Isolation transformer refit - worth proceeding? - Page 1
Forum section: electronics / repair

author: skyhawk
author_rank: Contributor
timestamp: April 28, 2015, 12:58:55 am
content: Hi guys,Perthis threadI got myself an oldHammond 171Eisolation transformer off Kijiji, intending to use it as a lab isolator for protecting myself and my shiny new oscilloscope. This unit differs from the unit I linked to in that it has no switch or over-current protection device, and has a duplex receptacle on the secondary side rather than a short tail.I just got the thing apart intending to figure out where and how to mount a breaker and switch, and holy oh my god this thing looks and feels crusty. Prior to disassembly it buzzed like a bastard (but did work) when plugged in. I can't find a manufacture date anywhere on it. I suspect it may be older than I am. The lamination material is hard as rock, there's no give and I feel like I'm going to start breaking pieces off every time I touch it. Based on the amount of paint that came off I'm the first person to have disassembled this thing since it left the factory.I have a feeling I should write this thing off as a mistake rather than spend even a single additional penny on it. I'm looking for opinions from people more familiar with the construction and failure modes of transformers than I am. What do you think?

author: tautech
author_rank: Super Contributor
timestamp: April 28, 2015, 01:17:45 am
content: If the lamination/cover bolts were loose it might hum louder. But they do hum anyway.Looks just like a fine old school transformer. Handle gently.If you know an electrician, get a leakage test done with a Megger or insulation tester....just for piece of mind.Consider a surface mount box for the socket and switching.Then you might have sufficent space also for a "push button" type breaker.

author: nctnico
author_rank: Super Contributor
timestamp: April 28, 2015, 03:22:24 am
content: Don't use an isolation transformer! There are too many things which can go wrong (thinking you are safe is one of them) and create an even more dangerous situation.Spend money on a proper differential probe and a sensitive GFI.

author: richard.cs
author_rank: Super Contributor
timestamp: April 28, 2015, 07:28:12 am
content: The transformer looks fine, they last pretty much forever if they're not overloaded. I'd be inclined to add overcurrent protection to the output but other than that I'd leave it as it is.Do make sure you understand what an isolation transformer is and isn't good for, in particular that once you hang a grounded scope off your DUT you've lost isolation again. That's normal and fine just don't go expecting it to be safe to touch.

author: dom0
author_rank: Super Contributor
timestamp: April 28, 2015, 08:00:03 am
content: Quote from: nctnico on April 28, 2015, 03:22:24 amThis forum actually has aignore user function. Engineering marvel!

author: nctnico
author_rank: Super Contributor
timestamp: April 28, 2015, 09:35:33 pm
content: Quote from: dom0 on April 28, 2015, 08:00:03 amWell go ahead and kill yourself in absence of life saving advice!Quote from: richard.cs on April 28, 2015, 07:28:12 amThat's normal and fine just don't go expecting it to be safe to touch

author: richard.cs
author_rank: Super Contributor
timestamp: April 29, 2015, 06:41:30 am
content: An isolation transformer allows you to take a smps where the 0V reference for the circuit (the bridge rectifier output) would normally bounce up and down by the full mains voltage and tie that point to ground. Having done so it becomes possible to make all kinds of measurements that are otherwise difficult to do. Changing the ground reference is a useful thing to do but doesn't improve safety - so what? That doesn't make the isolation transformer valueless.An isolation transformer can also be used to improve safety in a fully floating system if you are careful to ensure it stays floating. It's safer because two faults rather than one are necessary to pass a dangerous current through the user. That's an entirely different use and also perfectly legitimate.My point was that beginners often conflate these two uses which are fundementally incompatible. You can't do the first one and expect to see the safety benefits of the second. Indeed you don't get any safety benefits from the second one without being very careful in what you connect to it because the "first fault" doesn't have any obvious effect.

author: dmmartindale
author_rank: Contributor
timestamp: March 23, 2025, 12:36:36 am
content: I think the Hammond 171 series isolation transformers have been sold for many decades. I've got a couple that were installed in old arcade video games, probably from the 1970s or 1980s. They seem to be conservatively rated, specified to work at both 50 and 60 Hz, and should last nearly forever if not abused.The designs have changed over time, though. Sufficiently old versions have only a 2-wire power cord and 2-wire outlet. More recent ones (like yours and mine) have 3-wire cords and outlets. Both older versions have no fuse or circuit breaker protection (which seems odd). And they have no outer case. The iron core is exposed, and the transformer end covers contain the electrical connections. There is very little room inside the end covers, probably not enough to mount a fuse holder or a switch. The most 171 series is still sold, and current versions do have an input circuit breaker, but this version adds an extra metal box between the end cover and the transformer core in order to provide enough room for that circuit breaker. Hammond also switched from a receptacle in the end cover to a pigtail at some point, sometime after mine and yours were made.The ground connections are a bit odd too. Other isolation transformers I own do have an outer case, and they have a ground stud or bolt inside the case where input ground wire, output receptacle ground, transformer electrostatic shield, and the case are all connected together. The Hammond transformer does have an electrostatic shield, but the winding is connected to the core via a wire that is sandwiched between two laminations. The input cord ground is connected to a bolt in the input cover, while the output receptacle ground is connected to a bolt in the output end cover. There is no direct connection between input and output ground, or between either of these and electrostatic shield ground. The frame bolts that hold the end covers in place have insulating washers under their heads, so that should not be the connection between the two end covers. Both covers are in contact with the core, but the core laminations should have insulating varnish on them to suppress eddy currents.It is strange that yours buzzes loudly. Mine hum quietly when unloaded, and will get only a bit warm if left connected to AC but unloaded for several hours. Magnetizing current for my 300 VA units is 275 mA with no load. (Yours is rated 500 VA, so expect something less than double my values). If yours buzzes loudly, gets hot without load, or draws far more magnetizing current than expected, it may have an internal short somewhere.(Yes, I know the question is 10 years old now. But these transformers can last for many decades, so I hope this may be useful to someone someday).