What happens if a short goes into the oscilloscope? - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: August 10, 2020, 01:49:50 pm
content: I watched a bunch of youtube videos on oscilloscope safety. Here is one situation i am not confident regarding my understanding of what would happen in this scenario.- The Measurement source is a battery powered device which has a fixed known resistance and is outputting about 32w of between 6-7 volts.- The measurement source occasionally shorts out.- The device will blow a fuse before exceeding 10A if it shorts out.I would be putting the scope probes on the two outputs of the battery powered device i am measuring.Can this even be safe to run into an oscilloscope, or should I stick to a DMM with logging features?I have a Siglent 1052DL+.

author: atmfjstc
author_rank: Regular Contributor
timestamp: August 10, 2020, 02:10:33 pm
content: A short circuit in your device will not affect the scope. Any large currents will go through the short itself, not the oscilloscope, which basically appears as a very high value resistor. The only way the oscilloscope will ever be in danger is if the short somehow causes the outputs to exceed the maximum input voltage tolerated by the scope. Although if your device is battery powered as you say, the output voltage will never exceed the battery voltage barring some super exotic situations.

author: tggzzz
author_rank: Super Contributor
timestamp: August 10, 2020, 03:26:46 pm
content: There will be inductance in the wires connecting carrying the 4-5A current. In the absence of better information, assume 1nH/mm of wire, i.e. 1m -> 1µH.If that current is interrupted there will be a voltage induced across the inductor of V=Ldi/dt.Ifthe current changes by 5A in 1ns in 1m of wire, that implies there will be an induced voltage of 1e-6 * 5 / 1e-9 = 5kV. That would be sufficient to damage a scope's input!Obviously you can choose those figures to make any point you want

author: brucehoult
author_rank: Super Contributor
timestamp: August 10, 2020, 08:38:07 pm
content: Quote from: tggzzz on August 10, 2020, 03:26:46 pmWouldn't the inductance itself work to prevent such a rapid change in current?

author: tggzzz
author_rank: Super Contributor
timestamp: August 10, 2020, 08:46:54 pm
content: Quote from: brucehoult on August 10, 2020, 08:38:07 pmYes. The induced voltage is "an attempt" to prevent the rapid crrent change.

author: Mp3
author_rank: Frequent Contributor
timestamp: August 10, 2020, 11:51:49 pm
content: Quote from: tggzzz on August 10, 2020, 08:46:54 pmThank you for that.Where I am stuck now, is knowing my scope has a maximum input voltage of 400V be it AC or DC at 1 megohm on a 10x probe.These are the ideas I can think of to deal with this situation.... Is either #1 or #2 valid or is my only option #3 to guarantee scope safety?1) Start shopping for a differential probe.2) Be very strict about sending my probes through an isolation transformer (I have an audio isolation transformer that is quite gigantic and heavy)3) Not use an oscilloscope at all, and use a logging DMM instead

author: vk6zgo
author_rank: Super Contributor
timestamp: August 11, 2020, 12:31:28 am
content: Unless you are probing the device output & there is a huge length of wire between where the short is & that point, the induced voltage from a real world short won't amount to anything of concern.If the short is internal to the device, it will be so close to where your probe is attached, the inductance will be miniscule.Even with 1 metre between you & the short, the chances of it happening in 1ns are so remote that if it did happen, my suggestion is to go & buy a Lotto ticket, as you obviously have an affinity for statistically unlikely events.

author: tautech
author_rank: Super Contributor
timestamp: August 11, 2020, 12:49:50 am
content: Quote from: Mp3 on August 10, 2020, 11:51:49 pmNot quite.Your scope has 400V rated inputs directly into the BNC.Probe attenuation lifts the max V applicable to the probe tip which for a 10x probe theoretically lifts it to 4KV although I know of zero 10x probes that are rated that high where 600V is more common.For the sake of safety it's a good idea to limit 10x probes to a max of 1/2 their rating.For a battery powered device under test (DUT) without any electrical connection to mains ground you can connect a single probe and reference lead most anywhere however when using 2 probes they must share the same reference point.Always keep in your mind probe reference leads are connected to mains ground (BNC shells) and decide on where to put their clips so to not cause shorts known as ground loops.Checking before connection with a DMM is good practice until it becomes obvious where it's safe to connect or not.Take it slow and steady and always ask if unsure.

author: ledtester
author_rank: Super Contributor
timestamp: August 11, 2020, 01:10:38 am
content: Quote from: tggzzz on August 10, 2020, 03:26:46 pmIs simply high voltage enough to cause damage? Doesn't there need to be sufficient energy behind that potential?

author: bdunham7
author_rank: Super Contributor
timestamp: August 11, 2020, 02:27:17 am
content: Quote from: Mp3 on August 10, 2020, 11:51:49 pmNone of those are necessary. Things get a bit ridiculous some times and I'm not sure who is serious, but don't get overly concerned about nothing. You said you had a battery powered device putting out 32 watts and 6-7 volts, fused at 10 amps. Without knowing what your device is, I'm 99.999% sure that even if it powered by a car battery and unless it has an ignition coil or Tesla coil involved, there's no way it will damage your scope even with a 1X (direct) probe no matter what happens. If you tell us exactly what the device is and what is in it maybe I can be 100% sure.Just make sure that if you use more than one probe, they all are grounded at the same potential and preferably at the same spot. Or, as I recommend for beginners, take all the ground clips off except one when you use multiple probes.

author: bdunham7
author_rank: Super Contributor
timestamp: August 11, 2020, 02:41:18 am
content: Quote from: tggzzz on August 10, 2020, 03:26:46 pmThen dV/dt would be 5*10^12 V/sec. Quite an extraordinary pulse. But even a Dirac function has a definite amount of energy, in this case 12.5 uJ, so...QuoteNo, it wouldn't.QuoteWhy not use a picosecond? Then you have 5M volts and nobody is safe.

author: tggzzz
author_rank: Super Contributor
timestamp: August 11, 2020, 07:28:38 am
content: Quote from: Mp3 on August 10, 2020, 11:51:49 pmWe can't answer that because we don't know what you are trying to achieve, and only a little about how you might go about achieving it.Have you, for example, taken into account the scope's voltage measurement specifications?Is the "occasional shorting out" part of the test? (If not, shouldn't you avoid that?)Every piece of equipment has input voltage limitations.Some of the other calculations mentioned are as dodgy as the one I mentioned! Having said that, I agree the induced voltage probably wouldn't damage the scope - but is it worth the risk? Certainly I'm very careful when putting any equipment on any high energy source, for some of the reasons others have mentioned.If you could use a DMM, then presumably you aren't interested in high frequencies. In that case a simple low-pass filter would be sufficient to limit the peak voltage visible to a scope. You could even add a voltage suppressor. Tell us your problem/goal, as well your solution. In most walks of life, technical or non-technical, if we know the reason for the question you will probably get a better answer. Either the answer will be more pertinent to your needs, or perhaps it can suggest a better alternative that you haven’t even considered. Don’t ask “Can you give me a lift into town?” Do ask “Can you give me a lift into town, so I can replace my broken frobnitz?”. The answer might be “There’s a spare frobnitz in the attic”, thus saving time, money, the environment – as well as making some space in the attic.

author: Mp3
author_rank: Frequent Contributor
timestamp: August 11, 2020, 10:48:45 am
content: It is a simple custom made battery charge pack to put out a specific voltage. I'm trying to get an idea of when the shorts occur and under what conditions. It'll require me to mess around with the device and TRY to get the shorts to happen.I suppose I could use a zener diode as a voltage suppressor, but as bdunham7 said, I can't possibly imagine anything is getting screwed up with a fused output to prevent any ridiculous amounts of current from hitting the scope inputs.I'm not interested in high frequencies, so I may as well save myself the trouble and use a logging DMM. All I want to do is keep an eye on when the shorts occur without needing to stare at my DMM screen the entire time.I guess, once again, I don't have a use for my oscilloscope, which I bought 2 years ago and never once used since anything i want to do with it is apparently too dangerous for it. If I'm being trolled by tggzzz, then great, you kept the oscilloscope on my shelf for another day.

author: tggzzz
author_rank: Super Contributor
timestamp: August 11, 2020, 11:09:48 am
content: Quote from: Mp3 on August 11, 2020, 10:48:45 amSo, there is a lot of stored energy. Have you considered that the shorts might be inside the battery?Lots of stored energy can be a problem if it is released uncontrollably, e.g. with the Boeing 787-8 batteries orhttps://spectrum.ieee.org/energywise/energy/batteries-storage/dispute-erupts-over-what-sparked-an-explosive-lion-energy-storage-accidentI rather doubt your device is on the scale of that in Arizona, but if someone with Boeing's resources and expertise have problems, that should raise questions in your mind.Realise that amateurs think about things mightwork. Professionals also think about how things mightfail.QuoteNobody has mentioned anything about current hitting scope inputs.QuoteNow you are just being offensive, and a twat.That kind of comment is unlikely to encourage people to spend time trying to help you.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 12, 2020, 05:38:14 am
content: Quote from: tggzzz on August 11, 2020, 11:09:48 amYour original answer comes across as just that--------trolling!The suggested time of 1ns is ridiculous in any practical situation, & you haven't adressed the possibility that to such high frequencies, the shorted leads may appear as a transmission line, not an inductor.

author: SilverSolder
author_rank: Super Contributor
timestamp: August 12, 2020, 06:47:01 pm
content: Quote from: Mp3 on August 11, 2020, 10:48:45 amPersonally, I would use the scope with the x10 probe, and a nice slow sweep speed so one can see a history of what happens in the circuit over about a minute while you are messing around. I think it is likely to show useful information.The risks being mentioned are real, but in the same sense as there is a real chance that you may die in a car crash on the way to the supermarket. Does that stop you going to the supermarket...There is a non zero risk of breaking your instrument on almost any circuit, if you try hard enough. Like there is a non zero risk of coming a gutsa on your 1,000cc motorcycle as you open the throttles wide and go through the gears. Does that stop the motorcyclist from twisting his little pig's ear as hard as he can?

author: Mp3
author_rank: Frequent Contributor
timestamp: August 12, 2020, 11:01:00 pm
content: Thank you for putting it in perspective,SilverSolder.Quote from: tggzzz on August 11, 2020, 11:09:48 amIf that was your idea of "help", then no I don't want it, and I don't think anybody posting in the Beginners section for a legitimate reason wants "help" like that either.Keep your trolling to the other boards.

author: EmanuileGesuster
author_rank: Newbie
timestamp: December 09, 2022, 06:45:48 am
content: Quote from: vk6zgo on August 11, 2020, 12:31:28 amProne to statistically unlikely events) Well said, he can buy a lottery ticket of course, but it is better to go straight to thelightning roulette lucky numbersgame, and there try your luck. Maybe he'll get an electric shock after the last bet and lightning will give him a million-dollar jackpot. A man worries about electricity without realizing what voltage, current and how it works. If you don't know physics, the world will seem like a miracle

author: EPAIII
author_rank: Super Contributor
timestamp: December 10, 2022, 10:39:34 am
content: My thought exactly. There is going to be capacitance as well as resistance in the whole device/probe/scope circuit. The inductance may very well try to generate a high Voltage spike for a nano second, but the capacitance will try to swamp it by taking time to charge and the resistance will assist in that effort.I have used scopes for over 55 years and never seen the input of one damaged by a spike. Actually, I have never seen the input of one damaged at all. PERIOD! I am sure it happens sometimes, but I have worked with more than one idiot so those times must be VERY rare. You are probably more likely to damage a scope by tripping over it's AC cord.Quote from: ledtester on August 11, 2020, 01:10:38 am

author: tggzzz
author_rank: Super Contributor
timestamp: December 10, 2022, 10:51:48 am
content: Quote from: EPAIII on December 10, 2022, 10:39:34 amWhy not remove the adjectives and insert some numbers by doing a quick spice simulation?QuoteIt is easy to damage scopes (or spectrum analysers) with a 50ohm input. It is less easy to damage 1Mohm/20pF inputs, but it can be done.Damage does not necessarily mean complete failure; it can result in parametric shifts.I also note a key paragraph that ledtester chose to omit in order to make his point...Quote from: tggzzz on August 10, 2020, 03:26:46 pmQuote

author: radiolistener
author_rank: Super Contributor
timestamp: December 10, 2022, 10:55:33 am
content: Quote from: Mp3 on August 10, 2020, 01:49:50 pmIt depends on the circuit under the test. In most case, for a usual low Voltage electronic devices, it will be safe. But if it consists a large inductor, high Voltage transformer, powerful motor, solenoid and other things which can produce high Voltage spike, it can damage your scope easily with a high Voltage pulse.For example, if you try to measure output of a car distributor which is powered from 12 V battery, you can easily damage your oscilloscope. The same thing if you try to measure electric shocker output, despite the fact it is powered from 4 V and consume about 1-2 Amps, it can damage your oscilloscope very easy.And it doesn't matter if you're using 1 MΩ or 50 Ω input mode. In both cases you can damage oscilloscope. But for 50 Ω you can damage it more easy, because 50 Ω mode input is much more sensitive for a high Voltage.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 10, 2022, 12:27:12 pm
content: Quote from: EPAIII on December 10, 2022, 10:39:34 amI am one such an idiot, & I still couldn't damage the Oscilloscope!Many years ago, we were trying to find why there was no output from the PA of a MF AM radio transmitter.Aha! thought I, using my "up all night" brain:- "We can hang a x10 'scope probe off the output tube grid connection, & wind the RF drive right down to minimum, ('to be safe') & see if there is drive there!"No sooner said than done!-----I clipped the probe on, closed the Tx door & turned it on.There was a blinding flash, & all of the probe forward of the guard ring was vapourised."Ohhh, Bugger!"Checking the 'scope, it was OK, the probe having given up its life in its stead.At that point, we had a coffee, & looked closer at the book.The drive control didn't reduce drive to a "very low level", as I had surmised---there was still plenty there to kill the probe!Anyhow, we had now proven we had drive, so could troubleshoot the PA stage further.One thing I learnt from this,----my brain isn't at its best at 2am!

author: radiolistener
author_rank: Super Contributor
timestamp: December 10, 2022, 01:45:04 pm
content: Quote from: EPAIII on December 10, 2022, 10:39:34 amI remember, one time, I burned out input of my DMM from battery powered device. Just wanted to check Voltage, I expected it is something like 500-800 V, but I was wrong and it has about 7-10 kV, so my DMM input was burned out...Oscilloscope input is often burned out when someone connect it directly to a radio transmitter output. It has just about 50-100 V peak, but frequency can be more than 10...30 MHz.For 50 Ω input the things are much worse, you can burn out it even with a battery powered handheld walky-talkySo, be careful...Quote from: vk6zgo on December 10, 2022, 12:27:12 pmUsually in such case oscilloscope probe just release magic smoke. It seems that your transmitter has pretty high power...

author: vk6zgo
author_rank: Super Contributor
timestamp: December 10, 2022, 02:10:08 pm
content: Quote from: radiolistener on December 10, 2022, 01:45:04 pm5kW MF Broadcast transmitter---also, it wasn't the 200ohm Transmtter output, it was across a parallel resonant circuit, so the voltage across it would be substantially higher.As I said, it was in the very early morning, following an already busy day, so my brainpower was at a low ebb.