Test equipment safety question: isolation transformer options?

[audiotubes]

I saw Dave's how not to blow up your scope video and other similar videos.

I started looking around for isolation transformers and the ones I saw are all two wire for the DUT. How can you test a device that has a 3 prong plug?

Also, isn't it possible to make an isolation transformer just by using a 1:1 transformer? I don't understand the high prices on the few units I saw.

Thank you.

[Jwillis]

The point of an Isolation transformer is to remove the earth ground so the test device or devices float separate from each other. There is no direct link between them. By adding earth ground connection they are no longer separate (isolated).
This is a very good video on the subject of isolation transformers with experimentation.Its a detailed explanation on the types and purpose.
   

[audiotubes]

Thanks, I'll look at that video tomorrow.

Can devices designed for three wire power run normally without the ground connected?

[coppercone2]

you should make sure you have a device that needs it before you use it. With alot of test equipment you can just probe to the chassis for repairs. It usually is good enough to figure out what is wrong. at least that is my experience I have yet to use one.

[theHWcave]

I put an isolation transformer in my lab https://youtu.be/HdTagUVHR8s. I technically didn't "built" the transformer, instead I bought it, I just put it into an enclosure and wired it up. Yes, they are expensive but sometimes you can find a bargain as I did. Also look on eBay for used ones, especially those yellow boxes with isolation transformers mainly  for building sites but make sure its really an isolation transformer with separated primary and secondary windings. Sometimes they sell transformers with just a single winding to get 2x115V out of 230V. You can't used those.  Next, you need to be clear how many watts you need, the price climbs steeply the more power is needed.  Normally I hang the DUT on the isolation transformer and leave the scope on normal mains (and grounded). This is safer because you usually touch the scope buttons and sockets way more than those of the DUT. Also it allows connecting USB cables/network cables to your scope which you can't if you hang it on an isolation transformer. However, if your DUT draws kilowatts, you may have no viable alternative and run it directly from mains and isolate the scope with its <100W instead.

BTW, in a crunch and not as a permanent solution, you can use two normal isolation transformers back to back, say 230Vin 12V out, connect the 12V to another transformer's 12V and you get 230V out on its primary, doubly isolated but there are some losses and of course the 12V need to be beefy, for 100W output the 12V better be able to support 10A on both transformers.

Lastly, even with isolation transformer, there are still plenty of ways to blow up the scope. My favourite one is forgetting that the 2nd, 3rd ... channel all use the same common as the 1st channel. Makes for some spectacular shorts.

To be honest, these days I am using my isolation transformer a lot less since having a 2nd battery powered scope is nowadays quite affordable and a true alternative for an isolation transformer (I am using an OWON HDS272 but there are plenty of similar ones. Something you may want to consider. Just make sure the USB cable to the charger/PC is NOT connected to the scope while measuring...
 

[themadhippy]

Also look on eBay for used ones, especially those yellow boxes with isolation transformers mainly  for building sites but make sure its really an isolation transformer with separated primary and secondary windings

yellow ones tend to be 240v in 110v output so not much use for a 1:1 isolation transformer unless you use 2 back to back

[audiotubes]

This is safer because you usually touch the scope buttons and sockets way more than those of the DUT. Also it allows connecting USB cables/network cables to your scope which you can't if you hang it on an isolation transformer. However, if your DUT draws kilowatts, you may have no viable alternative and run it directly from mains and isolate the scope with its <100W instead.

Great point, thanks

To be honest, these days I am using my isolation transformer a lot less since having a 2nd battery powered scope is nowadays quite affordable and a true alternative for an isolation transformer (I am using an OWON HDS272 but there are plenty of similar ones. Something you may want to consider. Just make sure the USB cable to the charger/PC is NOT connected to the scope while measuring...

I didn't even think about that. It's a good suggestion since I don't do much general A/C device repair. I'll look into that.

I want to look into fixing an air fryer that failed. It now shorts the mains after working ok a small number of times. It's a good excuse to buy a new gadget or two 

[coppercone2]

just FYI if you are SURE chassis is earth and the frequency and precision is not high, you really should just use chassis.

and you can make a good ground point for the probe with this
https://www.pomonaelectronics.com/products/hardware/banana-jack-6-spade-lug. Just screw it under something and keep the ground hooked up into it. or a big ass alligator clip

Like so long you are sure you got ground I don't see the damn point. I almost consider my isolation transformer a waste of money. I like building the wooden boxes for it out of scrap cabinets though.

get a GFCI outlet instead.