Oscilloscope help

[Anthrax-VX]

alright so i screwed up somewhere, ive watched the videos on how not to blow up your oscilloscope, which i didnt do thankfully, but something else happened.  I have a 3d printer mainboard called a duet wifi, it has pwm fan control headers, i am powering the duet with a meanwell psu 250 watt, i did check for resistance between earth ground and negative and at the board, there seems to be roughly 10k ohms resistance,  i went ahead and put the fan header connected to a breadboard, and breadboard connected to fan, everything works from this point.  i used the breadboard so i can connect some leads to view the pwm signal, negative to ground on scope, and positive to scope, i verified that the negative was indeed the negative and the positive was indeed the positive.  i went to connect the negative to the scope first, and right when they connected, the mosfet popped for the fan header, where did i go wrong? do i need to connect the negative probe to the main negative input on the board? and not in between board and fan? lost and dont want to make any more mistakes, i already have to put in a new mosfet lol, luckily the rest of the board still works including the other fan headers

[alsetalokin4017]

I think so. If I am following your description you have this:
mains wall plug > Meanwell DC power supply > printer mainboard > fan header board > breadboard > fan
and you are using the breadboard just so you can connect the scope to observe the PWM signal to the fan.
I think you probably created a ground loop when you connected the scope probe "negative" (ground reference cliplead) to the PWM output, but without seeing the fan board circuit I can't be completely sure.

Remember that the scope's "negative" side, including all exposed chassis parts like the BNC connectors and especially the scope probe ground reference leads, are connected through the mains cord to the mains ground.  But the power supply's DC negative output is not normally connected to ground, unless you deliberately or accidentally connect it that way (like for instance clipping the scope probe's ground clip to it).

So I think that when you grounded one side of the PWM output with the scope probe's reference clip you may have shorted it to ground, thus overloading the mosfet. Maybe. Again, without seeing the schematic for the fan controller or your _exact_ hookup, I can't be sure.

A safe but more complicated way to look at your PWM output is to use a 2-channel differential connection without any "grounds" on the scope probes. Actually physically remove the ground clipleads (they just snap out from the probe body) so they can't dangle into something, and connect one probe to one side of the fan board output and the other probe to the other side, and tell the scope to perform the subtraction in Math.

[Anthrax-VX]

yeah i think i had a ground loop, the power supply i was using wasnt a bench power supply, i was using a meanwell switching psu that the printer uses, i dont know why i didnt use the lab bench psu but i just hooked it up exactly the same way except with the lab bench power supply and its working fine, and yeah i did measure a 10k ohm resistance between the negative rail and the earth ground, but new to all of this so wasnt sure if that was a strong enough short AND i thought as long as i was careful to connect it only to negative i would be ok, but guess not lol, thanks for the help and ill be sure to use the bench psu more often haha

[helius]

and yeah i did measure a 10k ohm resistance between the negative rail and the earth ground, but new to all of this so wasnt sure if that was a strong enough short

If two nodes have a resistance of 10 kohm between them, they are not the same potential at all. The clip of the scope probe is not the "negative probe", it is ground, the same as the ground pin of the wall socket. You did not create a "ground loop", you created a short to ground and the damage could have been total.

Consider a humble light bulb. Let's suppose it's a new, efficient model with only 12 W power usage. On 120 VAC, this means the resistance between the terminals is some 1.2 kohm. Question: what will happen if you connect the scope probe's clip to the live side during operation? The resistance of that point to ground is only a tenth of your circuit, so it must be safe, right??

[Anthrax-VX]

learning every day i suppose if you hook that scopes ground lead to the live side of a 120 ac, you would be shorting the 120v live side to ground,  I suppose i was confused / still am a bit, because of the DC part.  So i shorted the negative in the dc circuit straight to ground? thankfully the weak link was just the mosfet, all else seems to be working correctly and have since connected it to the bench psu and no issues, and i did verify there was no connection at all from the negative on the power supply to ground, and did verify by checking the chassis as it was a direct short to ground.  ive watched the videos on how not to blow up your scope but i suppose im still wrapping my head around it.  In your example i can clearly see that would be a short of 120v to ground. (unless im wrong )

[ledtester]

What were you trying to measure? Lots of battery powered multimeters can give you a readout on frequency and duty cycle of a PWM signal.

[Ian.M]

Its easy if the circuit you are testing and *ALL* external connections to it are fully floating as whatever point you connect to becomes ground.

If you are working on low voltage DC circuits and aren't 100% *CERTAIN* the point you are connecting to is true ground,  there's a lot to be said for using a simple test light between the point you wish to connect to and the scope ground, before connecting the scope ground lead.  If it lights up, its not safe to connect the scope ground there.  https://www.amazon.com/KKmoon-Circuit-Electrical-Indicator-Alligator/dp/B07RM16MSD

For checking mains circuits before connecting the scope ground, you really need a DMM with a low impedance voltage range so you can check the voltage between the point you want to connect to and true ground without being confused by stray voltages.  Check on ACV and again on DCV.  If you don't have a DMM with low impedance voltage ranges, make up an adapter with a 10K >3W wirewound resistor in parallel with the DMM jacks.  N.B. the resistor  could be overloaded if you probe the wrong node in a voltage doubling power supply, but will survive OK for the couple of seconds it takes to measure the voltage.