Medical Isolation Transformer Modifications - Page 1
Forum section: electronics / general

author: SteveyG
author_rank: Unknown
timestamp: February 21, 2015, 02:30:35 am
content: If anyone is interested, I've just uploaded a new video showing how I modified my isolation transformer to have a UK BS1363 socket and some 4mm banana plugs on it.I plan to do some proper electronics content in forthcoming videos, but someone may find this one interesting.Thanks

author: SteveyG
author_rank: Unknown
timestamp: February 21, 2015, 09:49:19 pm
content: I've also published a video explaining why an isolation transformer can be a useful bit of kit to have at your workbench

author: helius
author_rank: Super Contributor
timestamp: February 21, 2015, 10:49:47 pm
content: "Universal" transformers can also be very useful. Depending on how they are wired, they can act as voltage conversion autotransformers, isolation transformers, and isolating conversion transformers. You do need to set up your own sockets, but that should be pretty simply for readers here.

author: SteveyG
author_rank: Unknown
timestamp: February 22, 2015, 12:17:46 pm
content: Quote from: helius on February 21, 2015, 10:49:47 pmIndeed. It's also possible to modify a pair of 230V to 115V step-down transformers used on building sites to make a 230-230V isolation transformer. You can normally pick them up really cheap second hand and are good for 3kVA.

author: dexters_lab
author_rank: Unknown
timestamp: February 22, 2015, 03:57:30 pm
content: nice video Steve, thanks for sharingsubbed!

author: electr_peter
author_rank: Unknown
timestamp: February 22, 2015, 07:30:46 pm
content: Nice videos with clear explanation of the benefits of isolation transformers.On the topic of oscilloscopes and isolation transformers - beware that it is much better to use differential probes than isolation transformer in general. Scope+isolation transformer references DUT to the earth and defeats RCD/GFCI (thus creating potentially very dangerous situation).https://www.eevblog.com/forum/projects/isolation-transformer-and-oscilloscope/msg590315/#msg590315

author: SteveyG
author_rank: Unknown
timestamp: February 23, 2015, 03:34:36 pm
content: Quote from: electr_peter on February 22, 2015, 07:30:46 pmIt depends on the configuration of the transformer. You can recreate a standard mains arrangement by linking the neutral on the secondary to your outgoing earth (making sure the incoming earth is ONLY used to earth the transformer chassis), just as you would do with a generator for example. You can then use a plug-in RCD if you wanted to, although if you're powering your scope from the transformer the earth loop impedance would be so low that one is technically not required.

author: electr_peter
author_rank: Unknown
timestamp: February 23, 2015, 06:30:00 pm
content: My point was specifically about connecting DUT to isolation transformer and then probing DUT with mains referenced scope. If you do that and then touch live part of DUT, you are in circuit and are not protected by RCD/GFCI in any way. It is not good practice, differential probes should be used instead (EDIT: differential probes and RCD/GFCI).For other purposes, new ground can be created in various configurations. Problem may arise if, say, there is two isolated power sources (mains and local generator) with incorrect switching or turn-on/off sequence. Weird things can happen if one does not take such things in consideration.

author: SteveyG
author_rank: Unknown
timestamp: February 23, 2015, 08:57:15 pm
content: Quote from: electr_peter on February 23, 2015, 06:30:00 pmAh yes, this is true, but you are no better off just using differential probes. You need both!I do have a differential probe, however without spending a decent chunk of money they are not always the most accurate or have bandwidth limitations.

author: Zucca
author_rank: Unknown
timestamp: March 19, 2015, 11:00:20 am
content: nice video Steve, subbed +1!Why did you not put a RCD/GFCI in the isolated side of the transformer?I mean like thisL---------- ------------------- ----------- L to DUT| | |--------------|MAINS | ISO TRAF | | RCD/GFCI | MAINS TO DUT| | |--------------|N---------- ------------------- ------------ N to DUT|---------------------------- EARTH to DUTExample:- You have successfully repaired something using the iso. tranf., everything works. During the repair you have for some reasons compromised the device earth (something live is touching earth)- Now you want to put repaired device in business, and plug it in the main- BUM! the RCD/GFCI in the house trips (and the wife takes off)in this case something was wrong in the device earth but you missed it because with the iso. tranf. the earth was floating...That said working with mains without a RCD/GFCI in the back is, as already stated, not the best.I am also thinking in this way if something goes wrong ONLY the RCD/GFCI in the iso. tranf. trips an the home stays up and running. Not bad... for the wife too... :-)

author: electr_peter
author_rank: Unknown
timestamp: March 19, 2015, 12:03:26 pm
content: Quote from: zucca on March 19, 2015, 11:00:20 amI would say GFCI/RCD on isolated transformer output adds almost no benefit (no real added safety, just "imaginary" safety), so it is pointless.You can check for ground fault in better ways. For live wire to touch metal case after repair (but not before repair) is a big screw-up and very easy to find and correct with simple measurements.

author: Zucca
author_rank: Unknown
timestamp: March 19, 2015, 01:48:22 pm
content: Quote from: electr_peter on March 19, 2015, 12:03:26 pmYeah... that GFCI/RCD will trip only is there is current flowing back on the EARTH DUT... not exactly "imaginary" but nothing to do with proper safety systems (Look what Tek says about the Iso transfwww.tek.com/dl/51W_10640_1.pdfandhttp://www.newark.com/pdfs/techarticles/tektronix/FFM.pdf).Anyway I like the idea to discover immediately if I messed up the earth system in the device, instead to get the home cut off (or the one receiving the repaired device... goodbye customers...). Another big plus for me is the 100% certainty that if it was working after my repair, it will work also on real mains (hoping that there is not an exoteric failure going on). No excuses, without any further measurements/tests.Normally that GFCI/RCD will catch beginner mistakes so I see why at the end it is pointless for someone...(BTW how many beginner mistakes I did in my life? eh eh).Of course if I already know that there is a problem in the earth system a DMM wll do the job in 99% of the cases.That said if I have an extra GFCI/RCD under my bench doing nothing I probably put it there, IMHO even if it doesn't bring too much it has no negative effects.Nice reading on the side:https://www.eevblog.com/forum/chat/workbench-safety-mains-isolation/PS: What about the TEK A6901?http://w140.com/tek_a6901.pdfI have one...

author: SteveyG
author_rank: Unknown
timestamp: March 19, 2015, 02:29:05 pm
content: Quote from: zucca on March 19, 2015, 11:00:20 amThanks for the sub! I did contemplate this, but plug-in RCDs are widely available in the UK, so I decided I'd just use one of those if required. However I always do a full test on any equipment before bringing it into service to make sure it is safe.

author: Zucca
author_rank: Unknown
timestamp: March 19, 2015, 02:35:20 pm
content: Quote from: SteveyG on March 19, 2015, 02:29:05 pmOh dear, now I need an isolation tester too...What about this one?eBay auction: #360808171592

author: Shock
author_rank: Super Contributor
timestamp: March 19, 2015, 02:54:35 pm
content: Did you add any short circuit or over current protection to the secondary at all?

author: Zucca
author_rank: Unknown
timestamp: March 19, 2015, 03:28:07 pm
content: Quote from: Shock on March 19, 2015, 02:54:35 pmMy iso. transf. has that alredy that built in (actually on the primary), I bet most of the medical iso trans. does.

author: SteveyG
author_rank: Unknown
timestamp: March 19, 2015, 05:26:10 pm
content: Quote from: Shock on March 19, 2015, 02:54:35 pmIt has a thermal fuse, but in hindsight, I should have added an MCB to the device. The IEC mains inlet and switch assembly is fused however, so it should not be possible to overload the device.

author: SteveyG
author_rank: Unknown
timestamp: March 19, 2015, 05:30:01 pm
content: Quote from: zucca on March 19, 2015, 02:35:20 pmYou could look for an electricians multi-function tester. The Fluke ones usually go for a good price used.