Will an isolation transformer have a noticeable effect on mains hum? Also critique my DIY transformer idea

Question

(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!

Answer 1

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.

Answer 2

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.