Electronics > Repair
Why do I keep blowing scope channels, when measuring ripple?!
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rwgast_lowlevellogicdesin:
Lc is only resonent in a parallel config right? The capacitor was in parallel across the dc and the inductor was in series on + side of the dc supply in an lc filter config.

Yes disconnecting the scope  when changing the circut is a safe way to make measurements like this and i wish i had done that.. but then lets say i used that circut on the output of my PSU becuase the scope made it look acceptable, i would have blown up whatever i had the psu connected too if i unplugged the board without first powering down the psu. The only way to really test that is to use those lightening arrestor or simulation? I guess fast flyback diodes?
Geoff-AU:
Oh you can definitely have a series-LC resonance too.

Parallel LC has a high impedance at resonance, series LC has a low impedance at resonance.


--- Quote from: tggzzz on July 01, 2024, 09:33:35 pm ---Vinduced = Ldi/dt. So work out the current change in the time interval.
--- End quote ---

In theory, for any circuit regardless of inductance, the voltage induced is infinite for a sudden disconnect because current goes from some non-zero value to zero instantly. 

In practice devices don't turn off immediately, a circuit that's physically opened will draw an arc (the resistance of which will act to decrease the current) etc.  Or a device will momentarily go into breakdown to close the circuit.

Whether a circuit is damaged or not really depends on how much stored energy is trying to dissipate.  If you have a 10uH inductance at <1A, the stored energy is low, it'll likely safely dissipate into various imperfections in the real devices surrounding it.  If you have 1.5mH, you start blowing things up.  Of course if you had 10uH at 1000A then you're back to some hefty inductive kicks again.

General comment regarding simulation, it's a great tool, but it's only as good as the information you put into it.  Part of 'experience' is knowing what information is relevant, and what can be skipped for expedience.


tggzzz:

--- Quote from: rwgast_lowlevellogicdesin on July 02, 2024, 03:04:55 am ---Lc is only resonent in a parallel config right? The capacitor was in parallel across the dc and the inductor was in series on + side of the dc supply in an lc filter config.

Yes disconnecting the scope  when changing the circut is a safe way to make measurements like this and i wish i had done that.. but then lets say i used that circut on the output of my PSU becuase the scope made it look acceptable, i would have blown up whatever i had the psu connected too if i unplugged the board without first powering down the psu. The only way to really test that is to use those lightening arrestor or simulation? I guess fast flyback diodes?

--- End quote ---

A cursory bit of research will indicate that parallel and series resonance exists - and is the basis for many types of filter.

You should also realise that a straight piece of wire has inductance, the rule of thumb being 1nH/mm. Hence if you have a, say, 1ft wire then you should model it as its DC resistance in series with 300nH. If it is coiled, even slightly, the inductance will be higher.

Given your moniker, I would expect you to have seen the consequences of that when using a standard "high" impedance scope probe on a logic circuit. For the basic theory and indication of consequences, see https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/

You will also note that some subsystems make a point that they can be "hot plugged" or "hot swapped", i.e. they can be inserted/removed while power is applied. That's a big hint that most subsystems don't guarantee that.

Finally, realise that a regulated PSU (i.e. any modern PSU) employs internal feedback to keep the output voltage constant as the load current varies. Different PSUs do a better or worse job, especially where the load is reactive - and all loads are reactive. Look at the PSU's specifications; if none, be suspicious.
tautech:

--- Quote from: rwgast_lowlevellogicdesin on June 30, 2024, 06:35:09 pm ---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?

--- End quote ---
Welcome to the world of Back EMF !

How high can the voltage of a collapsing field go, as high as it needs to to find a discharge path ! ! !

Disconnecting current sources while probing something is never a good idea and when inductors are involved, bad things can happen.
Some protection is available by using 10x scope probes instead of 1x BNC to grabber connections but safe measurement technique dominates.
 
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