I Want A Variac - Page 1
Forum section: electronics / beginners

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 08, 2020, 11:12:43 am
content: Yeah, I do. Not sure why, I just do.Partly because you can get those big red ones. Shiny. And they have built-in meters.But I don't want the 10 amp version, I want the 20 amp version. Because 20 is MORE than 10. And more is better.Maybe because I just bought a set of 50 amp, 1000 volt bridge rectifiers, which I don't really need. And with a variac I can vary the input to the bridge rectifiers. Cool.Aahhh...I just figured how I can justify it. If I didn't have a variac I'd need to buy hundreds of different ratio transformers to do the same thing, and that would be INSANELY expensive. So if I buy a variac I'm saving myself TONS of $$$.Sweet.

author: Zero999
author_rank: Super Contributor
timestamp: May 08, 2020, 11:31:34 am
content: Can you get 120V input, 0V to 240V output variacs? That might be more useful for testing foreign equipment or seeing what would happen if an American device is connected to the European mains.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 08, 2020, 11:34:05 am
content: Quote from: Zero999 on May 08, 2020, 11:31:34 amHell, I'm scared enough of 120v, I don't need none of that 240v stuff

author: digsys
author_rank: Unknown
timestamp: May 08, 2020, 11:40:18 am
content: ISTRONGLYadvise that you also budget for an Isolation transformer to connect to the Variac Input ! If you're running stuff direct of the output, it may save you.It can go on the output, but specs will vary with voltage. I've done it this way for many years.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 08, 2020, 11:55:32 am
content: Quote from: digsys on May 08, 2020, 11:40:18 amThere's a law against isolation transformers on my bench. I use differential probes on my scope instead. And I don't want to be isolated from earth ground because it's there for a reason, to provide a path to ground instead of thru my body.

author: Mp3
author_rank: Frequent Contributor
timestamp: May 08, 2020, 12:12:25 pm
content: Quote from: digsys on May 08, 2020, 11:40:18 amI don't really see how this is any better than using an isolated surge protector or UPS as your mains source

author: AVGresponding
author_rank: Super Contributor
timestamp: May 08, 2020, 12:18:05 pm
content: This thread at first made me chuckle, then latterly shake my head...They're a lot cheaper than they used to be, even 5KVA ones are less than a couple of hundred of your yankee bucks nowadays, and if you're gonna suck 20 amps, you might want to oversize it a bit, as they can get quite warm...I shook my head at the suggestion that an isolation transformer might save you. That isn't their function, on a test bench.It's to remove common voltage references between test gear and the DUT, to reduce the chance of magic smoke escaping.The best way to keep safe when working with mains voltages, is toa. Know what you're doing (sounds like you very much do)b. Be careful, but confident (sweat and nerves will both make it more dangerous)c. Use a GFCI/RCD/RCBO protected supply

author: madires
author_rank: Super Contributor
timestamp: May 08, 2020, 12:28:04 pm
content: This forum hosts a lot of threads about isolation transformers vs. RCDs. Both have their use cases. No need to start another endless discussion.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 08, 2020, 12:28:58 pm
content: Quote from: engrguy42 on May 08, 2020, 11:12:43 amSince the stock market is so depressed, you can invest all the money you save. Then when the market rebounds, and you sell, your shiny new Variac will be free or maybe even turn a profit for you.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 08, 2020, 12:30:02 pm
content: Quote from: madires on May 08, 2020, 12:28:04 pmActually I started it in the General Chat as a bit of a goof (y'know, along the lines of "post pictures of your cat" and "what did you buy today"), but apparently the boss thought it belonged here...

author: Zero999
author_rank: Super Contributor
timestamp: May 08, 2020, 12:37:22 pm
content: Quote from: engrguy42 on May 08, 2020, 11:55:32 amI completely agree with using differential probes.The reason why the mains is connected to earth is not to protect against shock from everyday mains voltages. The neutral is earthed to prevent it from floating at a much higher voltage than 120V or 240V, due to a lightning strike, or someone connecting a high voltage autotransformer to it and the secondary side shorting to earth. Unfortunately it provides a nice return path for current through your body, so makes it more dangerous under normal circumstances. This can be mitigated using an isolating transformer to break the return path or an RCD/GFCI to disconnect the power, if the imbalance between the phase and neutral current exceeds a certain threshold.The whole isolation transformer vs GFCI/RCD debate is an interesting one, with no simple answer. A GFCI/RCD safely cuts the power if too much current leaks to earth, but there can be problems with nuisance tripping and regular testing is required. Isolation transformers are passive and have no moving parts to fail, thus should be more robust, reliable, than an RCD/GFCI and of course there isn't an issue with nuisance tripping. On the other hand, if any part of the secondary circuit does become earthed, then the protection is lost and you're back to square one. There are circuits/devices to warn the user if the secondary becomes earthed referenced which can help to mitigate that risk, but the user needs to be paying attention to them.I consider an isolation transformer more for the added flexibility of having the variable voltage source floating, rather than purely for protection against shock. Use whichever form of protection you feel most comfortable with, as long as you're aware of its shortcomings.

author: engrguy42
author_rank: Frequent Contributor
timestamp: May 08, 2020, 12:45:01 pm
content: Geez, you guys are no fun....

author: ArthurDent
author_rank: Super Contributor
timestamp: May 08, 2020, 12:53:22 pm
content: Quote from: engrguy42 on May 08, 2020, 12:45:01 pmWait until we've had a few beers......

author: MarkL
author_rank: Unknown
timestamp: May 08, 2020, 03:03:48 pm
content: Quote from: engrguy42 on May 08, 2020, 11:12:43 amI would stay away from higher power no-name versions. I bought one of those 20A, red, shiny, no-name, made-in-China versions and it caught on fire at 15A with flames shooting out the vents. And the fuse didn't blow. A postmortem showed that there was no way the wiper assembly could handle 20A.You can get used ones for a little more from recognizable names like Staco. A much safer and more reliable route, IMO.

author: Gregg
author_rank: Super Contributor
timestamp: May 08, 2020, 04:41:49 pm
content: You should strive for a setup like Photoinduction had on his YouTube channel

author: Mp3
author_rank: Frequent Contributor
timestamp: May 08, 2020, 08:33:45 pm
content: It seems like this thread went over a lot of peoples heads here.

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2020, 03:57:46 am
content: Quote from: Zero999 on May 08, 2020, 11:31:34 amIn theory it is possible but I have never seen one. Most 120 volt variacs include a tap on the input so the output range can be 0 to 120 volts or 0 to 140 volts. I assume 240 volt variacs include a similar tap.

author: BravoV
author_rank: Super Contributor
timestamp: May 09, 2020, 04:58:29 am
content: Quote from: David Hess on May 09, 2020, 03:57:46 amExample mine, dual inputs 110 and 220 volts, output adjustable from 0 to 240 volts.Japaneese call it Slide Regulator.

author: tkamiya
author_rank: Super Contributor
timestamp: May 11, 2020, 08:49:06 pm
content: Please - no "Japs."We used to call it a slide-ack. (that was 40 years ago) It could have been a brand name. I had one back when I lived there. Mine was industrial powder coated brown.I have one here in US, too. I don't know why. I don't use it much. I like it because it's red. Few things to think about when you are getting one now, especially one from China. If you want 10A capacity, buy 20A rated one. De-rate by 50%. Based on size and weight - after all, it's a transformer, there is no way stated rating is correct.Also, analog meter that came with mine were awfully inaccurate as in not-even-close. I always use DVM along with mine.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 11, 2020, 10:00:17 pm
content: Quote from: engrguy42 on May 08, 2020, 12:45:01 pmMate, I'm here if you need to get the party fired up

author: jpanhalt
author_rank: Super Contributor
timestamp: May 11, 2020, 10:48:14 pm
content: Well, I keep a very old Variac branded autotransformer (black,probably late 1940's vintage, 2-pin plug) on my bench. My very old Weller 25 W and 80 W irons are plugged into it when a I want a lot of heat, but I don't want to melt copper.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 11, 2020, 10:56:57 pm
content: Quote from: MarkL on May 08, 2020, 03:03:48 pmWise words indeedALLmost of those blood colored varyzaps are half baked quality with suss wiring and assembly, flimsy, wonky carbon brush and dial coupling assembliesIf you do score one for a slab of loo paper money, open it and sort it out,and de-rate it by at least 60% if under constant load > 10 amps scribbled on the wishful thinking label = 4 amps real world crossed fingers useyes they are that crappy, seen a few, don't want to see any moreand have an analogue meter on it to see if any carbon brush skipping or arcingoccurs along the entire travel whilst under load and no loadUsed units are definitely the way to go, built for WORK, but need to be checked and serviced, carbon brush assemblies and brush to coil tension/presure? in top shape,and correctly wired, so no polarity stuff ups or wrong input tap etcwith careful use and in/out FUSING and or optional frugal current limiting on the output, a decent brand or refurbished old school unit won't disappointfwiw -if wired correctly- true isolation transformers (which won't work with RCD/GFCIs)and interference suppression wired isolation transformers (which 100% should/better work with RCD/GFCIs)have their place on the bench and hospital wards.For starters they limit the entire street transformer dumping on your DUT and or you,and isolate the bonded -LIVE Neutral- and Earth combo, embedded in the very ground that you are standing on,as well as most exposed metal housed devices around you.Yes, even with an IT you still run the same zappage risks, but not cop the full wrath of the street transformer that aims to please the consumer'sfinalfreak demandDifferential probes are nice, but way overpriced for a TooHungLow made gadget, that's got the same or less parts cost, contruction, and R+D than most of the ever changing CAT litter rated cheap meters they dump on electronics puntersDifferential probes are not truly isolated anyway afaict, just a fancy xxx probe, which if badly handled and the cable abused, good luck getting parts and schematics for that fried expensive DSO,..6 weeks out of warranty