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Old 16th June 2014, 04:28 PM   #1
Ripcord is offline Ripcord  United States
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Default Oscilloscope Question

I finished a bi-polar power supply and would like to look at it on the scope. I've attached a diagram of the basic concept. I'm confused about the probe ground clip. Since I'm going through a transformer, the power transformer in this case, is it safe to clip the probe ground clip to the 0v ground, or CT, of the power supply. Is the power supply floating? It confuses me. Thanks
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Old 16th June 2014, 04:34 PM   #2
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This used to be a common problem with live TV chassis. The way we got around it was to disconnect the earth tang in the power cord to the oscilloscope.

NOW, THIS IS NOT GENERALLY A SAFE RECOMMENDATION. BUT, if you are working on equipment with 0V raised away from mains Earth it becomes a necessity. It is OK with Class 2 scopes which have plastic cases but can be dangerous with ancient metal bodied equipment.
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Old 16th June 2014, 04:42 PM   #3
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Yes its safe. Your transformer provides isolation. Note that if the 0v point marked on your outputs really is connected to mains earth (and it would be normal to do so in many cases) then the scope ground lead can only be connected to this 0v point. There's no safety issue there, just that it would put a short across the rails if you clipped the ground lead to either the pos or neg rail (which is in some cases a valid test mode depending on what you are looking at. If that 0v point is "grounded" then the supply is still termed as "isolated" because of the transformer but technically its not "fully floating".
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Old 16th June 2014, 04:50 PM   #4
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I bought a mains isolation transformer for this.
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Old 16th June 2014, 05:00 PM   #5
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Quote:
Originally Posted by Mooly View Post
Note that if the 0v point marked on your outputs really is connected to mains earth (and it would be normal to do so in many cases)
As a rule, do you usually connect the variac output ground to the power supply ground when testing or just leave it hanging?(safely).
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Old 16th June 2014, 05:03 PM   #6
Mooly is offline Mooly  United Kingdom
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I think the op will be getting confused with all this

Important points...make sure the scope is wired as per the manufacturers recommend with regard to its mains lead (that is, is correctly grounded).

Realise that the ground clip of the scope probe is connected directly to mains earth.

So following on from that, if you are working on a low voltage power supply such as in post #1, appreciate that if the 0v line is connected to mains ground, then your scope lead has the ability to short out anything that is not this 0v point. For example, touching the scope ground clip on the pos rail will make a big bang... but there is no safety or shock hazard.
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Old 16th June 2014, 05:06 PM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Ripcord View Post
As a rule, do you usually connect the variac output ground to the power supply ground when testing or just leave it hanging?(safely).
The variac should be grounded if it has a mains safety ground.

Whether you connect the mains ground to your power supply 0v line is up to you. There is no safety consideration in either case.

In audio, connecting the 0v line to ground may cause a ground loop but that depends on what else you connect to your audio circuitry.
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Old 16th June 2014, 05:09 PM   #8
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A Variac is an auto transformer.
The Mains Input is directly connected to the Variac Output.
The output from a Variac is NOT Isolated.
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Old 16th June 2014, 05:19 PM   #9
AndrewT is offline AndrewT  Scotland
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Default Test Bench Isolation

If you want to measure at elevated voltages, then you must fit and use an Isolation Transformer.

The scope and all your equipment is fixed either by direct connection or via your hands/body to Mains EARTH.

The scope being Earthed will drap any isolated voltage down to Earth potential as soon as you connect the Earth Clip.

This makes all the Earthed parts of the test circuit safe to touch. You, your hands and your body are at Earth potential. The scope clip and the wiring/circuit the clip is connected to are also connected to Earth potential.

The Isolation Transformer allows the test circuit to float at a voltage that is independent of Mains Voltage, except for a little bit of capacitive coupling from pri to sec of the isolation transformer.

The elevated voltages are no longer directly connected to Mains. The Isolating transformer is doing it's job.

BUT, there can still be dangerous voltages in the test circuit. Valve/Tube gear typically have voltages exceeding 200Vdc across components.
Don't touch these HV parts with hands or conducting tools !!!!!!!

I am going to open a new Thread where the "expert Membership" can give SAFE instructions on how to build an Isolated Test Bench.
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Old 16th June 2014, 05:26 PM   #10
Ripcord is offline Ripcord  United States
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I'm beginning to become a whole lot less confused . Thanks you guys.
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