Variac with selectable isolation-Any recommendations (UK based)? - Page 1
Forum section: electronics / testgear

author: djsb
author_rank: Super Contributor
timestamp: November 02, 2021, 10:38:55 am
content: I'm looking for 240v (UK based) Variac that has selectable isolation. I have had a look at these but they are out of stockhttps://www.aldetronics.co.uk/product-category/variable-isolation-transformers/I am unclear about the advantages/disadvantages of isolation (apart from the household RCD circuit breaker not being activated in case of a fault). And of course, not being electrocuted or causing a fire. Can someone enlighten me and clear up my ignorance regarding the protection of oscilloscope inputs etc. I mostly work with low voltage stuff but I wan to be safe when working with High voltage.My budget is around £250 or so, but safety and reliability is the most important. Any advice welcome. Thanks.

author: HKJ
author_rank: Super Contributor
timestamp: November 02, 2021, 10:57:45 am
content: I use isolated mains all the time when testing to avoid being zapped and also sometimes to test at other voltages. You can test you equipment over the full range of mains voltage or at 110V (with wrong frequency).Another possibility is a AC generator, I have a BK Precision 9801, it has a cleaner sine wave output then the (isolation) Variac (Mains is not a sine wave and has not been for may years), but it do not like SMPS with large input capacitors.

author: Alti
author_rank: Frequent Contributor
timestamp: November 02, 2021, 11:27:35 am
content: You forgot to specify power requirement.I do own a variac but not an isolated version. What I can tell you from my experiences:-some kind of overload protection / current limiter is a must. Don't leave variac with DUT connected unattended. Start from incandescent bulb in series.-inrush of my 10A variac overwhelms B16 circuit breaker. You need some kind of inrush current control here.-cheaper versions have sliding brush which wears out. Get spare brushes together with variac. Roller brush versions work smoother, do not wear out.- make sure your variac is quiet at full load. Buzzing is annoying.- for oscilloscope AC measurement you have to tie one point of isolated variac to PE with thick conductor. There is always going to flow tiny current from primary-secondary coupling. Do not use probe leads for that.-keep all probes ground clips clipped only at single point, to one potential which has to be PE at all times. Do not try to minimize the loop by clipping individual grounds close to probed signals. I did clip grounds of two probes of my scope on both sides of MCB once - stupid mistake. Isolation won't prevent serious currents to circulate in between shields of two probes and when playing with variac it is easy to.

author: THDplusN_bad
author_rank: Regular Contributor
timestamp: November 02, 2021, 11:37:50 am
content: Good Day,just to be clear: The referenced "Variable isolation transformers" have variable output voltages, but do not offer any variable terms of isolationRule #1 is: Float the device under test/DUT, not the oscilloscope.You may also want to read over this excellent App Note from Tektronix: Fundamentals of Floating Measurements and Isolated Input Oscilloscopeshttps://download.tek.com/document/3AW_19134_2_MR_Letter.pdfThis document provides good visuals, too:https://www.tek.com/document/application-note/three-facets-floating-measurement-solutionsAnd as a general introduction into Electrical Safety may I recommend this technical training course from FLUKE:https://www.fluke.com/en-us/learn/online-courses/electrical-measurement-safetyStay safe!Cheers,THDplusN_bad

author: Gyro
author_rank: Super Contributor
timestamp: November 02, 2021, 11:49:37 am
content: Just a note in terms of cost. I would normally expect that a separate isolation transformer and variac would be cheaper (and more flexible?) than an isolated variac - they are more difficult beasts to build. Accidentally burning out a standard variac is less of a loss.Don't rule out ebay - but avoid Chinese 'cheapies' with exaggerated current ratings. [Edit: although the pickings look pretty thin at the moment, you need to bide your time!]Scrub that. Checking around the usual suspects, there's little, if anything, in it compared to the prices you've found.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 02, 2021, 01:05:44 pm
content: One thing I find strange is that the cheaper variacs all seem to have an analog voltmeter that's basically useless, as opposed to an ammeter which is very good to have.

author: Stray Electron
author_rank: Super Contributor
timestamp: November 02, 2021, 02:44:41 pm
content: Quote from: Gyro on November 02, 2021, 11:49:37 amI agree, use a separate isolation transformer and a separate variac. The transformer is useful for many other things than just for use with the variac.A New transformer will probably cost an arm and a leg! But industrial sized and rated isolation transformers are easily found in industrial surplus places in the US and they sell for next to nothing. Despite what many people think, they're worth almost nothing as scrap since most of their weight is iron and the copper windings are usually pressed on and frequently epoxied as well so they're extremely difficult to take the windings off of.I paid $20 for the last isolation transformer that I bought. It weighs about 35 pounds and has dual primaries and dual secondaries so it can be set for 2 to 1 ratio or 1 to 1 ratio. IIRC it's rated a 1 kVA. A LOT of industrial machines in the US have isolation transformers in them, not only to prevent electrical noise form passing into or out of the machine but also so that they can be configured to run on 220 or 440 volts. Most of the industrial transformers around here are rated for 50 or 60 Hz.Look for something like this one on Amazon. <https://www.amazon.com/Acme-Electric-T253009S-Transformer-Isolation/dp/B007V6DLXO/ref=sr_1_17?dchild=1&keywords=480v+transformer&qid=1635864144&qsid=130-8639653-1073613&sr=8-17&sres=B008FM7ZBC%2CB0052MY200%2CB008HOYMOQ%2CB06XD87TQR%2CB07L9KP8R3%2CB07KW58LKR%2CB01ETSEP0O%2CB00JV3J36O%2CB075VN3F9J%2CB005T8TODM%2CB00LZ7BCLS%2CB008HOVDFC%2CB08Z8GB1M6%2CB007V6CGXK%2CB007V6DLXO%2CB01LY4YBJT>

author: TheBay
author_rank: Unknown
timestamp: November 02, 2021, 09:33:16 pm
content: Get a separate isolation transformer, the Screwfix one is fine.

author: Hydron
author_rank: Super Contributor
timestamp: November 03, 2021, 10:40:03 am
content: I DIY'd one (well, did some serious fixing up of someone else's effort) with a separate Variac and transformer. Probably wouldn't have made the effort if I'd seen the OP's link - those prices are pretty good (and the £250 one seems to be in stock now, albeit only 1).If you want to go cheap then eBay will have some second hand stuffFinally as for meters, you can eBay reasonably priced current/voltage/power/pf/VA panel meters that can be modded to read down to 0V (they use a capacitive dropper as a power supply so won't work below about 80V - separate the PSU bit from the measurement section and feed it from the Variac input and it'll happily measure the output down to 0V). If you're lucky they'll fit in the existing cutout!

author: woody
author_rank: Frequent Contributor
timestamp: November 03, 2021, 10:50:51 am
content: Make sure the one you choose has inrush current limiting. The cheaper ones do not have that which leads to circuit breakers doing their thing as I found out the hard way.Paul

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 03, 2021, 11:16:24 am
content: If you only need a small amount of current, a cheap isolation transformer can be made by wiring two identical transformers back-to-back

author: djsb
author_rank: Super Contributor
timestamp: November 03, 2021, 12:03:15 pm
content: Thanks for all of your replies so far. There's a lot to think about before I buy anything. For the moment, I'm going to use my Airlink 500VA 230v to 110v toroidal transformer. I can set the voltage on the HP8563E to 120v, and I'm hoping that I have some degree of safety. I use this transformer with my 110V PACE MBT350 soldering station.I will probably get a Variac AND a separate isolation transformer later on.