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Old 16th October 2011, 08:47 AM   #1
akis is offline akis  United Kingdom
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Default Oscilloscope probes earthed?

My oscilloscope probes as well as my tone generator's "common" are both earthed!It makes it very hard to measure floating values as well as inadvertently creating shorts!

Is that supposed to be so?
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Old 16th October 2011, 08:50 AM   #2
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some equipment has an earth lift . see if it has one
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Old 16th October 2011, 09:00 AM   #3
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Yes,the test equipment is supposed to be earthed.
Get an isolation transformer to use on the Device-Under-Test.
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Old 16th October 2011, 12:14 PM   #4
Frex is offline Frex  France
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Hello akis,

For measuring a floating voltage, you can use differential probes.
It is more secure than remove the earth of your equipment that remove also
your own protection if an issue occur on it..

Using an isolation transformer can avoid some problems, but not if you want
to measure something connected directly on the ac outlet !

Regards.


FRex
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Old 16th October 2011, 12:57 PM   #5
akis is offline akis  United Kingdom
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Hi. My tone generator's common is earthed, so as soon as I connect it to the circuit under test the circuit's own common becomes earthed too. The oscilloscope is earthed which as you said makes dangerous to try to measure AC from mains transformer's secondary windings on PSUs which may use earth as common too.

I will just learn to be careful now, do not attach the shield of the oscilloscope probe unless I am sure of what I am doing.
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Old 16th October 2011, 01:20 PM   #6
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You can measure floating voltages using Differential Mode with your 'scope so long as you remain within the limits of your 'scope inputs.

Set the two input channels to the same gain/attenuation and invert one channel, then use the Add function.
The voltage you then want to view is only between the two tips of your test probes.

This method is also described as 'Common Mode Rejection' and there is a useful description of this in the Manual for the Tek 7A18A dual input plug-in on page 2-8:
http://bama.edebris.com/download/tek.../tek-7a18a.pdf
Probably also described well elsewhere.

Same principles for any 'scope with these functions available for the input channels, but be aware that you need 2 input channels to view one signal in this mode.
Very useful for looking at either Balanced line signals or Bridged Amplifier Outputs etc where the wanted signal is not directly referenced to a common ground......

hope this assists
Mik
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Old 16th October 2011, 02:14 PM   #7
Pano is offline Pano  United States
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Yes, the two probe technique works well. I have to use it often with bridged Class-D amps.
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Old 23rd October 2011, 04:08 PM   #8
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Mik,
I had been wondering the same thing, I am at the same level as akin with my testing skills it would be nice to have the moderators add a test procedure thread and add your advice as a sticky.

Thanks
Bill
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Old 26th October 2011, 04:06 PM   #9
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Please be aware that you will still need a Mains Isolation Transformer if you want to look at the AC Input side of Power Supplies for example!
Mik
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Old 27th October 2011, 04:17 PM   #10
cbdb is offline cbdb  Canada
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Please be aware that you will still need a Mains Isolation Transformer if you want to look at the AC Input side of Power Supplies for example!
Mik
Why? I dont think so, the 2 probe method still works.

NEVER LIFT YOUR GROUND (on anything) A lot of oscilloscope chassis are tied to ground so if you lift it and put a single probe across the amp rails, and touch the scope case, poof your DEAD! 50volts is more than enough to kill you!
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