Electronics > Repair
Why do I keep blowing scope channels, when measuring ripple?!
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rwgast_lowlevellogicdesin:
I had a 25mhz Siglent DSO I got second hand back in 2012 or so, it was my first real scope. I blew channel 1 testing the ripple of a PSU, i don't remember the exact circumstances, I don't even think I knew the channel was broken till a day or two later. The signal had some weird attenuation issue. At the time I just ordered a new Siglent SDS1104x-e in 2016ish. Later I took the 25mhz scope apart and there was a blown resistor in the front end. I think I had shorted the power supply I was testing with the probe in 1x mode. Shouldn't the 10Mohm impendence stop the scope from drawing any of the current if the psu gets shorted?

Last night I blew out channel 1 of my SDS1104x-e, I was testing some custom filtering on  the output of a cheap switched lab PSU. The PSU was loaded down to about 720ma @ 5V. The scope was in 1x mode, and I was using a BNC to alligator clip lead across the load resistor. I got most of the noise out of the thing down from 70mv PK to PK to 12mv PK to PK. I was messing around though and changed the last inductor in the filter(the inductor is a series inductor on the + side of the DC output right before the load) from a 10microHenery to a 1500microHenery I wound on a #26 core years ago. It killed all of the noise on the wave form except a slower periodic ripple that looked to be bjt/fet overshoot, I looked away for a second and then looked back and my trace was gone.... I reset the scope and re caled it, and there is still no trace on channel 1. I can see the trace if I couple the channel to ground, but it disappears when using DC or AC coupling. I can also see it briefly if I scroll the voltage range knob quickly but then it disappears in a split second. Im pretty sure I must have blown a resistor or clamping diode in the front end use to bias the input. Now I may have accidently shorted the PSU output for a second and didnt realize it, but is there anyway that big inductor could have blown the channel out?

Im pretty sure I can open the thing up and find the problem in the front end section and replace the 0202 passives or whatever, but ive had some other issues that have made me want to upgrade to a new 1104x hd or 2204x hd. The thing is I don't want to spend that kind of money on a scope and blow the front end up again.. im not sure why i keep blowing front ends and why aren't they equipped with fuses, signal integrity?
Wallace Gasiewicz:
Never, Never, Never use 1X probe. throw it away.    Use 10X.     
There are very few uses for 1X. You have just discoverer the most common use.
rwgast_lowlevellogicdesin:
Well i was only using a 1x becuase #1 clips were more convient, but #2 i watched a video about ripple and common mode noise and i remmber dave saying you want to make the measurements at 1x.

It looks like i ended up blowing a jfet and a few uhf transistors, not sure if thats it since i dont have a scope to trace the signal path.
J-R:
The scope is rated for 400V pk-pk so 1X seems reasonable, but maybe not once you factor in a voltage spike from the inductor.
Ian.M:
+1
Momentarily mechanically interrupt a DC circuit that has a large inductor, and you can expect a transient of several kV.  The scope input breakdown voltage is lower than that so all the energy got dumped into the scope's input circuit with predictable results.

I'd suggest getting a couple of these https://www.amazon.com/Coaxial-Lightning-Arrestor-Protector-Protection/dp/B07WLNL76F
and using one on each scope input whenever you are messing with PSUs and inductors.  Their nom. breakdown voltage is 90V, (min. 75V) so if the DC rail is above approx. 35V (to maintain a factor of two margin), use a x10 probe, not a x1 one, or use an appropriately rated external DC blocking capacitor.
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