Connecting my Siglent scope ground or not...

[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]

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.

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.

[rrinker]

 Also, as much fun as it is to play with new toys - see if you can't find some sort of servicing guide - they tend to shove them in behind the control panels on washers and dryers. My GE still had it - which provided a schematic of the circuit as well as a block diagram and some troubleshooting information - there were several LEDs on the board which provided diagnostic blinks which were listed in the manual so I ended up not using so much as a multimeter to fix mine. There are several appliance repair sites online that in addition to having the repair parts, have the manuals as well.
 I know, I probably just totally ruined the fun of finding the fault when all you end up having to do is watch an LED blink. On the bright side, there's no chance of it blowing up your nice new scope, or yourself, in the process.

[james_s]

That looks like a switching power supply on there with a transformer that provides isolation, but there is also some obviously live stuff on there like those BIG bulk filter capacitors. I would be extremely careful trying to do any sort of live testing with a scope, a power board like this is a dangerous place for a beginner to be poking around.

There may well be a schematic stashed behind the control panel, but it's extremely likely that any kind of circuit board will be a "black box" on the schematic.

[rrinker]

 True. I had to completely replace the motor on mine even though there was absolutely nothing wrong with the motor - the actual fault was the "braking" resistor was fried. Since this was considers a 'safety' issue, it wouldn't go into fast spin with the defect. Board was not removable from the motor and assembled as a unit there was zero chance of gaining access to just replace the failed resistor. Freaking joke how everything is made disposable these days. Don't recall the value of the resistor but I think it was maybe a 10 watt rating - so what, $1 for the part, maybe $2 - instead I had to spend quite a bit more for a whole new motor with PCB.

[james_s]

I think I would have found a way to get it apart, or reach in there somehow and tack on wires to connect an external resistor. It's rare that I encounter something assembled such that it keeps me out, even epoxy potted bricks I've dug into and repaired.

[syl20bou]

Thanks again to all for your answers. I was able to validate with my DMM that there was near zero AC and DC difference between the earth connection and the 5V supply negative lead on the board.

Then I did what I originally intended to, which is look at how noisy the 5v was. And it looks indeed very noisy to me. Attached pictures. I suspect that such noisy supply can be the source of my problem.

What do you think?

[tautech]

Rail noise is only a problem when it’s enough to go below logic low and trigger a low state. Also, use the blue ‘Print’ button so screen shots capture the scope menu settings.
Further, the reference lead can act as an antenna and provide waveforms that aren’t really there so sometimes it’s better to use the reference spring instead of the reference lead.

[GeoffreyF]

Using a DMM, measure the voltage between the Scope ground and the ground of the device you are testing.  Consider a connection between the frame of what you are testing and the equipment ground for the scope (after using the DMM to assure this is safe).  The consequence of not assuring this is ruining a scope probe or the scope itself.  In addition to safety, you want a consistent ground for your measurements.

[soldar]

... 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.

You need to connect both or any information is worthless. Here is how I would go about this and all you need is a resistance meter.

First, confirm the earth from the wall plug is good. It should be good but I have seen cases where it was not connected just floating. Measure resistance between the ground in the outlet and a pipe or other grounded metal object and you should get a very low reading. (The house I am presently in was missing a connection and all the upstairs grounds were not actually connected to ground, just floating.)

With the washer disconnected from the mains and the board plugged in measure the resistance between the negative of the board and the earth-frame of the washer. Chances are they are connected together and that is your answer.  If all readings between the board and the frame are in the many megaohm range then the board is floating. I think this is unlikely in a washer, but it is possible.

If the board has its own PSU connected to the mains then you need to be extremely careful while working on it.

Even then if it has a SMPS chances are the output negative is grounded. In this case I would cover or protect the input part so there is no chance of touching anything there by mistake.