Superb and very useful isolation transformer for only £40 - Page 1
Forum section: electronics / testgear

author: Fraser
author_rank: Super Contributor
timestamp: June 02, 2016, 12:00:06 pm
content: I am placing this in the test equipment area as it is part of my test bench safety systemI have used 230V isolation transformers before and have a very nice 230V / 230V medical grade unit that came with one of my Faxitron X-Ray units. It is not needed on the Faxitron so I used it on the test bench.I stumbled upon an even better isolation transformer last week..... it is a 115V/230V input and 115V/230V output with medical grade safety isolation.How is this different to my 230V/230V version ? Well you can independantly set the input and output voltages so it can take 230V and produce 115V output at 1000VA, or it can take 115V input and produce a 230V output at 1000VA. Very useful to me as I work on both 230v and 115V kit.The unit was priced at £40 + £10 postage so was very reasonably priced for such a high quality transformer. The auction contained both an external and internal picture which I attach, along with pictures of my unit.The unit contains a quality torroidal isolation transformer and a 24V DC power supply. Presumably this transformer was designed for a specific equipment that required medical grade mains safety isolation plus a 24V DC supply.The manufacturer of the 24VDC/ 1.5A power supply PCB is HORSODAN ELEKTRONIK. A specialist in this type of product.http://www.horsodan.dk/The unit weighs 12kg, but thanks to its torroidal transformer it is compact.Sadly the three units that the seller had advertised have now sold, but I am writing this post to highlight the nice quality and availability of these medical type isolation transformers and even models that can up or down convert the supply voltage.I am well chuffed with my purchaseFraser

author: nctnico
author_rank: Super Contributor
timestamp: June 02, 2016, 12:25:02 pm
content: Do note that this transformer isn't intended for a repair work shop! The input & output earth are connected and it has multiple outputs. This unit is obviously intended to interrupt ground loops and filter mains noise for sensitive medical equipment.

author: lukier
author_rank: Unknown
timestamp: June 02, 2016, 12:45:40 pm
content: Darn, always late for good eBay dealsI could use isolation transformer sometimes, but for now instead I use RCD and Tektronix A6902B which is only 20 MHz, but good for up to 3kV (with the huge probes attached), need to calibrate it anyway. There is Tektronix P5205 HV diff. probe on its way to me, but this was bought as parts/repair with both cables cut so it is still uncertain if it is repairable.

author: Fraser
author_rank: Super Contributor
timestamp: June 02, 2016, 12:47:41 pm
content: nctnico,Well spotted and thank you for the warning. Well noted.Still love this transformer for the money thoughFraser

author: woodchips
author_rank: Frequent Contributor
timestamp: June 02, 2016, 12:48:24 pm
content: Surely the input and output earth have to be connected together, how else would you get an output earth? With a floating secondary then whether you then earth one end of the winding to earth reference the secondary is up to you, but that would normally be done.What importance does medical grade have? If the windings have an electrostatic shield then you won't get any leakage from primary to secondary, so the secondary won't start floating at odd voltages. But that is why you earth one end of the secondary in any case.Useful transformer but don't really see what is so special, for 115V one of the site 55-0-55 transformers do fine, tweak the earthing if wanted, but cheap and good quality.

author: Fraser
author_rank: Super Contributor
timestamp: June 02, 2016, 02:01:43 pm
content: Three words..... Quality, quality, quality

author: pascal_sweden
author_rank: Super Contributor
timestamp: June 02, 2016, 02:13:53 pm
content: I am a bit confused now.First it was perfect... and then came some remarks... are the remarks valid or not?Isn't there any isolation transformer out there without any of the described flaws?Even if it costs double or more, it would still be good to have.What about any drawings to explain the shortcomings?Or what about any references to isolation transformers that don't have any of the described flaws?

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: June 02, 2016, 02:50:34 pm
content: Quote from: Fraser on June 02, 2016, 12:00:06 pmIt's still off-topic here and should have gone in 'other tools & products'.@mods: please move

author: Fraser
author_rank: Super Contributor
timestamp: June 02, 2016, 03:04:25 pm
content: I disagree.... Test equipment includes power supplies ! This is a form of POWER SUPPLYYes it is an isolation transformer, but it is also a step down transformer and a DC 1.5A power supplyFraser

author: rsjsouza
author_rank: Super Contributor
timestamp: June 02, 2016, 03:13:52 pm
content: Well, I can't disagree with Fraser... It is a dual output power supply: 115/230VACon one output and 24VDC/1.5A on another...

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: June 02, 2016, 03:16:14 pm
content: Quote from: Fraser on June 02, 2016, 03:04:25 pmNo, it's not. An isolation transformer is a safety device, not a test instrument.And this forum has separate categories for a reason, and it's expected from each of us to respect that.

author: Fraser
author_rank: Super Contributor
timestamp: June 02, 2016, 03:22:17 pm
content: Wuerstchenhund,I get a feeling you like to split hairs and cannot tolerate any deviation from your nominal.Such is life and not everyone will want to die in a ditch over such a trivial deviation from your perfect world. Certainly not me. I am an active member of this forum and take some offence at being accused of not respecting its policies.I will let the Mods decide what to do with the post. It was intended to help people and not cause disharmony on the forum.Life goes on in its imperfectionFraser

author: nctnico
author_rank: Super Contributor
timestamp: June 02, 2016, 04:13:34 pm
content: Quote from: pascal_sweden on June 02, 2016, 02:13:53 pmAsk yourself why you want to have an isolation transformer. Really forget about safety when repairing/developing mains powered devices. There has been a long thread about that recently with documents from both the BBC and Tektronix saying the same; look it up.

author: Jasper
author_rank: Contributor
timestamp: June 02, 2016, 09:25:04 pm
content: I can't quite tell from the innards photo, but if the two grounds are connected together, then why would there be two separate grounding posts with different symbols on the back? It looks to me as if you can connect them together externally if you want, but this is not required.

author: nctnico
author_rank: Super Contributor
timestamp: June 02, 2016, 10:49:05 pm
content: By the way: the outputs are not switchable between 115V/230V. That extra selector is for the 24V power supply.

author: Fraser
author_rank: Super Contributor
timestamp: June 03, 2016, 10:29:09 am
content: nctnico,The picture of the internal parts was not greatI attach some better pictures that I just took of the internal configuration.I am pleased to advise that one voltage selector serves the transformer INPUT and the other serves the OUTPUT. The 24V DC power supply is auto voltage selecting (as I would expect). I tested my unit and the output voltage may be selected independent of the input voltage which is why I consider this to be more than JUST and isolation transformer. It is a versatile part of the test bench that can power 230V or 115V DUT's and offers a useful 24V DC @1.5A for use via a Boost Buck converter for all manner of voltages. It makes a pretty nice addition to the test equipment line up for testing and development work.It is easier to work out the configuration when you have the unit in front of you. The input voltage selection is just the normal two windings that may be placed in series or parallel. The output also has two windings that may be placed in series or parallel. The 'neutral' (blue) end of the windings is taken to chassis ground and the 'Live' (grey) end of the windings is connected to a Lucar connector splitter. The split output is connected to the 24V DC power supply, that has a 100V - 240v input, and the multi socket mains output strip.As I said, all easy when you have the unit apart on the bench. Not so easy when looking at the poor quality image from the auction that I previously provided.Fraser

author: rsjsouza
author_rank: Super Contributor
timestamp: June 03, 2016, 10:38:00 am
content: Fraser, too bad it is "only" 12kg or I would love to take this super nice transformer out of your hands...

author: Fraser
author_rank: Super Contributor
timestamp: June 03, 2016, 10:51:38 am
content: I confirm that the secondary of this transformer is indeed connected to earth/chassis so it is not a workshop type isolation transformer as ncnico earlier warned. It is however very easy to modify it to a fully floating secondary, thanks to the nice wiring layout and use of LUCAR spades and receptacles for the chassis connections. I would need to check the DC PCB for 'earth via chassis' connections though.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: June 03, 2016, 11:02:25 am
content: Video on adapting a medical isolation transformer for workshop use as an isolation transformer......Fraser

author: nctnico
author_rank: Super Contributor
timestamp: June 03, 2016, 11:20:45 am
content: IMHO you really shouldn't promote the use of an isolation transformer. Especially not as a cheap way to create a false sense of safety because there are so many things that can go wrong with using an isolation transformer especially in the hands of a novice or someone not experienced in working with lethal voltages.Lethal voltages should never be 'made' or considered 'safe to touch'. If you work on circuits with lethal voltages always use differential oscilloscope probes or a DMM with the proper CAT ratings.

author: Fraser
author_rank: Super Contributor
timestamp: June 03, 2016, 12:11:03 pm
content: Isolation transformers have been used ever since I started in electronics aged 7 and I see nothing in my comments recommending novices, or anyone else uses one to protect their lives. I state that I use one on MY bench as a safety system. I know what I am doing and working on live chassis equipment mandates using an isolation transformer as part of the service protocol issue by the OEM !I have offered no insight into the USE of such a transformer deliberately as anyone wishing to use one needs to fully understand the issues and correct procedures. There are many ways to seriously harm yourself in our business, Darwin awards will continue to be issued to those who misuse equipment.Maybe times have changed ? Isolation transformers were considered an essential element of a test and measurement bench when I was trained in Maritime elecronics. I own two differential probes for my oscilloscopes, but they do not offer me ANY protection from accidental contact with a live conductor whilst adjusting a component on a live equipment. The one hand in the pocket rule applies of course.In the UK we mandate the use of isolation transformers in bathroom applications for reasons of safety. They are a well respected safety device.I do take your point regarding novices though. At no point have I suggested that the isolation transformer detailed here totally protects them from exposure to a lethal electric shock event. Sadly a novice IS capable of making life threatening mistakes. That goes with the territory if working on utility mains equipment whilst it is connected to the supply.SO FOR ALL NOVICES ... DO NOT USE AN ISOLATION TRANSFORMER WITHOUT FIRST ENSURING YOU ARE COMPETENT TO DO SO.OK all done, we can sleep well tonightBut seriously, I take no offence at your comment. It is well meaning and justified where Novices are concerned. I accept that. But please do not condemn those of us who have been trained to use such isolation transformers for doing so. There is a whole world out there beyond low voltage electronics that mandates the use of an isolation transformer.And PLEASE, no one say that the RCCD or ELCB is all that is needed...... if you want to trust your life to one of those, go ahead. It is a safety system but it is a brave person or relies wholly upon such an protector. They work..... most of the time. You do not want to be the unlucky statistic where they do not however.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: June 03, 2016, 12:26:37 pm
content: For those interested in the UK Health and Safety Executive guidance on Electrical safety during testing and the correct installation of isolation transformers, I attach the leaflet here. It is worth noting that the transformer I have detailed DOES NOT comply with the single socket outlet rule and is so is not suitable for commercial use. In my controlled environment I may use it at my own risk however.Fraser

author: janoc
author_rank: Super Contributor
timestamp: June 03, 2016, 12:37:39 pm
content: Quote from: Fraser on June 03, 2016, 12:11:03 pmBetter save your buck and work without like real men!*ducks and runs*Seriously nice transformer, though!

author: nctnico
author_rank: Super Contributor
timestamp: June 03, 2016, 12:41:54 pm
content: Quote from: Fraser on June 03, 2016, 12:11:03 pmIn the past RCDs/GFIs where extremely expensive so an isolation transformer was a cheaper alternative. BUT it needed a carefully controlled environment like an isolated bench, isolated equipment, etc. Again: once you connect a grounded oscilloscope probe to a live circuit behind an isolation tranformer suddenly the device is ground referenced again and this situation may not be obvious to the person working on it. Things get worse when you connect another instrument which may or may not be grounded. And RCD or GFI may fail but using an isolation transformer opens up a whole can of worms.Pintek has a whole range with CAT (safety) rate differential probes:http://www.pintek.com.tw/product_classify/landersound-tail/index.php?Product_Site_Classify_SN=17072&Company_SN=6002When using one (or more) of these it remains perfectly clear what is safe to touch (your equipment) and what not (the device under test).

author: Fraser
author_rank: Super Contributor
timestamp: June 03, 2016, 01:05:05 pm
content: Fair comment.I do own a couple of differential probes and use them on SMPSU related repairs.As you say, great care must be taken when using such a transformer and i do advocate removing the safety earth from test equipment as that can cause a 'floating bench' situation which does have the potential (no pun intended) to be dangerous.I own decent multimeters and portable oscilloscopes so can effectively work with these without the need for an earth connection.As you say, the topic can be a can of worms.... no doubt about itFraser