Isolation Transformer - Page 1
Forum section: electronics / general

author: Jbouss
author_rank: Contributor
timestamp: November 09, 2022, 03:50:51 am
content: Hi All,This may be a silly question, but I am trying to understand isolation transformers. I was researching them and with my limited experience, I am hoping for more understanding as to what they do and why you would use them. I am under the impression that they use the voltage from the source, but isolate the load from the mains, particularly the load ground current leakage.I am working on a project using 5 power supplies connected to a GFCI breaker. Since GFCI breakers trip at such low current in the states (6mA), I keep tripping the breaker. My thought is, if I am understanding the isolation transformer, I can connect the transformer to the GFCI breaker, and connect the power supplies to the transformer. By doing this the breaker will not trip.Is this correct? Either way, now I am very interested in the purpose of an isolation transformer.Thank you all, this forum has been very helpful any time I have had questions.

author: fourtytwo42
author_rank: Super Contributor
timestamp: November 09, 2022, 11:37:20 am
content: It sounds to me like you have an actual earth leakage fault. Adding an isolation transformer would be potentially dangerous as you would hide the fault.A normal PSU has Y capacitors of 2.2nF producing an earth leakage of <0.1mA so even though your breaker is an unusually sensitive 6mA (normally 30mA) that should not be anywhere near enough to trip it from that cause.Have you tried isolating the fault ?Are you powering on all the power supplies simultaneously ?

author: themadhippy
author_rank: Super Contributor
timestamp: November 09, 2022, 01:45:14 pm
content: Quoteerm did you miss the bit about being in the usa, whilst 30ma is our standard goto value things get done differently else were. With such sensitive devices i wouldn't be at all surprised to find 5 psu causes them to trip

author: Jbouss
author_rank: Contributor
timestamp: November 09, 2022, 02:33:16 pm
content: Themadhippy is correct, the PSUs trip the breaker in standbye at times. While running, I believe the Y caps are the cause for the breaker tripping with a much higher leakage then 0.1mA. I think these caps are used for CE certification, and since breakers are rated differently in the UK, the Y caps are dumping to much to ground.Fourtytwo42, safety is a concern, I wouldn't want a true gnd fault to occur, with no protection.Thank you, both of you, I appreciate the feedback.

author: madires
author_rank: Super Contributor
timestamp: November 09, 2022, 02:58:54 pm
content: The class Y caps are for EMI/RFI suppression, not specificly just for CE certification, also for FCC and all the other regulators in each country.

author: uer166
author_rank: Super Contributor
timestamp: November 09, 2022, 07:15:25 pm
content: Quote from: Jbouss on November 09, 2022, 02:33:16 pmRemember that if you put an "isolation transformer" where the output and input grounds are tied, and the output Neutral is bonded to GND, you're simply defeating the upstream GFCI, deleting any/all protection. I'm assuming the upstream job is a standard 5mA outlet GFCI. Ignoring the fact that 5mA is painful as shit and is a concern, you could add a transformer with PE/N bonded at output (creating a separate source), and add a 20mA GFCI immediately after, between transformer and the power strip/PSUs.Beforedoing that you need to measure/confirm the actual leakage value to make sure it's not something to worry about.A much better way than fucking around with transformers is simply replacing the upstream GFCI with a 20mA job if you know 100% that this is just an overzealous false trip without a safety issue.

author: Jbouss
author_rank: Contributor
timestamp: November 10, 2022, 01:53:41 am
content: Appreciate this insight.I understand Y caps are not specifically CE but still believe they are the cause for tripping the GFCI.

author: Jbouss
author_rank: Contributor
timestamp: November 10, 2022, 01:55:56 am
content: Agreed, I ruled out a transformer. The first thing I need to do is find out what my leakage actually is, unfortunately, I don't have a leakage clamp, so I will have to tap into gnd with a meter. Once I determine this, I will move on to a solution. Thank you.

author: bdunham7
author_rank: Super Contributor
timestamp: November 10, 2022, 02:18:03 am
content: Quote from: uer166 on November 09, 2022, 07:15:25 pmWho does that?I'd like to know exactly what the PSUs are. If they are typical mostly-isolated (linear or switchers) with a metal case or internal chassis that is grounded, then a isolation transformer that passes the ground is a perfectly fine way to solve the OPs problem. It doesn't sound like he particularly wants to probe the PSU guts, just use them normally. But I'd ask exactly what model(s) of PSU he is using before saying all that for sure.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: November 10, 2022, 02:31:36 am
content: I remember using isolation transformers in the 70's when servicing TV's.This was mainly due to a lot of manufacturers running 'live chassis'. If you plugged a scope probe ground connector onto a live chassis set you would get lots of sparks and no more scope probe.So adding an isolation transformer allowed you to ground the chassis to run the set and happily probe the circuitry with a scope.

author: uer166
author_rank: Super Contributor
timestamp: November 10, 2022, 06:08:05 am
content: Quote from: bdunham7 on November 10, 2022, 02:18:03 amI sure do, for purposes such as step up (120V to 240V split phase conversion). It's also a valid way to defeat upstream GFCI as I described, you better know exactly what you're doing, as usual.If you're truly using it fully isolated, you're just creating your own IT mains system with its' own set of challenges and issues.

author: madires
author_rank: Super Contributor
timestamp: November 10, 2022, 12:49:18 pm
content: Quote from: Jbouss on November 10, 2022, 01:53:41 amYep, each class Y cap passes some current form live/hot to earth/ground. You can calculate the current by applying Ohm's law and the cap's reactance: Xc = 1 / (2 pi f C).

author: Jbouss
author_rank: Contributor
timestamp: November 10, 2022, 05:45:38 pm
content: They are cheap switching supplies, kps305d. The leakage is 2.0-3.0mA depending on the load. Better linear supplies would leak less, but I am not sure the gfci would hold even then. I am ok with 'bypassing' the gfci with the isolation transformer, but do not want an unsafe situation if something were to short gnd. I guess I could put a reg breaker in series with the transformer.What would happen if a PS shorted to gnd on the output side of an isolation transformer? Will the system still be safe with no path to earth gnd? Albeit the supplies are fused, so the supply would shut down quick.

author: Jbouss
author_rank: Contributor
timestamp: November 10, 2022, 05:51:48 pm
content: I will admit this would not be a conventional use for an isolation transformer. Thats why safty is my biggest question.Oscope DUTs and old CRT TV testing are typical situations were an isolation transformer can save equipment and even life's.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 10, 2022, 05:56:59 pm
content: The sensible thing to do is to investigate why the PSU keeps leaking so much current, or just get another PSU without this fault.6mA leakage through Y capacitors for a single PSU is not normal. Unless it's some special industrial PSU.

author: Jbouss
author_rank: Contributor
timestamp: November 10, 2022, 06:06:15 pm
content: The supplies are only leaking 2-3mA, times 5 is the issue. Regulations allow PSU manufacturers up to 3.5mA leakage for PSUs.

author: coppercone2
author_rank: Super Contributor
timestamp: November 10, 2022, 06:33:34 pm
content: put seperate ones