Can two inputs make an isolation transformer? - Page 1
Forum section: electronics / projects

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: December 10, 2018, 07:36:07 pm
content: I just acquired a toroidal transformer with two 120V input coils.I figure that the inputs are indended to be wired in parallel for double current or series for double volts.Is there any reason that one can not be used as an isolated output? Perhaps insulation is a problem?Sorry the specs are scant.

author: tecman
author_rank: Frequent Contributor
timestamp: December 10, 2018, 07:50:36 pm
content: It is okay to use as an isolation transformer. Just be aware that you can only use it at 1/2 the rated power, since each primary coil is designed to handle 1/2 of the power.One issue is that there will be no shielding between these two primary windings. Therefore there will be come capacitive coupling between the windings, which can result in some voltage present on the second winding with respect to ground (on the first winding). While generally low current, you can get a nice tickle from the secondary winding to ground. A proper isolation transformer will have an electrostatic shield to prevent this from occurring.paul

author: Gyro
author_rank: Super Contributor
timestamp: December 10, 2018, 07:53:03 pm
content: Firstly, as a "Medical Isolation Transformer" it's intended for the purpose - and a very good one. The two input primary windings are intended to be wired either in parallel (100-120V supply) or in series (200-240V supply). You need to connect both primary windings to achieve the rated wattage of the transformer.As for the question I think you're asking - can I use one of the primary windings as an isolated secondary? The answer is adefinitely not. Firstly, for the reason above, and secondly (as you guessed) because of insulation. The primary and secondary windings are formed in very particular and very well insulated layer sections, with a protective ground screen (green/yellow), also insulated, between them. The two primary windings are together, in the same layer section.You need to use the secondary windings, as intended.

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: December 10, 2018, 08:05:44 pm
content: OK, thanks.But if they can be wired in series wouldn't that create the same potential difference between the input coils as if they were wired for isolation? I would think they would need enough insulation either way.Maybe they are not intended to be wired in series?

author: Gyro
author_rank: Super Contributor
timestamp: December 10, 2018, 08:16:54 pm
content: Yes, they are insulated enough for series / parallel configuration across the mains (a few hundred volts), that's not the same insulation quality as between the primary and secondary windings of a mains transformer (several kV) and a safety isolating transformer in particular.Stick with the intended secondary windings: Red-White and Blue-White (Check!).P.S. You've got a very nice quality 300VA safety isolating transformer there, the medical ones are the best you can get. Be nice to it!

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: December 10, 2018, 08:35:49 pm
content: Ah, if I understand you are saying that the insulation could break down with a high voltage event.I wasn't really thinking of high voltage isolation from mains for safety but just being able to probe something with an oscilloscope, only a few hundred volts isolation.QuoteWe always hurt the ones we love.

author: Gyro
author_rank: Super Contributor
timestamp: December 10, 2018, 08:42:49 pm
content: Yes, correct.No, that's when accidents happen (just probing something with a scope!), youneedthe safety isolation capability. The two primaries are possibly just relying on the insulation thickness on the enameled winding wire (as a single 240V winding would).I don't understand your reluctance to use the proper secondary windings, they should be the same voltage.

author: Kilrah
author_rank: Unknown
timestamp: December 10, 2018, 09:39:27 pm
content: Yup don't understand, you're wanting to use it for exactly what it's made for, but in a twisted way instead of the intended one for apparently no good reason...https://www.plitron.com/default-factory-specs/medical-grade-toroidal-transformers/

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: December 10, 2018, 10:05:56 pm
content: As I said, the specs are scant and I do not know yet what the secondary voltages are. Of course I would rather use them if they are the full voltage.You think that a medical isolation transformer only isolates and does not step up/down? Great if I could be sure.

author: Gyro
author_rank: Super Contributor
timestamp: December 10, 2018, 10:31:27 pm
content: Yes, they should be the same voltage as the primaries [Edit: otherwise the output voltage would probably be stated on the label] - they don't have 100/120V taps because the ratio can be adjusted with the primary ones. If you are not able to safely measure the voltages, then you should be able to check their resistance - they should beapproximatelythe same as the primaries (different winding diameters so not exact).Using a single secondary will get you 150VA. Again you use parallel or series for full wattage 120 or 240V. IMPORTANT: If wiring both secondaries, you will need to make sure you connect them in phase otherwise you will get 0V (series) or an effective short (parallel).It doesn't sound as if you are that experienced in mains wiring from your previous replies, if so, please be very careful and ask for advice. Be safe.

author: electrodacus
author_rank: Super Contributor
timestamp: December 10, 2018, 10:38:36 pm
content: I think you where searching for this

author: Gyro
author_rank: Super Contributor
timestamp: December 10, 2018, 10:56:12 pm
content: Perfect.Those embedded thermal switches (they may well be non-resetting) are another reason not to take liberties with it!

author: ArthurDent
author_rank: Super Contributor
timestamp: December 11, 2018, 01:35:29 am
content: As others have said, it may already have a split 120/240 volt secondary. The unused wires are probably unused 100 volt primary taps. Looks like it has a shield between primary and secondary and low capacitive coupling. Use an incandescent 40 watt bulb in series with the line when you first power it up to make sure of your connections. It should be good for up to 2.5 A.https://www.plitron.com/default-factory-specs/medical-grade-toroidal-transformers/

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: December 11, 2018, 02:47:12 am
content: QuoteThanks. I went back to the store today and found what I think is the correct schematic. Exactly the same but with a ground shield.Those certainly look like they are supposed to depict 1:1 windings.

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: December 11, 2018, 02:51:49 am
content: QuoteI'm actually less experienced than it sounds. I'll be careful. The best thing I got going for me is that mains freaks me out.Thanks for all your help.

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: December 11, 2018, 03:11:12 am
content: To clarify my intents, this was a case of grabbing something before they are gone and then figuring out what I got. Now these look like they are nice isolation transformers so I went back and bought two more. The last two. Couldn't just leave them there.I also picked up these two. Labels are good. CJLTT.Thanks again, everyone.

author: Kilrah
author_rank: Unknown
timestamp: December 11, 2018, 08:44:12 am
content: Quote from: scuzzyTerminator on December 10, 2018, 10:05:56 pmMy post above had a link to the full specs and schematic??

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2018, 10:22:18 am
content: Quote from: scuzzyTerminator on December 11, 2018, 02:47:12 amThere are several different types of insulation:Functional: can only withstand enough voltage for normal operation. Failure of such insulation will not pose a shock hazard.Basic: used between an earthed enclosure and live parts or a non-earthed enclosure and some live parts with another layer of basic insulation in between.Reinforced or double: used between a non-earthed enclosure and live parts.Functional insulation is the bare minimum required to allow the device to work.Basic insulation needs to be significantly over-rated, because its failure can result in electrical shock, although not so much as reinforced insulation, as there's enclosure is earthed or has another layer of basic insulation, offering more protect.Reinforced insulation needs to be twice as good as basic insulation, because there's only one layer between it and the user.

author: 001
author_rank: Super Contributor
timestamp: December 11, 2018, 10:27:01 am
content: why medical isolation transformer use torroidal construction?in 80`th all medical trs has primary and secondary coils at DIFFERENT bobbins to eliminate parasitic capasitance

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2018, 10:33:25 am
content: Why not use a toroidal construction for a medical grade isolation transformer?There's an earthed screen between the primary and secondary, which both offers additional protection and prevents capacitive coupling between he secondary and primary windings.

author: 001
author_rank: Super Contributor
timestamp: December 11, 2018, 11:09:16 am
content: Quote from: Hero999 on December 11, 2018, 10:33:25 amYeaAnd it adds capasitance between sec and ground so still shock hazadrd

author: Gyro
author_rank: Super Contributor
timestamp: December 11, 2018, 11:16:26 am
content: Quote from: scuzzyTerminator on December 11, 2018, 02:51:49 amThat's a useful defense mechanism in keeping you safe. Your best bet would be to properly mount it in a metal case. For the mains input it would be best to use an IEC connector to avoid the trip hazard of a trailing mains lead when carrying. The Input ground pin should be wired directly to the case, ensuring good connection with a tag, serrated washers and lock nuts. The protective ground wire from the transformer and the ground from the Output socket should go to the same point.The Input Live connection should go straight to a panel mounted fuse holder, sleeve all connections. Try a 3.15A Slow blow fuse to cope with input surge current. You may want to add a double pole switch and neon indicator, of just go straight to the transformer primaries, wired as the diagram. You could either use soldered connections with plenty of heatshrink sleeving or a mains rated terminal block.The secondary wiring is the critical part for a safety isolating transformer - if either of the output connections accidentally comes into contact with the case (ground), it will completely nullify the purpose of the transformer. Wire the secondaries in parallel (0-0, 115-115, assuming your mains is 110-115V), straight to the output connector. If you need to extend the wires then usemultiplelayers of heatshrink sleeving on the joints, or use a terminal block with an additional layer of insulating sheet, with decent overlap, underneath it.It sounds as if you got a very good deal if you went back for the other transformers. They look useful and clearly marked. Make sure you parallel the primaries again for proper operation.If unsure of anything, post photos as you go.

author: Gyro
author_rank: Super Contributor
timestamp: December 11, 2018, 11:20:59 am
content: Quote from: 001 on December 11, 2018, 11:09:16 amAlthough if it is a medical grade one, steps (insulation thickness etc.) will have been taken to minimize capacitance - there are maximum specs for these things! It is a reputable brand - there is no shock hazzard.

author: 001
author_rank: Super Contributor
timestamp: December 11, 2018, 12:55:03 pm
content: Quote from: Gyro on December 11, 2018, 11:20:59 amBut what capasitance value is ok?

author: Gyro
author_rank: Super Contributor
timestamp: December 11, 2018, 01:11:33 pm
content: All covered (all approvals and specs) in the web page already quoted if you bother to look..https://www.plitron.com/default-factory-specs/medical-grade-toroidal-transformers/.For the OP's 300VA model, capacitance is less than 0.78nF, Leakage current less than 13uA. Below the threshold of human perception. As I said before NO shock hazard.

author: 001
author_rank: Super Contributor
timestamp: December 11, 2018, 02:55:02 pm
content: Quote from: Gyro on December 11, 2018, 01:11:33 pmI see it is variable value from unit to unit, isnt it? Is some max value standartizied?At least, can You say few words herehttps://www.eevblog.com/forum/metrology/how-to-measure-capasitance-and-leakage-in-transformers/?

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2018, 05:04:00 pm
content: Another thing is the secondary will be capacitively coupled to earth, rather than the mains, so the leakage current should be half.

author: David Hess
author_rank: Super Contributor
timestamp: December 11, 2018, 07:54:04 pm
content: Quote from: scuzzyTerminator on December 11, 2018, 03:11:12 amUsing the separate primary windings works but the reasons it does not work well have been covered so I will not repeat.If you have more than one of the same transformer then you can solve all of the problems. Wire two identical transformers back to back and ground one lead of each of the low voltage windings between them. Now you get full power and the grounded low voltage windings act as the electrostatic shield in addition to any shielding built into each transformer. The disadvantages are twice the weight and space, twice the power loss which is not a big deal, and twice the series impedance which may even be considered an advantage since it helps limit surge current.I did this with a couple of 1000+ watt transformers and then wired the outputs to separate outlets so I actually have two outputs which are isolated from the power line and isolated from each other. Or I could convert either way between 120 and 240 with isolation if I had a need to do so.