Isolation Transformer and Oscilloscope - Page 1
Forum section: electronics / projects

author: nyo
author_rank: Contributor
timestamp: January 18, 2015, 01:17:41 am
content: Hi guys,I posted a video to YouTube -http://youtu.be/QOpBGMn5ECAand posted documentation on my sitefelipes.net.It's about modifying a medical isolation transformer...I would love some feedback, you can post it here, my site or the youtube video, you can even email it to me, my email is at my site.Thanks,Hope it's useful to someone out there.

author: nctnico
author_rank: Super Contributor
timestamp: January 18, 2015, 01:45:19 am
content: If you want to apply for a Darwin award then please follow the instructions provided by Nyo.To be clear: don't use an isolation transformer if you value your life and that of others. Buy differential probes!

author: wraper
author_rank: Unknown
timestamp: January 18, 2015, 02:07:23 am
content: Quote from: nctnico on January 18, 2015, 01:45:19 amUsing isolation transformer for DUT (not oscilloscope) with regular probes is safer than using differential probes on live device. Live device will happily zap/kill you if touched by accident regardless if you probe it with oscilloscope or multimeter

author: nctnico
author_rank: Super Contributor
timestamp: January 18, 2015, 02:11:15 am
content: I guess you never heard of a GFI (ground fault interrupter) which is much safer than thinking that every device is floating until you accidentally ground the floating mains somewhere.Isolation transformers are a thing from the past when GFIs didn't exist or where very expensive.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 18, 2015, 02:36:30 am
content: There's some videos in this post:https://www.eevblog.com/forum/testgear/isolation-transformer-and-gfi/msg298793/#msg298793

author: crystal
author_rank: Contributor
timestamp: January 19, 2015, 11:32:38 am
content: The guy on this video explains how to eliminate the ground problem and to meausure mains voltage. Is this the safe way to do it, has anyone tried this:

author: nyo
author_rank: Contributor
timestamp: January 19, 2015, 11:44:18 am
content: Hey crystal,On the video he is using the differential mode. The differential method is very good for safely probing stuff with an oscilloscope as long as you know what you are doing. The isolation transformer does a little more, it removes the path to earth ground on secondary side making it safer to work on equipment connected to it. There is a great video that goes into detail about this, check out Todd Harrison's videohttp://youtu.be/11Yve2ijWyk

author: Rerouter
author_rank: Super Contributor
timestamp: January 19, 2015, 12:13:15 pm
content: There must be 5 or 6 threads where i have gone over this topic in detail....An isolation transformer does not make the scenario any safer to you or your test equipment, it is merely a tool that still has to be used for the correct purpose and in the correct way, with limitations.The differential probe is safer to you and your equipment as you only need to question A. is the voltage between the test points too high, and B. is the voltage on either input too high from ground, once again they have there limitations and need to be used in the correct manner, if you must be probing live, then wear something non-conductive back to your elbow joints on both arms,To set the matter straight for those that cannot afford, if you have a 2 channel scope, it should have a subtraction function (or add + invert) for the 2 channels, with this you have a differential probe, you leave the ground clips disconnected and away you go, you will loose some bandwidth and noise can get in easier, but it is safer than getting the though in your head that the isolation transformer protects you, just like getting the though that an RCD or GCFI will save you, if current does not flow through you to ground, it will never trip.If you want to probe things live, you should try and learn all this stuff, for your own safety and others, there is no magic device that fixes every problem, everything is a trade off it just comes down to knowing which ones you can make,

author: madires
author_rank: Super Contributor
timestamp: January 19, 2015, 01:20:13 pm
content: Quote from: nctnico on January 18, 2015, 02:11:15 amHow fast is your RCD/GFI? Can it fail? I've seen more failed RCDs than failed isolation transformers. The best approach is to use several security measures, e.g.- isolation transformer for DUT- differential probes for scope- rubber gloves- one hand in the pocket

author: nyo
author_rank: Contributor
timestamp: January 19, 2015, 01:38:11 pm
content: Quote from: Rerouter on January 19, 2015, 12:13:15 pmI agree completely. I posted the video because I did the modification myself and I decided to turn the camera on while I was doing it and write some documentation. By no means do I encourage the use of any of these or even probing live mains circuit if you don't understand all of this. Doing the wrong thing could seriously hurt you or at the least damage your equipment... careful guys, the more info you can get the better, do your homework

author: electr_peter
author_rank: Unknown
timestamp: January 19, 2015, 03:50:24 pm
content: This topic has been discussed multiple times before, isolation transformers are not magic safety devices - do not trust them blindly, use differential probes.Isolation transformer has one meaningful use with oscilloscopes, but it is still dangerous.As oscilloscope and DUT are both grounded, you can only clip oscilloscope ground clip to the earth and measure signals with respect to the earth. This may not be what you want.If you power DUT from isolation transformer, you can clip oscilloscope probe ground clip anywhere and measure what you want.However,by doing this you once again make DUT referenced to earth(floating becomes earth referenced) andDEFEAT RCD/GFCI PROTECTION FOR DUT. See simplified diagram. Note that GFCI/RCD is not effective in this configuration even if you add it after isolation transformer.Isolation transformer did not add safety in this case, it made things more dangerous!!! Please use real differential probes or pseudo differential probes technique( sum[1CH; 2CH inverted] ) instead if you mind safety.Isolation transformers are mainly used to provide isolated power or add some noise filtering. They do not add safety by default.Also, install GFCI/RCD, but do not trust it too much:for one, GFCI/RCD may fail to operate (could be device failure, could be some faulty wiring, etc.)GFCI/RCD are designed to trip at 5-30mA or higher currents in short period of time. However,GFCI/RCD do not limit instantaneous maximum current during the fault in any way, which could be 100mA-500mA or more!!!Do you really want to try that?

author: calzap
author_rank: Frequent Contributor
timestamp: January 19, 2015, 04:23:33 pm
content: A bit of warning on "isolation" transformers. Not all transformers carrying the label of "isolation" provide isolation in the sense discussed in this thread. Some only provide noise reduction from the incoming mains. An example is the Tripp-Lite Isolator series. Their literature does state that the secondary "neutral" is bonded to ground, but doesn't provide a warning that the receptacles on the device can't be considered electrically isolated from the incoming mains. I bought one and was suspicious when I saw the outlets were of the three contact type. Opened it and was horrified to see one of the transformer secondary wires connected to the case; then I read the fine print. I later modified and tested it to be a true isolation transformer. I'm not the first to do this; there are examples on the net.Mike in California

author: bogart219
author_rank: Contributor
timestamp: December 07, 2018, 11:10:54 am
content: Hey fellows, Reviving an old thread. I have this 1 KVA control step down step down transformer. I measure no continuity between the primary and the secondary. Is it safe to assume that this is a isolation transformer? I called Dongan and asked them but the guy I talked to seemed confused whether it was or not. what do you think? When I apply 120 volts to the 480 primary setup, I get 120 volts on the secondary.

author: Ian.M
author_rank: Super Contributor
timestamp: December 07, 2018, 02:01:19 pm
content: Probably a that's what the diagram on the label indicates, but as an isolation transformer is safety critical, you need to be 100% certain of the integrity of the insulation between the primaries and the secondaries. If its a split bobbin transformer, visual inspection may be sufficient, otherwise you need to tie all the primary terminals together, and the same on the secondary side and do a Hi-Pot test, at a test voltage or 1KV or higher.Once you are certain its insulation is OK, the best way to wire it would be to parallel the nominally 240V primaries, and put the nominally 120V secondaries in series (link H1-H3, H2-H4 and X2-X30). Then its wired 1:1 and is good for 1KVA @ 240V so 4.16A. If you use it for 120V input isolation, wire the supply as-if it was 240V and the whole series secondary will give you 120V out, but you don't get any more current capability than the 4.16A its rated for.

author: bogart219
author_rank: Contributor
timestamp: December 08, 2018, 10:13:06 am
content: Hi Ian, thanks for the info. I have a 500 volt megger. I will do that test.