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max input vltage

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max input vltage

Postby iamamp360 » Mon Dec 17, 2012 6:18 am

I just purchased a 3206B with standard probes. The input range of the scope is +/- 20V. The manual states specifically not to exceed 100 volts or risk damage. it also states it's not intended to read "mains" power voltages. The probes are selectable X1 and X10 and are rated to 600VDC.
115VAC(rms) is +/- 163V peak. A probe switched to X10 would present +/- 16.3Vpeak to the oscilloscope input. I don't understand what the problem with this would be.
Please explain.
Al Paulson
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Re: max input vltage

Postby PeterF » Wed Dec 19, 2012 3:17 pm

I must make this very clear, it is very dangerous to attempt to measure the AC mains voltage directly using the standard x1/x10 probes.
Firstly, the ground lead of the probe is directly connected, through the probe, scope & USB lead, to the chassis of your laptop or PC. A desktop PC or a laptop being charged is connected to mains voltage earth. When the ground lead of your probe is connected to one of your mains voltage terminals, you have a 50/50 chance of connecting to the live terminal in which case you are shorting mains to ground through your scope & PC. Most likely, after a large bang and flash, you will have a dead scope, PC and possibly operator! Even if you only connect the ground lead to the neutral terminal, that frequently has several volts on it with respect to mains earth, enough to draw a large current and still do some damage.
Do not try to get round this "Shorting to ground" by running your Laptop on batteries with no connection to the mains charger as the full mains voltage will appear on any exposed metal connection on the laptop, with the potential of shock.

The only way to safely (& legally) measure mains voltages is to use either transformer isolation or an active differential probe:-
Note that this advice also applies to benchtop oscilloscopes.

Please contact us again if you have any other questions.
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