Mains isolation transformer and variac in series? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 05, 2012, 10:19:15 pm
content: I am having trouble to find a UK mains (240v 50Hz) isolation transformer with variable voltage output for safe testing of unknown mains operated gear at a sensible price. I already have a small open frame variac, if I bought a hefty basic 240V / 240v mains isolation transformer is there any reason not to feed the secondary to the input of the variac, and have the variac's output terminate in a mains socket, and put the two units in a case? At a later date I could buy a heftier variac should I need more current capacity. Thanks.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2012, 10:31:46 pm
content: Sounds OK to me.But what does "safe" testing mean in this context? Usually it means limiting fault currents to a safe value. You do this with the variac by slowly winding up the voltage and observing carefully for anything that looks out of place.You could do this just with the variac alone, and IMO have little to gain from the additional transformer. Put the device under test on the bench, make sure it is properly earthed, don't touch it, and probe it carefully for unexpected voltages in wrong places. Also do some continuity and short circuit tests on the device while it is unplugged and look for faults that way before energizing it for the first time.

author: krivx
author_rank: Frequent Contributor
timestamp: July 05, 2012, 11:14:56 pm
content: The isolation transformer allows you to float the ground on your DUT. You can now probe it with an oscilloscope without the danger of attaching the ground clip in the wrong place and blowing a probe, a scope channel or your hand.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 05, 2012, 11:16:43 pm
content: I have used this combination of an isolation transformer before a variac for a long time.The isolation transformer will protect against a short to ground. (possibly through you.)I don't trust the variac by itself it being an Autotransformer.These days you probably could just rely on an RCD to protect against a short to ground.You definately need something like this if you are playing around with mains voltage switch modes.I like to have the transformers close by as then you can hear when the transformers are heavily loaded, must be harmonics generated by the core saturating.When I hear the loaded transformers then I stop winding the voltage up. It is much more intuative than relying on reading a meter. You can hear there is something wrong.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2012, 11:17:06 pm
content: Quote from: krivx on July 05, 2012, 11:14:56 pmI see.

author: digsys
author_rank: Unknown
timestamp: July 05, 2012, 11:45:30 pm
content: The systems I build have the Variac FIRST. Being an autotransformer, the common Neutral can be an issue.THEN you put your Isolation transformer next ! The beauty of this method is that - you can have multiple taps oreven multiple tranformers at any power level, up to Variac Max. It means you can whip up a 2nd LV tap for anisolated DC supply, for testing. The Isolation tranformer(s) secondaries only draw their power required.You can also have a STEP up winding, quite safely. I use these in HV battery packs, testing / charging.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2012, 01:31:09 am
content: Quote from: Chris Wilson on July 05, 2012, 10:19:15 pmAt the risk of sounding pedantic,what you are proposing is placing the variac & isolation transformer incascade------in other words,the output on one unit becomes the input of the other."Series" connection of transformers implies a very different type of connection,where either the primary or secondary winding (or both) are connected in series with the single tapped winding of the Variac.I would agree with digsys,that the Variac should come first,with its output feeding the primary of the isolation transformer,if only because that is consistent with the normal use of a Variac to adjust the input voltage to the primary of a power transformer in a traditional transformer type power supply.

author: Psi
author_rank: Super Contributor
timestamp: July 06, 2012, 01:46:27 am
content: I uses that setup for my tesla coilIsolating transformer --> Variac --> coil(So the coil is isolated from the house wiring and i can control the voltage.)

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 06, 2012, 09:12:25 am
content: OK, thanks for the heads up on this, not sure now whether to put variac pre the isolation transformer, or post, but ast least i know the set up is sound. I see where I slipped up about being "in series", I did of course mean cascaded, but *I* knew what i meantReally in series would be quite a different scenario, as you say! The goal was to allow safer scoping, and to allow unknown old gear to be slowly powered up, to monitor current draw and to maybe give old electrolytics time to reform.As an aside, if I put a current draw meter in the case, is there such a thing as an auto ranging current display? I did condider just having a low / high power switch, but an auto ranging display would be neater. Thanks again.

author: digsys
author_rank: Unknown
timestamp: July 06, 2012, 09:58:14 am
content: QuoteEither way will work. Having the transformer second is safer and gives you a completely simplified output, you don't have the mess of acommon "neutral".QuoteIMO, the stages are in series, so you could easily argue either way. I'd look at them as stages.QuoteWell, not really, unless you hack up a DMM etc or make your own. Why not a FSD analogue meter with banana sockets to plug inan external meter. Or you could use a ~1ohm in line and pick off a voltage=current, which will make it easier to play with.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 06, 2012, 11:32:43 am
content: I think it's arguable which way is safer, I consider variacs unsafe and therefore it's nice to have them isolated too.Quotedigsys: I am not sure what the problem is here.

author: Psi
author_rank: Super Contributor
timestamp: July 06, 2012, 11:35:54 am
content: Quote from: HackedFridgeMagnet on July 06, 2012, 11:32:43 amhehe

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2012, 11:52:05 am
content: Quote from: digsys on July 06, 2012, 09:58:14 am

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 06, 2012, 12:00:51 pm
content: I have just found a good used 500VA isolation transformer on a well known auction site and bought it as I am lucky enough to have a wife who gets me ludicrously cheap shipping via her workplace. It makes bidding on things with high freight charges more viable if the seller is happy for me to arrnage my own courier collectionI am hoping that should have enough ooomph for most ham radio related needs? Some of the new ones seem to have very low power handling capacity. My variac is 3 Amp, so until that combo proves inadequate I just need to find some sort of case to slap them in and sort a socket out, and ideally a means of displayiing current and voltage by meter or numeric display. Thanks for your help!

author: madires
author_rank: Super Contributor
timestamp: July 06, 2012, 12:30:56 pm
content: Hi!Quote from: krivx on July 05, 2012, 11:14:56 pmBut you stiil need to be very careful! Whatever you connect to the probes ground clip is connected to earth (via the scope). If you touch - by accident - something with a voltage much lower or higher than the grounded point you'll close the loop (remember: you're connected to earth). Better use a differential probe or set up two channels of the scope being differential (invert one channel, then enter add-mode and remove the grounding clips from the probes).CheersMarkus

author: digsys
author_rank: Unknown
timestamp: July 06, 2012, 02:22:01 pm
content: QuoteRefer the 2 options -In option A, the OUTPUT has a potentially DIRECT connection to the Isolated 240V. Our "student" has dialledup 12VAC and is happy that it's safe !! BUT one side of his output is at A2 or N2.The "neutral" (in rabbit ears) was to convey the fact that this connection is still at mains reference, just being lazy.Option 2 is totally isolated at the 12VAC.Some may think this is pedantic, but I live with SAFETY in education all the time. Anyone is quite FREE to voicewhatever opinion they desire, and it's saved here on the forums for ever !!

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2012, 08:09:32 am
content: Quote from: digsys on July 06, 2012, 02:22:01 pmOK, I can see that in "A" you can get a shock if you :-(1) Are across A2 & N2,or the equivalent points on the Variac winding so you receive the whole 240 volts from the isolation transformer secondary.(2) Are across the entire Variac winding,so you may receive a shock of greater than 240 volts.(3) Are between the wiper & the A2 connection for a 228 volt shock(4) As(3) but between the wiper & the extended (unused) part of the Variac winding for a shock of greater than 228 volts.(4)Are across the tap & N2 when the last (12v) section of the Variac winding becomes open circuited,elevating the wiper point to 240 volts.All these possibilities except (4)are extremely unlikely because the Variac is heavily insulated.In no way is there a lack of insulation between the output of this setup & the "Street" mains.In the case of "B"If you stay at the output of the Isolation transformer,you can get a shock if:(1) The Variac is wound up enough & you get across the two sides of the transformer secondary while there is a dangerous voltage present.(2) As in (4) above,the last section of the Variac winding becomes O/C,increasing the voltage to the Isolation transformer's primary,although having most of the Variac winding in series with its primary will stop it seeing the full 240volts.Neither of these setups are a safe method of obtaining low voltages for permanent use!I agree that the connection of "B" is the better one,but more from considerations of practicality than anything else.The most logical setup is to have the Isolation transformer & Variac as separate units,so they may be used where only one or the other is required.

author: digsys
author_rank: Unknown
timestamp: July 07, 2012, 09:23:35 am
content: QuoteIF you wanted adjustable low voltages, then you'd use a Fixed 240:12/24VAC etc secondary anyway, then you're 100% safe.The OP wasn't asking for a LV design, all this is for "what comes first"The other advantageof "B" is - the output is a pure inductor, whereas "A" is an inductor, a carbon contact, and a HV aerial.Try designing a synchronous bridge or any design requiring a "clean" L

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 07, 2012, 02:29:15 pm
content: Well back on the subject of what comes first.I should describe my situation whereby the variac has a three pin plug on the input and a 3 pin plug on the output. The incoming active is not accessible.I would imagine this is fairly common.I consider the variac a dangerous piece of equipment because it can supply lethal voltages and is often used in powering equipment that is in test or being fixed, equipment that may have exposed conductors.As such I think the variac should always be isolated.The trouble with B may come if somebody else needs the isolation transformer for something else and takes it away, and leaves the variac plugged in to mains but not isolated.Anyway in this day and age we have RCD's to protect us.Does anyone make a variable transformer with the slider on the secondary?'This would probably be a good thing and render our discussion superfluous.

author: saturation
author_rank: Super Contributor
timestamp: July 07, 2012, 03:11:04 pm
content: I use the B or A setup as needed, B by default for more safety. These days this is the only way to get low cost variable AC. These old devices remain popular and integrated both functions, on eBay the hammer bid them up ridiculously, almost to like-new prices because no such devices are current made:http://www.ebay.com/itm/SENCORE-PR57-ISOLATED-METERED-VARIAC-VARIABLE-AC-POWER-LINE-SOURCE-SHOCK-TESTER-/110869399288?pt=LH_DefaultDomain_0&hash=item19d054c6f8http://www.ebay.com/itm/Heathkit-IP-2718-Heathkit-IP-5220-Variable-Isolated-AC-Supply-Heath-kit-/140779320201?pt=US_Radio_Comm_Device_Power_Supplies&hash=item20c719f389