How to test old mains isolation transformer to make sure it is working correctly - Page 1
Forum section: electronics / beginners

author: kiwimaker
author_rank: Newbie
timestamp: May 28, 2015, 09:39:28 am
content: This is my first post on EEVblog Forum after many, many hours of watching Dave's videosI have been trying to fix a mains powered device, and decided it was probably best to get a isolation transformer to avoid accidentally zapping myself.I purchased an old one from an auction online. How can I test to make sure it is working correctly and the transformer is not damaged, and I am indeed isolated. I thought of putting some DC voltage through it and measuring to see if any passes through the transformer (which I assume there would be a fault if any was able to). Not sure if that is a good idea (or what an appropriate voltage to test it would be). How would you test to make sure you are isolated from the mains?Here are photos of it:

author: Ian.M
author_rank: Super Contributor
timestamp: May 28, 2015, 10:18:12 am
content: First check leakage resistance between the primary and secondary. I would be very suspicious of anything under 10M. Then do an insulation test at 1KV. If you don't have the equipment and skills to do that, get it PAT tested, with one end of the secondary grounded.

author: kiwimaker
author_rank: Newbie
timestamp: May 29, 2015, 12:36:27 am
content: Thanks for the advice. I checked the resistance between the live input and live output sockets using my Brymen DMM. It was unable to find any resistance. Or did you mean for me to check something else?

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2015, 10:20:48 pm
content: What do you mean by you were unable to find any resistance? If the multimeter read open circuit then fine but if it read near zero Ohms then it's bad.Also check the resistance between the primary and earth and the secondary and earth. It should read open circuit.Check the resistance between the earth conductor and the metal case. It should read closed circuit or near zero Ohms.

author: ez24
author_rank: Super Contributor
timestamp: May 30, 2015, 04:54:04 am
content: Different brand and different voltages but check this out

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 30, 2015, 06:27:38 am
content: QuoteIf you mean use a ohm meter, that's fine. But if you mean using any DC current above milliamperes, don't do it. Transformers don't like DC.

author: helius
author_rank: Super Contributor
timestamp: May 30, 2015, 06:55:04 am
content: Quote from: ez24 on May 30, 2015, 04:54:04 amNice video, but I wish he had been more explicit that although the isolating transformer has no reference to ground, as soon as you connect your probe ground clip, you have referenced ground to that point. You will be sorry if you use two probes and clip their grounds to two different circuit nodesThe isolation transformer also loses its personal safety aspect when you connect the ground clip like that.

author: Rerouter
author_rank: Super Contributor
timestamp: May 30, 2015, 07:35:24 am
content: the correct way is to borrow or rent an insulation resistance tester (trade name is a "Megger") and test between primary and secondary to the rated voltage or atleast double what you intend to use it to withstand,e.g. you want to use it on mains, and to ground the active to measure somethign wtih the scope, great, test it to 500V, the device uses a capacitive multiplier and you want to measure something involved there, great, 1000V (these are 2 very common settings on a megger),for most things you want better than 1Mega ohm resistance,

author: Ian.M
author_rank: Super Contributor
timestamp: May 30, 2015, 10:24:08 am
content: It may have an earthed interwinding screen, so you also need to do insulation tests to earth from both windings.Finally, you need to determine how the earth pin on the secondary's output socket is wired. As this is effectively an independent supply, it should be open circuit (or tied to output neutral), but it is not uncommon to find its tied to input earth which is extremely dangerous for bench work. If that's the case, you'll need a custom output lead that disconnects E which must never be used for other equipment so should be secured with a cable clip under one of the lid mounting screws.As Helius points out, introducing a ground to the device under test is problematic. I strongly advise testing for voltage between ground and the point you are thinking of clipping a ground lead to *BEFORE* connecting it. Use an analog meter as capacitive coupling is likely to give misleading voltage indications on a DMM. Also, *NEVER* try to ground an active node in a SMPSU circuit. The stray capacitance is likely to cause your switcher to blow up, and even if it doesn't, the switching spikes can break down your transformer's insulation.

author: onlooker
author_rank: Frequent Contributor
timestamp: May 30, 2015, 01:20:30 pm
content: Also, check if there are any burn signs, or unusual heating up when plugged in. The other is to check if the current leakage is still in spec.

author: Doc38343
author_rank: Contributor
timestamp: June 02, 2015, 04:50:20 am
content: Quote from: Ian.M on May 30, 2015, 10:24:08 amI bought the exact same isolation transformer as in that video. It iis also medical grade so i took it apart and disconnected the earth mains from the secondary neutral as in the video. HOWEVER the earth mains is still connected to the ground terminals on the output socket. I am confused reading so many different conflicting opinions about grounds in relation to isolation and safety. So here are my thoughts based in logic of combined information:1. I am better off if the earth ground is completely disconnected from the secondary/output.2. There are some circumstances where a earth pin grounded secondary could be needed, but not when troubleshooting a live circuit board.3. Act as if I am in equal danger when working with any live voltage circuitry.4. to protect my new oscilloscope test potential connections with my VMM before i connect the probes. and this is certainly a case where i do not want earth ground anywhere near the device under test especially the metal chassis of what I'm working on.5. it is best practice to keep my oscilloscope Grounded to earth main thru normal ground pin in socket (not to use an adapter to defeat the earth pin)Just seeking clarity on this confusing subject. (it seems so simple to understand in Daves video but the one above is leaving the mains earth ground connected to the secondary output recepticals)I simply trust Daves lesson more than i trust the others i've seen, as far as workbench electronics go.

author: johansen
author_rank: Super Contributor
timestamp: June 16, 2015, 09:08:57 am
content: i would lift the ground pin on the isolation transformer, and ground your dut through the oscope probe.. and the oscope should also be floating.this way, you can't shock yourself unless you can dump enough current into your body at the 60 hz line frequency.. usually this is difficult, but at 400 hz it isn't, you're going to get shocked through your own capacitance.and you can take other precautions to reduce the capacitance of the system such that it is safe to float your oscope and the isolation transformer at the switching node of a supply, you can also thread your oscope leads through a common mode choke to make this easier.if you're worried about resonance destroying the isolation transformer when doing the above.. put a dozen common mode chokes in series with line and neutral....you can also ground the output of the isolation transformer through a resistor to reduce the Q if somehow you end up with a resonance at the frequencies of interest.