Electronics > Beginners

Connecting my Siglent scope ground or not...

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syl20bou:
Hi,

I studied electronics in 1980 and barely practiced since then, hence this beginner's question.

I recently watched an EEvblog Youtube video about the risk of connecting a scope probe ground to a device. I understand that the way the device relates to ground/earth has to be considered before connecting to it.

I used my new Siglent 1104x-e scope to diagnose a problem on a Frigidaire washer/dryer combo and was able to eliminate the possibility of my water level sensor to be in fault. I did so by powering the sensor alone on my bench and got a very clean square 45Hz PWM that varied as I changed the pressure in the tube (which it's supposed to do).

I measured the same pin in-circuit (inside the washer) to verify if the signal was getting to the main board properly but I did not dare connect the ground of my scope to the negative of the power supply, not knowing how it is related to the earth. The signal was present but very dirty. Now I wonder if it really is that dirty, which may explain the problem. For that, I guess I need a proper connection of the scope - i.e. with ground.

That's where I need help. I attached a picture of the board and want to get an opinion as to whether it is safe to connect my probe ground to the supply negative. I see two transformers on the board probably indicating that the supply is lifted from the ground.

What do you think? How can I validate if it's safe with my Fluke 83 first?

Thanks!

wraper:
You should connect it to ground to make a measurement. But it's only if it's isolated from mains - on secondary secondary side of PSU. If it's a primary side connected to the mains, you shall not measure it with usual probes to begin with. Peak mains voltage exceeds probe ratings, not to say it's unsafe to do so.

syl20bou:
Thanks for the quick reply wraper. I understand that.

My question is more 'how' do I verify that it is isolated from the mains?

And if I need to  'not measure it with usual probes to begin with' as you mention, then what is my alternative?

Thanks.

KrudyZ:
A sticker that says "HEATSINK LIVE" is usually a good indication that you want to be very careful around an assembly.

tautech:

--- Quote from: syl20bou on December 22, 2018, 02:46:05 pm ---Thanks for the quick reply wraper. I understand that.

My question is more 'how' do I verify that it is isolated from the mains?

And if I need to  'not measure it with usual probes to begin with' as you mention, then what is my alternative?

Thanks.

--- End quote ---
Some basic circuit block understanding helps one hell of a lot however just remembering that every part of any circuit must form a loop for it to work.
So that might indicate where to probe for the info you need you then need to find a safe reference point for your measurement and that’s where your DMM comes in....your reference point for a grounded scope must always be at the same mains ground potential, period.
So with a DMM find the different PSU rails and mark them then measure their differences to mains ground....use the BNC on the scope as the DMM reference if you must and then you can be sure a close to zero measurement will indicate it’s safe to clip the probe reference (gnd) clip.

Once you gain circuit block understanding and develop methods to find safe measurement reference points you can proceed into unknown circuits with a minimum fuss and bother.
Keep asking questions and be safe.

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