(Uber-edit - it has come to my attention that I am an ignoramus. This post is being left in place not to highlight my stupidity in particular, but to serve as a warning for others who think transformers are just about winding some wires around each other right? See the separate chat discussion with @periblepsis that explored the depth of my ignorance and taught me stuff!)

I am planning to build a transformer to isolate the device under test for my oscilloscope, so I don't accidentally destroy it. But it occurred to me that it might also help with mains hum in audio processing. Do you think I will see any benefits there?

(edit - also want to point out I'm not planning to use this as an everyday appliance. Only when I specifically want to test something, or maybe do some audio work (if it helps with mains hum). So yes I know it's not super safe, and I'll need to keep an eye on it. But do you think it will work at all? I am probably gonna try it anyway, so what are your predictions?)

The other question I wanted to ask was about my design, because I have a couple concerns. (editing list after reasonable concerns have been raised)

I'm basically gonna use (edit) 6 things (all of which I already have at home)

  1. A box
  2. Standard outlets
  3. A piece of I-beam
  4. A big spool of high-gauge plastic coated copper wire
  5. A temperature sensor with display
  6. 2 Fans for cross ventilation

Here's my design. This is very much a bushcraft kind of design and I understand that, and if you tell me this is a really bad idea I won't do it, but I think it will probably work.

I have a piece of I-beam from a construction job that's cut to the size of a large transformer. I'm going to use that as the core of the transformer, and double-wrap this spool of wire in a 1-1 input/output configuration to give me an isolation transformer from mains.

The box will be fitted with two fans, intake and outtake, to circulate air. In addition I will nuke the project entirely if I test the wire under load and find that it gets too hot.

I know that I will not get very good efficiency using self-wrapped plastic coated wire. I'm OK with losses. My main concerns are temperature and efficiency. I'll of course test it out and make sure it doesn't overheat. But will the losses be so big that I should just not even try it?

If/when I do it I will update this post with some measured results. I'm interested in feedback and success/failure predictions. Thanks in advance for any positive or even negative feedback!

  • \$\begingroup\$ @All - The long comment chain here has become a discussion and has exceeded what is reasonable for comments. Therefore comments have been moved to chat & should be continued there. Here is the chatroom link. -- As this bulk moving of comments to chat can only be done once per question, any further comments posted here might be deleted without notice. Keep it in chat now, please! \$\endgroup\$
    – SamGibson
    Commented May 21 at 19:52
  • \$\begingroup\$ I was wrong about everything. This question doesn't even make electro-magnetical sense. An I-beam piece? I swear I wasn't trolling. Anyway this project was doomed to failure so I'm glad I didn't attempt it. But I think I still might, now that I have learned something. I will document this if and when it actually happens. For now we can all be content in the knowledge that I am not going to attempt to build this Frankenstein's monster. StackExchange won one. \$\endgroup\$
    – neural
    Commented May 21 at 20:45
  • \$\begingroup\$ @neural You can also consider using a battery powered scope. These are safe. \$\endgroup\$ Commented May 22 at 12:40
  • \$\begingroup\$ I own a mains powered scope \$\endgroup\$
    – neural
    Commented May 22 at 18:17

2 Answers 2


Usually isolating the scope is the worst idea ever. When people measure something that is dangerous like some device with unisolated mains using a scope, they forget that the scope metal parts and other probes and circuits they connect to become also live and dangerous.

A far better idea is to isolate the measured device, not the scope.

Also whether or not isolating the scope is the right solution to audio hum depends on why the hum is there to begin with. Perhaps the solution is something else, like just avoiding ground loops when measuring, or using different kind of power supplies etc.

Also the plan fails to show any calculations for the plan. You can't simply have an idea to wind up some wire over some metal bar and connect it to mains and expect it to not blow up the fuse, melt the wires, or otherwise being dangerous. So it's not about being efficient, it's about how you need to do it to begin with so it is done even approximately right.

Strong safety suggestion is to not try it without first doing some numbers like how many turns of wire you need. The calcultions will also show if the idea is possible or not with the materials you have.

  • \$\begingroup\$ Yes, I understand that the device under test needs to be isolated. But I don't have an isolation transformer. \$\endgroup\$
    – neural
    Commented May 21 at 19:16
  • \$\begingroup\$ As far as calculations go, I think 1-1 is pretty straightforward, no? I'm going to calculate the amount I can wind with the length of wire I have and use all of it. I estimate I can get about 10-20 turns on each side, something like that. And as far as safety goes, I plan to put it through some tests and see how it performs. I don't need it to provide a lot of power, I'm not going to be using it for my refrigerator, but for small appliances like my laptop and any things I want to test with my scope \$\endgroup\$
    – neural
    Commented May 21 at 19:18
  • \$\begingroup\$ Btw - edited my OP to make it clear I understand that the device under test is the one that needs isolating. Point is I need an isolation transformer. I'm not trying to make you look silly in your reply, sorry to retroactively edit like that, but I realize that my original wording was ambiguous \$\endgroup\$
    – neural
    Commented May 21 at 19:26
  • 1
    \$\begingroup\$ @neural 10 to 20 turns is not going to work. It will blow up your fuse. Have you ever taken a look at mains transformers - they have hundreds if not thousands of turns. Also your laptop already has an isolated power supply so since it is safe to use the laptop without getting an electrical shock, it is also safe to connect your oscilloscope to the laptop directly. Isolating the laptop mains with an isolation transformer achieves nothing. Unless you are repairing the power supply itself. \$\endgroup\$
    – Justme
    Commented May 21 at 20:05
  • \$\begingroup\$ I've been convinced that I am so far off in space that I shouldn't even attempt this launch. Thanks for being one of the people talking me off the cliff. I need to learn more and come back later. \$\endgroup\$
    – neural
    Commented May 21 at 20:36

I am planning to build a transformer to isolate the device under test for my oscilloscope, so I don't accidentally destroy it.

Something is amiss if you think you'll destroy a device under test with an oscilloscope. These concerns arise when working on circuits with dangerous voltages, like the primary sides of switching power supplies, and you're not qualified to do any of it anyway. And then you are worrying about destroying the device under test, the oscilloscope, and potentially destroying yourself as well - and the last should be the primary concern. Otherwise, isolation won't save you.

If you'll be shorting close-by pins with the tip of the scope probe and damaging things that way - also known as "the probe slip" - the isolation transformer is not a concern, as the current flows through the probe tip alone and the probe doesn't need to be even plugged into a scope to do the presumptive damage.

I'm not sure what sort of application you have in mind where a scope could damage the device under test where an isolation transformer would actually help. Nothing comes to my mind, frankly. I've been using scopes since I was in elementary school, and my own kids are now older than that, and it simply was never a concern. I highly doubt that you need that. When I needed to measure primary or otherwise high-voltage circuits in power supplies, I used isolated differential probes, current probes (inherently isolated), or high-voltage scope probes.

it occurred to me that it might also help with mains hum in audio processing

Audio isolation transformers, as well as fiber-optic audio isolators are there for this use.

I'm OK with losses. My main concerns are temperature and efficiency.

If you need to actively cool your transformer, you better be working in electric power distribution/power utility, otherwise you're doing something seriously wild.

You can't use a solid piece of construction iron as a core for an AC transformer. It'll work well for an electromagnet that doesn't turn on/off too often (much less often than several times per second!). But as soon as you wind a transformer on such a core and put 50 or 60Hz into the winding, there will be serious eddy current heating of the core, and the "transformer" will be mostly an inductive core heater with a bit of transformer action tossed in as an afterthought.

AC transformers, as well as audio transformers, must not use large conductive volumes in the core. For fairly low frequencies such as you have in mains use (50-60Hz), making the core from isolated sheets of steel (called laminations) is sufficient to keep eddy current losses minimal. For higher frequencies, cores that use powdered materials with insulating binder are needed, or poorly conductive cores made out of ferromagnetic ceramics.

  • 1
    \$\begingroup\$ It's the oscilloscope I'm worried about. If you test a device connected to mains with a scope, you can ruin the scope if you put the probes in the wrong spot. \$\endgroup\$
    – neural
    Commented May 21 at 22:21
  • \$\begingroup\$ @neural The only devices where this is a real concern are switchmode power supplies. In transformer-powered audio gear, the transformer primary side is well separated from everything else and you’d be hard pressed to probe mains accidentally. Otherwise, high power audio amps have high voltages on the secondary and isolation won’t do anything there. You should be using non-switchable 10x scope probes rated for mains and then you won’t damage anything at all. DO NOT use switchable x1/x10 probes. And don’t connect ground clip to anything that’s not at earth potential. \$\endgroup\$ Commented May 22 at 12:43
  • \$\begingroup\$ The ground clip probably should stay at the ground star point, and you may want to use a differential probe anyway if unsure. Much easier to deal with than the dangers of misapplied isolation transformers. \$\endgroup\$ Commented May 22 at 12:44

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