Toroid ISB060W Isolation Transformer Modifications - Page 1
Forum section: electronics / testgear

author: Housedad
author_rank: Frequent Contributor
timestamp: April 26, 2017, 01:00:48 am
content: I ordered a Toroid ISB060W ISB-060W Medical Grade Isolation Transformer off of Ebay and it arrived today. It needs the ground modified to be able to safely use it for DUT on the electrical workbench. It is a 600VA transformer. The toroid makes it lighter and smaller than other types like the Tripp-Lite.All that has to be done to this unit is to remove the grounds from the outlets. The neutral is already not tied to ground in this model. Most models out there from various manufacturers have the neutral tied to ground. 2 c14 to 5-15R adapters are on their way so I can use regular cords with the unit. The ground wire coming out of the toroid just grounds the transformer core.After it was done, The grounds and neutrals are electrically isolatedPic 1: Business side of unit.Pic 2: Specs stickerPic 3: Opened, top viewPic 4: Opened, side viewPic 5: Ground soldered at outlet. No need to unsolder them here.Pic 6: Ground attachment at case. The top two on the left stud are the outlet grounds.Pic 7: Outlet grounds disconnected, shrink wrapped and zip tied to another wirePic 8: Some MOV's on the input line

author: mmagin
author_rank: Frequent Contributor
timestamp: April 26, 2017, 03:04:32 am
content: I got a somewhat similar looking unit, except the transformer seems to have been potted, all the connections have a healthy dose of hot glue, and it has inrush current limiters with a time-delay relay to short them after a couple seconds.Also mine was riveted closed, though one set of rivets got sheared off due to poor packing and shipment!While I like the secondary line/neutral entirely floating for test bench purposes, I'm not so keen on lifting the ground as a matter of course. If I really want to do that I use a special cord with the ground cut.

author: Housedad
author_rank: Frequent Contributor
timestamp: April 26, 2017, 03:17:41 am
content: If you keep the ground on the secondary, then what good would the unit be for bench testing? Leaving the ground connected to the DUT defeats the purpose of the isolation transformer on the bench.

author: helius
author_rank: Super Contributor
timestamp: April 26, 2017, 03:47:22 am
content: Like many things, it depends. You might have isolated the P/S of the DUT while the signal section is grounded by way of external cables anyway, so lifting that section's ground is pointless. You need to be familiar with the equipment to know whether any scenario is safe, which is one of the limitations of the isolation transformer as a repair tool that you just have to accept.

author: pascal_sweden
author_rank: Super Contributor
timestamp: April 26, 2017, 08:49:09 am
content: Maybe a stupid question, but why doesn't the manufacturer implement your modification already from in the factory, if it is such a better improvement?It is hard to believe for me, that you can't buy something off-the-shelf with the proper isolation.Or are all the manufacturers stupid, and they don't really know what they are doing, in such a way,that end users have to do modifications to their devices?Really confusing for me at least.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 26, 2017, 08:52:50 am
content: Thanks for the tear down pictures.Interesting, that some wires have crimped terminals plugged in and others are soldered.I would have expected all to be crimped terminals in a medical rated device.

author: Housedad
author_rank: Frequent Contributor
timestamp: April 26, 2017, 02:23:42 pm
content: Quote from: Shock on April 26, 2017, 02:11:00 pmThey call it an isolation transformer, but it seems the real confusion is about what you want to isolate. Technicians want the ground isolated out, while medical and audiophiles want the surges and spikes isolated out. This unit that I bought is only going to be used for bench work, so I don't want the grounds at all.

author: Shock
author_rank: Super Contributor
timestamp: April 26, 2017, 02:37:35 pm
content: That grounds for dismissal.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 26, 2017, 04:21:15 pm
content: Housedad, why do you want to remove the ground lead? What scenario are you thinking of where having the ground removed is a good thing?I can't think of a single instance where removing the ground lead makes sense. However, removing the ground from one side of the AC line makes perfect sense. That's what your isolation transformer is designed to do.There are a few message threads that talk about isolation transformers. Have you read them?Ed

author: nctnico
author_rank: Super Contributor
timestamp: April 26, 2017, 04:26:11 pm
content: Quote from: edpalmer42 on April 26, 2017, 04:21:15 pmProbably not otherwise he would not have waster his money and effort. In short: isolation transformers are a bad idea in an environment which is not setup for that purpose. Better get a differential probe.

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2017, 05:31:47 pm
content: Quote from: Housedad on April 26, 2017, 03:17:41 amThe *neutral* is still isolated from ground. Usually the problem is the ground loop through neutral back to the panel where neutral and ground are connected.Quote from: Housedad on April 26, 2017, 02:23:42 pmAnd I want the neutral, or hot if they happened to be reversed, isolated from ground.

author: mmagin
author_rank: Frequent Contributor
timestamp: April 26, 2017, 05:42:24 pm
content: Quote from: Shock on April 26, 2017, 02:11:00 pmI suspect part of it in the medical application is to avoid the secondary neutral being above ground potential in cases where the primary neutral goes above ground due to wiring faults or other unusual situations. Also (in the US), the sort of outlets these are plugged into generally provide a ground that is insulated from the metal box/conduit that might feed it.

author: Housedad
author_rank: Frequent Contributor
timestamp: April 26, 2017, 06:21:12 pm
content: The last thing I want is a ground reference working on tube radios and power supplies. If you guys want to have one, then enjoy but make sure your medical insurance is up to date and you have your affairs in order first.

author: bitseeker
author_rank: Super Contributor
timestamp: April 26, 2017, 06:59:58 pm
content: Quote from: Housedad on April 26, 2017, 01:00:48 amI remember an earlier photo that you posted of this unit that showed a notice that the secondary neutral was not grounded. That's interesting since, as you said, most have neutral and ground tied together on the secondary. This is the first one I've seen that was specifically built this way.Quote from: Housedad on April 26, 2017, 02:23:42 pmYes, there are different levels of isolation depending on what you need. The type of DUT, nature of faults, etc. all have a bearing on what may or may not be safe using any particular isolation/configuration.Adding a switch to enable/disable only the ground connection on the secondary side (i.e., neutral and ground still isolated) can be useful for verifying DUT grounding after repairs and before going fully live.

author: _Wim_
author_rank: Super Contributor
timestamp: April 26, 2017, 07:14:01 pm
content: Quote from: nctnico on April 26, 2017, 04:26:11 pmA differential probe protects you fromprobingthe "wrong" part, an isolation transformer protects you fromaccidently touchingthe "wrong" part. So it is best to use both together.

author: nctnico
author_rank: Super Contributor
timestamp: April 26, 2017, 11:45:28 pm
content: Quote from: _Wim_ on April 26, 2017, 07:14:01 pmThe error in that reasoning is that in a 'lab' which is not setup for using an isolation transformer it is very easy to make the DUT earthed again without the protection offered by a chassis ground and GFI.

author: _Wim_
author_rank: Super Contributor
timestamp: April 29, 2017, 02:20:32 pm
content: Quote from: nctnico on April 26, 2017, 11:45:28 pmWhy do you think the GFI would not work anymore? A GFI just checks the difference in current between hot and neutral, and if a current would find a "new" route via your body to ground, it will still trip just as it would when no isolation transformer was used.When you touch both hot and neutral at the same time, a GFI will not offer any protection, but that is also when no isolation transformer was used (that why they say "always measure with one hand in your pocket", so you cannot touch both hot and neutral at the same time...)

author: nctnico
author_rank: Super Contributor
timestamp: April 29, 2017, 02:32:13 pm
content: Quote from: _Wim_ on April 29, 2017, 02:20:32 pmDraw the schematic and show the current flow! Hint: you are dead wrong here.

author: _Wim_
author_rank: Super Contributor
timestamp: April 29, 2017, 02:40:04 pm
content: You are right, should have tought this more though! So I agree that is a disadvantage then when using an isolation transformer

author: technogeeky
author_rank: Frequent Contributor
timestamp: June 02, 2017, 02:53:56 am
content: Quote from: _Wim_ on April 29, 2017, 02:40:04 pmPlease, one of you two explain the correct reasoning so that we can benefit.

author: djnz
author_rank: Regular Contributor
timestamp: June 02, 2017, 05:02:17 am
content: Quote from: technogeeky on June 02, 2017, 02:53:56 amA GFI works by comparing the current in the live/'hot' and the neutral wires and if they are not the same amount, it deduces that someone is getting shocked (as that forms an alternate path, stealing some of the current).With an isolation transformer, when if you are getting a shock on the secondary side, the primary side still has balanced currents in the live and neutral wires, and because the residential GFI / RCD sees only the currents on the primary side it won't trip.

author: Electro Detective
author_rank: Super Contributor
timestamp: June 02, 2017, 05:26:59 am
content: and in all/most? cases still won't trip at the primary end even if the secondary neutral is tied to earth/ground and chassis too.Then of course the transformer is no longer a true isolation transformer.

author: technogeeky
author_rank: Frequent Contributor
timestamp: June 02, 2017, 03:46:07 pm
content: Quote from: djnz on June 02, 2017, 05:02:17 amOh! That's where I was mistaken. I thought you guys were discussing the merits of putting a GFCI outlet on the secondary side of the isolation transformer. And/or how that's related to grounding the secondary or not.So, how about those things?

author: nctnico
author_rank: Super Contributor
timestamp: June 02, 2017, 05:42:00 pm
content: If you ground the secondary side of an isolation transformer you can use a GFI but also skip the isolation transformer. IOW: forget about isolation transformers and get a differential probe (which you only connect/disconnect when the device is powered down).

author: Electro Detective
author_rank: Super Contributor
timestamp: June 02, 2017, 11:16:32 pm
content: With the correct understanding of how transformers and incoming mains power works,you can rig up an isolation transformer to give you isolation and safety to any degree you desireBut you have to modify and LABEL it accordingly,make up some adapter cables for certain tasks and LABEL those,and ensure the transformer is not left in a half assed 'jerry rigged' stateand have a means to EASILY default back to 'medical isolation' mode, when it's not used to isolate DUTs,so it will therefore work with GFCI/RCDs on the secondary and primary sides again etc in case someone uses it when you're not in the lab or mancave,(or in the event you cash out in the future, and an unaware 'new player' grandson/daughter joins you in the ground before their time)i.e. if you're not 1000% sure of all the possible wiring variations, and the good, bad and ugly possible with some of them, then take the advice of many at EEVblogand buy adifferential probeonce, so you don't have to always think twice(doctors, hospitals, and undertakers have enough work to do, and they don't come cheap)