Safe, good value, reliable variable AC transformer recommendations? - Page 1
Forum section: electronics / beginners

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: July 01, 2017, 06:55:55 pm
content: HelloI need a good, relatively inexpensive way of getting variable AC voltages up to line (240VAC). I have looked into variacs, amazon electronic "SCR" chinesium transformers, and of course buying a range of different value "set" transformers.To be honest, variacs look hugely expensive for one that wont kill you, and the SCR ones all look like theydefinitely will kill you(especially if doing anything requiring decent power).Am I missing something obvious with an easy way to get variable AC?Can I chain together "set value" AC transformers in series and get their voltage to add?Cheers chaps and ladyfolkses

author: Benta
author_rank: Super Contributor
timestamp: July 01, 2017, 07:19:33 pm
content: Yep, isolation transformer followed by a standard variac does the trick.

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: July 02, 2017, 11:29:11 pm
content: Thanks guys!Those both look rather pricey though, any ideas of where or how to get something reasonable, say a max of £50 for both? Or am I off in pink unicorn land again?

author: tpowell1830
author_rank: Frequent Contributor
timestamp: July 03, 2017, 01:32:12 am
content: I would look on ebay for a used, name brand Variac (not chinesium). These units are designed safe and will accomplish variable AC. Also you could post a WTB on this forum.

author: digsys
author_rank: Unknown
timestamp: July 03, 2017, 01:52:29 am
content: Quote from: BentaThere aresomesituations where we put the isolation transformerafterthe variac. Usually where you are working / monitoring on the primary side.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 03, 2017, 06:04:09 am
content: Quote from: digsys on July 03, 2017, 01:52:29 amDoesn't that place a strain on the isolation transformer if designed to be run 240 in / 240 out, and the variac dialed up anywhere from 0 to 240v?Or the available current is less/more?

author: digsys
author_rank: Unknown
timestamp: July 03, 2017, 10:23:23 am
content: Quote from: Electro DetectiveNot at all. You can't run them at a higher voltage due to saturation / depending on where on the BH curve they were designed, normally ~1.1TCurrent is mostly determined by wire gauge, which doesn't change. And in the simplest description - AC constantly goes from 0V to 240V :-)When testing P/Supplies, we sweep all transformers with a variac, same thing. For the first ~ 0-30V, linearity may be off a bit, and efficiency down, but that's all.

author: Ian.M
author_rank: Super Contributor
timestamp: July 03, 2017, 11:21:48 am
content: Caution:Its common for Variacs to have a tap between 80% and 90% of the winding that can be used for the line feed. If wired this way, the output voltage can exceed the inpuut voltage by a significant amount (useful for testing devices under simulated high line conditions), but if you use an isolating transformer after the Variac, the extra voltage risks saturating it.N.B. Variacs have a rated current that must not be exceeded at any wiper position. If you crank one down to 10% of input, you dont get ten times more output current capability. If you need lots of current at a low voltage you need to use a fixed step-down transformer, with a secondary voltage a bit higher than the maximum desired, after the Variac to boost the current available. Again, don't use the tap on the Variac for the feed!Quote from: SpannersToSparks on July 02, 2017, 11:29:11 pmCloud Cuckoo land for sure. 20 years ago, you could get a reasonable bare isolating transformer for about half that and the cheapest used bare Variacs came in around that price. By the time you have added a suitable case, and a circuit breaker, you'll be lucky to be able to put anything together for a budget of double that even buying dodgy sold as seen transformers off EBAY. The price also rises steeply if you need more than a couple of Amps.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 03, 2017, 11:29:13 pm
content: Thanks guys, good to know from people that have gone thereSpannersToSparks,isolation transformers do come up in the used market and you can snag one at a good price, sometimes with good amperage.Stepdown 240 to 120 ones are more common and fairly cheap, and apparently two of these wired -back to back- will give you a 240 > 120 > 120 >240 arrangement,but they MUST be isolation types, not autotransformers!FYI: I haven't had the need to do it, but apparently others have due to necessity, that's all I know.OTOH Variacs have never been cheap, and buying a used one 'as is' or untested can end up being a money pit and pita,especially if the carbon wiper brushes are worn, uneven, or the top of the surface they mate with is bad news too,winding breaks, partial shorts, insulation damage,or a hard to spot cracked housing frame that supports the selector dial to add more woesMine is an old one and got lucky as it just needed some elbow grease, polarity check and a good testingIf you don't need a lot of wattage, a new cheapie variac with a warranty may fit the bill, 500 va I thinkbut I recommend to 'take it apart'first and inspect and properly secure any -Made In Ch!na- oopsiesGood luck

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 03, 2017, 11:49:08 pm
content: Opper and stet prices have really skyrocketed.Power line transformers, which emplolarge amounts of both are now prohibitively expensive.

author: edavid
author_rank: Super Contributor
timestamp: July 04, 2017, 12:03:54 am
content: Quote from: schmitt trigger on July 03, 2017, 11:49:08 pmNope, still cheap on US eBay:http://www.ebay.com/itm/SUPERIOR-0-140V-3-75A-VARIAC-/371992654336http://www.ebay.com/itm/Vintage-RCA-TV-ISOTAP-WP-25A-275VA-Variable-Isolation-Transformer-/263008582860I realize this doesn't help the OP - perhaps he should try a radio rally or boot sale

author: Electro Detective
author_rank: Super Contributor
timestamp: July 04, 2017, 01:34:55 am
content: OP SpannersToSparks won't get anything cheap living in kangaroo central either

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 04, 2017, 03:10:58 pm
content: edavid;I agree with you, the way to go is purchase an used device.

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: July 04, 2017, 06:15:29 pm
content: Thanks for all the great advice guys, as always EEV Blog is on fire (in a good way)What do you make of this one?https://translate.googleusercontent.com/translate_c?depth=1&hl=en&prev=search&rurl=translate.google.co.uk&sl=es&sp=nmt4&u=http://www.satkit.com/b2c/index.php%3Fpage%3Dpp_producto.php%26md%3D0%26ref%3DVARIAC-TDGC2-2K&usg=ALkJrhiRQWoOOP52AkZtqCSnGcexU4ZCBQThey do a range of Amperage models from 1 to 20, am thinking 8 amps will be more than enough for what I want as most of the stuff I am looking at has a 5 amp fuse in the plug anyway?Then its just a search for an isolation transformer.... is this needed because there is effectively no isolation with the variacs "one long coil"?Cheers chaps, you're all awesomeSpanners

author: Zero999
author_rank: Super Contributor
timestamp: July 04, 2017, 06:19:37 pm
content: Quote from: SpannersToSparks on July 02, 2017, 11:29:11 pmWhat power rating do you require?I doubt £50 would even cover the cost of raw materials for a reasonable size transformer.I had a quick search and this seems like a bargain:http://www.ebay.co.uk/itm/Variable-Isolation-Transformer-Safety-Single-Phase-like-Variac-Isolated-240V-/122425852209

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: July 04, 2017, 06:21:01 pm
content: Quote from: Electro Detective on July 04, 2017, 01:34:55 amI wish I lived in Kangaroo centralI am actually in Dirty Pigeon Alley, sipping tea and avoiding the pollen(actually nah I'm fine, finally found something that treats the hayfever!)

author: Gyro
author_rank: Super Contributor
timestamp: July 04, 2017, 06:35:18 pm
content: Quote from: SpannersToSparks on July 04, 2017, 06:15:29 pmHi Spanners,That does look alarmingly cheap - Eur54 for an 8A 2kVA Variac.There has to be a catch, the one I can see is that its input and outputs are on Terminals rather than the mains connectors that you might expect. That might not go down at all well from the safety perspective (for instance, if you accidentally mis-wired it or shorted your input connections). It's still a very big variac for the price, maybe with some 'recontruction'....That site seems to have very low prices generally, for instance an induction heated 100W solder station - also Eur54! Not one I've seen before.Maybe someone here has some direct experience of this seller?Chris

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: July 04, 2017, 06:44:42 pm
content: Quote from: Hero999 on July 04, 2017, 06:19:37 pmGood question. The driver to buy it is to try and nurse an old boiler motor into health, mainly as a learning experiment. However I keep having a need for a nice variable AC source so I figured I would get one.The boiler motor isnt going to draw much more than 3A as thats what the fuse is on the whole thing, so I am guessing 240 x 3 x 0.8 = 576 Watts? The unit is 2KVA which according to my hastily googled maths is, at that power factor, 1.6kW?

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: July 04, 2017, 06:46:54 pm
content: Quote from: Gyro on July 04, 2017, 06:35:18 pmI see what you mean, yeah could be fun with croc clips floating about and the like. I think I would go with crimped ends and a bit of heat shrink, and not switch anything on til I'm 100%?

author: Ian.M
author_rank: Super Contributor
timestamp: July 04, 2017, 06:50:16 pm
content: Yes, their 8A model looks possible:http://www.satkit.com/b2c/index.php?page=pp_producto.php&md=0&ref=VARIAC-TDGC2-2KDerated to 5A with a 5A circuit breaker on the output it should be OK.I don't like the meter or the terminals - its probably best to treat it as a Variac kit and fit a normal UK lead and socket, and reasonable quality panel meters for voltage and current. Some metal-bashing to make up a new front panel box for it will be required.One thing to be cautious of is the input voltage - nominal 220V in may put it a bit too close to saturation at 240V in, especially if you are in a rural area and if the mains supply runs on the high side except when nearby agricultural users drag it it down by running heavy loads. Worst case: you may have to add a 12V 100VA transformer wired as an autotransformer to buck the 240V UK mains to a more euro-friendly 228V.Incidentally its more versatile to have the Variac and isolating transformer as separate units. Its not uncommon to need to soak-test a device at a higher or lower voltage than nominal while you get on with some other job that also requires isolation for your safety.Hint: If you use the Variac to power US 115V devices, for **** sake tape (or otherwise lock) the adjusting knob in place so you don't bump it and accidentally over-voltage them

author: Benta
author_rank: Super Contributor
timestamp: July 04, 2017, 08:29:00 pm
content: For isolation transformers, here's a tip for you:Check around in your area if dentists are closing, or perhaps being refurbished.Each treatment room (with chair) usually has a 1...2 kVA isolation transformer. Icing on the cake is, that they are medical grade, meaning very high isolation and very little leakage capacitance pri/sec.This mainly works if you are well connected in the community and have the option of driving by and waving cash under the nose of the foreman/electrician in charge (they are usually happy if they don't have to lug it away). Negotiate a scrap price (they'll be scrapped anyway), and you'll have a top transformer.Other types of clinics could also be options, but I have no info on their equipment.True scavengingI love it!

author: bson
author_rank: Unknown
timestamp: July 05, 2017, 08:52:10 pm
content: If your power needs are modest, such as for bench use to simulate a 25V secondary at < 10W, then an audio power amplifier can be used. Just feed it 50/60Hz in from a function gen, at a level that produces the secondary voltage you want to emulate.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 05, 2017, 11:42:58 pm
content: Quote from: bson on July 05, 2017, 08:52:10 pmProbably good for up to 35 to 50 volts? assuming the amp has some output protection and can handle the load/s.OTOH in the same setup, a bridged output from an audio amplifier (with a rear bridging switch) should give a higher voltage and 'wattage' and should work fine at low 50 hz frequencyand bonus isolation from earth/ground too, since both positive lines are now a push/pull? floating 50 hz AC output and not tied to the amp chassis/earth/ground.Then you can go one further by driving a step up transformerIt's been a while since I played with this stuff,so please'don't try this at home'unless sure..and put some fast blow fuses in lineif you feel like a gamble