simply get one of these, plug it after a surplus isolation transformer you can get on eBay and assemble a variable one myself. I monitor amps and watts by plugging a killawatt as a meter, and for accuracy check readings against a DMM.https://www.eevblog.com/forum/beginners/pro-and-con-and-info-of-low-vs-high-frequency-transformer/msg85653/#msg85653Quote from: Axel1973 on February 03, 2012, 06:48:37 am

author: Zero999
author_rank: Super Contributor
timestamp: February 06, 2012, 08:37:03 pm
content: Quote from: mikeselectricstuff on February 02, 2012, 06:26:42 pmWhat if a break forms in the Variac's winding?If the isolation transformer is after the Variac the magnetising current taken by it will may reduce the voltage to a safe level but if it's connected after the isolator it won't be the case.In any case, I wouldn't recommending the use and isolation/variac transformer combination as a source of SELV.

author: Axel1973
author_rank: Regular Contributor
timestamp: February 07, 2012, 03:30:19 am
content: VERY INTERESTING POINT!thanks

author: saturation
author_rank: Super Contributor
timestamp: February 08, 2012, 07:20:42 pm
content: A variac is a very old design, and its almost foolproof if not abused. It will last decades at least, many sold on eBay are over 20 years old. They typically powered variable AC supplies.http://sound.westhost.com/articles/variac.htmYou can cause it to malfunction by using it past its amp rating, and thus burn or fuse the brush to the windings. In another thread, I recommend getting these Chinese copies of Staco, and simply using it to 50% of its rated power, and it should be fine. Few electronic bench items require this much power, and many of these variacs are used in power applications.I'm not sure it will qualify as a SELV device, so it should be used knowing its limitations and primarily for lab testing purposes.

author: Unknown
author_rank: Unknown
timestamp: February 08, 2012, 10:08:31 pm
content: Without isolation transformer it definitely wouldn't qualify as SELV (what if someone reversed live and neutral?). With isolation transformer it may, I'm not an expert on these regulations. I imagine the adjustment range up to more than mains voltage may be an issue. It may also depend on the short-circuit current of the isolation transformer. A small transformer that will saturate at high current and with significant copper losses will be safer than a huge isolation transformer that can supply large amounts of current.

author: Zero999
author_rank: Super Contributor
timestamp: February 09, 2012, 12:13:34 pm
content: Quote from: alm on February 08, 2012, 10:08:31 pmOr the neutral could break.QuoteWhat's worse is some Variacs give a high voltage pulse when turned off regardless of the wiper position. I have a small Variac with a set of touch proof terminals and a neon lamp connected to the secondary. When the power is turned off the neon lamp flashes, even if the voltage is set too low to light the neon when the power is on. I haven't measured the voltage transient and I certainly wouldn't touch the output terminals even when using an isolation transformer with the voltage set to a safe level.I imagine the adjustment range up to more than mains voltage may be an issue. It may also depend on the short-circuit current of the isolation transformer. A small transformer that will saturate at high current and with significant copper losses will be safer than a huge isolation transformer that can supply large amounts of current.I'd be tempted to de-rate the isolation transformer to account for the higher than normal mains voltage but not go way overboard, 20% will probably be more than enough.Another option is to buy an isolated variable transformer:http://www.radioelectricindia.com/isolateddw.htm