Oscilloscope connected to AC netural

[tunedwolf]

I would have to agree with Nigel on this, lifting the earth is way safer in some cases, and not so in others, just ask anyone that has touched their probe gnd accidentally while pokin around a tv chassis at half mains. That said, what I have is my earth lead cut before the plug. I made up a lead that I bolted to my chassis gnd stud on the scope with a croc clip on it to bond to the DUT, I figure if it has be at any potential, better it's all at the same potential. No matter what I'm doing, even if I'm not using the scope, I always clip it on to the chassis of the DUT. I would recommend however, that your test bench be run from an Isolation transformer heavy enough to cope with all your test gear and any DUT you'll encounter, or envisage having on it and bond all your test gear. All the test gear I have on the bench is cross bonded, but not electrically earthed because of the Isolation transformer. Happy days...

 

[Nigel Goodwin]

An unearthed scope is fine providing ti's souble insulated, but I haven't seen many of those around.

No, you don't see any (or at least I never have?), but metal case or not, un-earthed is safer in far more applications than an earthed scope. Bear in mind a scope isn't live chassis, it contains it's own isolation transformer anyway.

 

[HiTech]

isolation transformers can be pricey if you buy new, but they can be found in a number of items if you search earnestly. I scavenged a nice 4 amp xformer from a piece of discarded office equipment. I also scarfed one from an industrial machine that supplies up to 8 amps and it also has secondaries of 90v and 130v which is nice for stress testing equipment. I mounted it into a case that has an outlet and binding posts, a 3-position switch, fuse and a meter. Yje last thing I want to do is destroy a Tektronix scope or a $50 probe!

 

[jimg]

I've found a TrippLite IS-250 (250 watt) on the web for $79.

Would you say this was a resonable price to pay and would the unit be good enough?

 

[HiTech]

looks like a nice unit and Tripp Lite is a solid company. Try poking around photocopy machine vendors who often have line isolators and conditioners lying around. I have a Tripp Lite 15amp EMI/RFI line conditioner that works well. Quality stuff.

 

[Nigel Goodwin]

I've found a TrippLite IS-250 (250 watt) on the web for $79.

Would you say this was a resonable price to pay and would the unit be good enough?

I don't see how it helps your problem?.

The transformer is used to isolate the unit under test - which in your case is live mains, so presumably you can't isolate it?. If you isolate the scope, you either still earth it (so have achieved nothing), or disconnect the earth lead (which is really no different to doing so without the transformer). The scope already has an isolating transformer inside of it.

 

[jimg]

Nigel-

You are absolutely correct. There is no connection between the ac lines and any part of the scope other than into the transformer. No isolation is necessary. It was just one of those persistant ideas that won't go away. 115v will kill you just as good through an isolation transformer, so other than keeping from tripping the mains circuit breaker, the only point of using one is if you short between the output of the iso and a real ground, there shouldn't be any current flow, so a little safer to test out the circuits, not for the scope.

 

[Hero999]

Please post a schematic.

If it's one of those transfomerless power supplies envolving a rectifier, zener and capacitive or reisitive ballast then you could temporarily disconnect it and replace it with a small isolted DC power supply.

 

[jimg]

That's exactly what it is. Everything works flawlessly with an isolated dc power supply, it's when it's attached to it's normal noisy ac environment that things go sour. Currently, the problem is a 3v d-type flipflop latch triggered by a 95 ms pulse from different locations over an unused mains wire. But that's nothing to do with this thread, other than connecting the circuit to the ground through the scope, so I'll post in the other thread when I get farther along. https://www.electro-tech-online.com/threads/power-supply-for-a-touch-switch.23176/

 

[Hero999]

I think an unearthed scope is pretty dangerious because the case will float at mains potential if you're monitoring something connected directly to the mains. You should always power the circuit you're measuring from an isolation transformer, never the scope for this very reason.