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Isolation Transformer for Oscilloscope Testing

Paul Malloy

Paul Malloy

Active Member
Looks like it's a wise idea to have an isolation transformer to do testing on Device Under Test (DUT).

I'm just a DIY. Don't want to damage my scope/equipment or myself.

What is really needed? Go for a new one, used one, modify one or build one?

Rather build one myself. Any good building link with a transformer part number?

Paul
 
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Do you want to isolate your DUT, or the scope? The one listed on Amazon is only rated a little over an Amp, so it could power a scope, but not a big amp/receiver.
 
Keep in mind that when using the isolation transformer, the ground wire (green/yellow) is carried through, so there is no ground isolation. You get isolation from hot and neutral using the transformer.
 
drjim said:
Do you want to isolate your DUT, or the scope? The one listed on Amazon is only rated a little over an Amp, so it could power a scope, but not a big amp/receiver.
Click to expand...
I understand you need to isolate the DUT, not the scope.

I think this explains it pretty well here:

EEVblog #279 - How NOT To Blow Up Your Oscilloscope! - YouTube

What amperage is suggested for an isolation transformer? I don't have a really big amp (50 watt) but I don't want anything right at that level in case I want to test something bigger.
 
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Paul Malloy said:
I understand you need to isolate the DUT, not the scope.

What amperage is suggested? I don't have a really big amp (50 watt) but I don't want anything right at that level in case I want to test something bigger.
Click to expand...

Grounding, bonding, and isolating can be a complex problem, depending on what type of measurements you're making. If the DUT has NO power transformer, then it should be isolated for safety. I've had to isolate the test gear when making distortion measurements using the most sensitive range of my THD meter or else I couldn't get it to null properly. Could be a ground-loop problem as I was using two different AC outlets, so I'll have to look into it a bit further.
 
(I have VIZ WP-28 1.5 Amp Isolated polarized 2 prong), and will mostly be using it for '70's ish 2 prong recievers, R2R, --think Marantz, Mcintosh, Pioneer RT707, etc....

Listened to the aussie guy repeatedly and hasn't sunk in yet. Great guy that Dave.

I get it I want DUT isolated --- Using Siglent 1202XE, Siglent AWG etc ...

The Viz is like '70s TV radio repair guy stuff that the guy took on house calls -- thinking I need something bigger .. definately need a better grasp on the concept before I go lighting my eyes up though...
Thanks guys -- such an important safety topic, and Thanks to the OP for the post. :beerchug:
Sorry no pics doggone new laptop no card reader.

:lurk: Thinking cap in place and listening .... very quietly...
 
Nice having an isolating variac. I have to use two discrete units.
TBH I found a DBT more useful than the variac but for one experiment I did to test the behavior of a Kill-A-Watt I needed the variac... (wanted to see what happens if you had something plugged in that had negative wattage. Turns out, it does not seem to behave correctly.)
 
Paul Malloy said:
Sounds like you shouldn't isolate your scope from earth ground? Sound like using a differential probe is how to protect the scope?
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From the video you linked, I understand the safe way (for the oscilloscope and the person testing a device) is to keep the DUT not grounded, without any connection to earth ground (around '30 of that video)
Still I'm not 100% sure why the isolation transformer has a 3rd pin connected to chassis and earth ground.
 
elnaldo said:
From the video you linked, I understand the safe way (for the oscilloscope and the person testing a device) is to keep the DUT not grounded, without any connection to earth ground (around '30 of that video)
Still I'm not 100% sure why the isolation transformer has a 3rd pin connected to chassis and earth ground.
Click to expand...
You can have a distinct earth ground system after an isolation transformer. It isn't so much practical with the table-top transformers being discussed, but is still possible. Normally, you want all grounds on a system at the same potential. This means you don't drive a ground rod outside the window of your rack of equipment. Multiple ground rods are fine, but they must be bonded together, ideally presenting the same potential at all of them.

If you have a shed or workshop detached from your house. You could feed it through an isolation transformer and then have a separate earth ground system that doesn't have to be bonded to your house main ground rod. Picture an amateur radio station in an out-building and you want to ground your tower and isolate that from the rest of your house. However, this will put you right back in the situation requiring isolation from ground when using your scope or other test gear.
 
Honestly it depends what you are working on and what you need to measure. Exactly once I've actually needed an iso transformer on something I was working on, and that was a switch mode supply. I needed to look at something on the primary side which is just rectified mains voltage. I fed it through an iso transformer with the scope still grounded in the usual way and it worked fine.
 
gadget73 said:
Honestly it depends what you are working on and what you need to measure. Exactly once I've actually needed an iso transformer on something I was working on, and that was a switch mode supply. I needed to look at something on the primary side which is just rectified mains voltage. I fed it through an iso transformer with the scope still grounded in the usual way and it worked fine.
Click to expand...
Well.. I bought my isolation transformer to work on switching mode power supplies, but I understand, in order to be protected from electrocution (my priority is my life, not the oscilloscope) I need to leave the SMPS floating, connected with just 2 prong to the isolation transformer, without connection to earth ground. Please correct me if I'm wrong.
 
Paul Malloy said:
Sounds like you shouldn't isolate your scope from earth ground? Sound like using a differential probe is how to protect the scope?
Click to expand...
If your DUT is isolated, you don't need to protect the scope, scope will be safe as the isolation also protects it. If you can't isolate the DUT (which there are many cases, none of which should fall into home audio use), then you will have to isolate the scope, but of course this does not protect the person working with the scope because the scope chassis can/will become live. In this case an insulated scope (e.g. plastic cased handheld scopes qualify with their plastic coated BNC connectors) then you'll be fine. Note that pretty much all wall powered Digital MultiMeters are designed this way with full isolation, they have the exact same problem as with oscilloscopes.

As for grounds, you should keep them all connected to the same earth ground. If there's a problem in the DUT where it shorts against ground then you have a different issue to resolve, but it will not cause a safety issue since the current is shunted to ... ground.

That is, if your ground is sufficient. If you don't have all your grounds connected tight (and your DUT ground is leaking), then you will be back to square one. GFCI/RCD outlets may help debug/protect against this situation and you should fix this first before debugging other issues.

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Hmm... I now have a pile of ancient wall powered DMMs. Most of them are truly completely isolated, and two that are optionally isolated - supplies a binding jack in case you have to ground the DMM. With it grounded, even these DMMs have to follow the same testing safety procedures as with using a wall powered oscilloscope.
 
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