How to not blow up my scope? - Page 1
Forum section: electronics / beginners

author: cfrolander
author_rank: Newbie
timestamp: December 04, 2010, 07:34:11 pm
content: In my school, when DSOs, we always knew what sort of signal we were expecting when we hooked up. Now I'm looking into reading the signals off of an automotive instrument connector. I'm expecting to see two hall effect sensors, but honestly have no idea what sort of voltages those might put out and spike to. How can I go hooking my scope up (Rigol 1052e) to this without having to worry about torching the scope? Also, how much can these scopes read safely?

author: Simon
author_rank: Unknown
timestamp: December 04, 2010, 07:51:08 pm
content: max is 400 V with the 10X probe, but you should have like 3000V catII input protection (as 300V for the scope X 10 on the probe

author: Unknown
author_rank: Unknown
timestamp: December 04, 2010, 08:16:24 pm
content: Quote from: Simon on December 04, 2010, 07:51:08 pmNot sure what you mean with 'should have'. The max input voltage is the minimum of the probe spec and the scope spec times attenuation factor. So for a 300V scope with 400V x10 probe, the max voltage is 400V, not 3000V (not sure about CAT rating, I would expect the lowest CAT rating to apply). If you exceed the rated probe voltage, the attenuation hybrid (simplest version would be resistor with cap in parallel) could short, which would put the full voltage on the scope input, and subsequently kill the input circuit.If you expect higher voltages, get probes with a higher rated input voltage (eg. x100 probes, like Tek P5100). Also keep in mind that the voltage rating is derated with frequency, a 300V probe won't be able to resist 300V at 100MHz.Quote from: cfrolander on December 04, 2010, 07:34:11 pmMake sure the voltage is well below safe max. For DC or low frequency, you could check it with a DMM, which is usually more robust (although you should also observe the rated maximum). No idea about automotive electronics, so I can't help you with what to expect. There are some high voltages in the ignition circuit, but I wouldn't expect those in instrumentation connectors.Quote from: cfrolander on December 04, 2010, 07:34:11 pmSafe voltages are usually printed on the equipment or in the documentation. There should also be a frequency derating curve in there, too.

author: tyblu
author_rank: Frequent Contributor
timestamp: December 04, 2010, 08:39:18 pm
content: Make sure that either the Rigol is floating (on batteries) or the car is grounded with it. They can float up to a few kV static.

author: saturation
author_rank: Super Contributor
timestamp: December 04, 2010, 11:03:17 pm
content: Fearing what alm describes, I wouldn't risk a $400 CAT I scope, the Rigol 1052E, measuring an unknown circuit. Car alternators do have well regulated DC output, and the spark plug discharges [ kV output] are isolated from the rest of the auto, as I know and have seen using my handheld scope; but what if it isn't? The Rigol is rated for the types of voltages expected in automobile work, but it maybe safer to get an instrument with more margin for higher voltage transients, such as a CAT 2 device or more.This is one situation to get a DMM-scope like the Uni-T or the Velleman, which are CAT II, and are battery operated, so there is no risk of having high ground potentials, as tyblu suggests. A 1 MHz Velleman cost ~ $120 and the 12 MHz version has been on sale at tequipment.net for $200 for many months; its a low cost way to explore an unknown electrical situation before risking a better scope. The Velleman cheapie has a reputation for use as low cost automobile scope.

author: Unknown
author_rank: Unknown
timestamp: December 04, 2010, 11:30:11 pm
content: I would be careful using floating scopes for potentially dangerous ground potentials, unless careful attention was paid to isolation (wouldn't trust Uni-T or Velleman without checking). People have been killed while using floating scopes with dangerous voltages, even professionals that knew what they were doing and had taken precautions.Ever wondered why Fluke uses those weird isolated BNC jacks? Floating scopes should be designed to protect the user from touching any 'grounded' parts. Manuals for many mains-powered scopes state that it should be grounded with a separate grounding wire when powered from batteries, this would include the Rigol if it had proper documentation (no idea if it's actually in there).Grounded parts on your car are unlikely to be dangerous, otherwise you wouldn't be able to safely touch the metal body either. But battery power is no magic bullet for safety. Using a cheap piece of equipment to check for damaging signals is a fine solution as long as it's not dangerous to the user, and it has enough bandwidth to record any dangerous transients.

author: saturation
author_rank: Super Contributor
timestamp: December 05, 2010, 12:31:58 am
content: I agree, there is always issues of the potential between all floating leads and each lead and earth ground. Maybe I should qualify 'unknown' circuit and limit it to equipment as risk for CAT I transients, such as this auto application.As example a Fluke scopemeter, were explicitly defined are the voltage limits between inputs and ground, both in voltage and frequency.Here's a manual and its listed under safety limitations:http://www.google.com/url?sa=t&source=web&cd=1&sqi=2&ved=0CBsQFjAA&url=http%3A%2F%2Fsupport.fluke.com%2Ffind-sales%2FDownload%2FAsset%2F1629083_6120_ENG_F_W.PDF&rct=j&q=fluke%20scopemeter%20manual%20pdf&ei=h9b6TIuuKoOclgfPopimDA&usg=AFQjCNHpH6aLaKDw4fCglWfg7SS43fym3g&cad=rjaAlas, neither the Rigol or Velleman is as thorough. Yes, battery power is not a panacea, but it should fare better than mains powered instruments; I don't think either device explicates it level of isolation but in the end the Rigol specifies its application only for CAT I 300 V devices and is TUV certified; the Velleman claims CAT II 600V but is not certified.Quote from: alm on December 04, 2010, 11:30:11 pm

author: Psi
author_rank: Super Contributor
timestamp: December 05, 2010, 10:47:48 am
content: hehe yeah, i learned that the hard way.Needed to measure the waveform across components in a circuit that was directly connected to the mains and couldnt be isolated as that was likely to change the waveform i wanted to measure.So i put the isolating transformer on the scope instead.It worked fine, until i had the scope probe ground clip on a component that was at 230v ac and a grounded object touched the scope caseBlew up quite a bit of the scopes horizontal and vertical drive section.Lucky it was an old 50mhz CRO made with opamps and 4000 series logic gates on a single layer pcbFound a circuit diagram on the net and after replacing 8 transistors it worked againSo yeah, if ya need an isolated scope, dont cut corners, get a proper one.

author: saturation
author_rank: Super Contributor
timestamp: December 05, 2010, 10:52:55 am
content: Thanks psi, yes, that's a good story of ground potential differences. Glad you were able to repair it.Quote from: Psi on December 05, 2010, 10:47:48 am

author: Unknown
author_rank: Unknown
timestamp: December 05, 2010, 02:04:46 pm
content: Quote from: saturation on December 05, 2010, 12:31:58 amIndeed, it explicitly states that both the voltage between probe tip and reference lead and reference lead and 'earth ground' (actually just whatever the potential the person holding it is at) are 1000V CAT II / 600V CAT III.Quote from: saturation on December 05, 2010, 12:31:58 amThis is for the probe tip, I can't find any spec for the reference lead (BNC shell). They do state that isolated probes should be used for voltages above 30V, but I believe the ground isolation of these probes is only something like 50V (can't find anything about it in the spec sheet, so I would assume the worst).Battery power makes it less likely that you kill the scope / device under test by shorting something, but for personal safety, it's usually worse, since you're often at ground potential. Grounded parts will always be safe to touch as long as you're grounded, floating parts can be connected to any potential in the device under test, even without your knowledge (fault somewhere). Fluke mitigated this risk by protecting you from any parts connected to the circuit (like in a DMM), but I'm not sure if the cheaper ones do. The Velleman appears to use normal uninsulated BNC connectors, which suggests they expect the shell to be at a safe potential.Quote from: Psi on December 05, 2010, 10:47:48 amGlad you were OK! Detailed schematics don't exist, and many parts are proprietary or not available.

author: TopherTheME
author_rank: Regular Contributor
timestamp: December 05, 2010, 05:18:20 pm
content: If you're worried about high voltage then just probe the connectors first with a DMM. If the voltages are low enough and you have you're scope grounded to the chassis and or negative battery terminal you should be fine.

author: saturation
author_rank: Super Contributor
timestamp: January 10, 2011, 07:55:16 pm
content: Given this some thought, and as you've suggested, for voltages above 30V given the unknowns with the Velleman, switched to using Rigol 2200 leads that are at least CAT II rated [ which I presume are TUV rated!]In the end I think its either your safety, or the safety of the device, should a high voltage penetrate a device that isn't suitably isolated, and isn't grounded.Quote from: alm on December 05, 2010, 02:04:46 pm

author: crnazvijezda
author_rank: Contributor
timestamp: October 16, 2013, 04:13:15 pm
content: Hi all,I'm newbie in oscilloscopes and have one simple question. I put this question also on Dave's videoand still waiting answer so maybe you can help.If I put oscilloscope to work on UPS it is not in same circle with power network.Is? there also chance to make some kind of short circuit and blow my scope?I think maybe it will be cheaper because I looking to save it when my 14 years old son working on it.Thanks in advance for answer!Tomy

author: Unknown
author_rank: Unknown
timestamp: October 16, 2013, 05:13:47 pm
content: A UPS will not generally isolate equipment from ground. If you unplug it and rely on its internal battery, then you are defeating the safety ground connection and floating the scope.Floating a general purpose oscilloscope is unsafe. It may save your scope, but it may not save your son. Operator safety should take precedence over equipment. In general you're not likely to blow up the scope unless you're working with high energy circuits. The ground connection is generally fairly beefy. It's usually the DUT that gets damaged by shorting it.

author: Simon
author_rank: Unknown
timestamp: October 16, 2013, 05:52:36 pm
content: Quote from: crnazvijezda on October 16, 2013, 04:13:15 pmif the scope comes into contact with a high voltage on it's ground side (which is not grounded any more) someone can get a shock. if you need ground isolation because you are measuring a floating voltage one way to do it is using both channels and using the math function of the scope to get the difference between the two. I did this recently and it worked great. You may get a bit more noise on the pseudo 3rd channel as you have the noise from both real channels but it's a great work around. DO NOT apply any voltage to either probe that is too high for the scope to handle with reference to earth although in theory it could work I won't vouch for it.

author: crnazvijezda
author_rank: Contributor
timestamp: October 17, 2013, 03:37:23 pm
content: Hi Simon,Thanks for answer!I'm not sure did you understand my question or do I understand your answerSo, if I measuring circuit as voltage regulator connected to power network with my oscilloscopeconnected also on same power network. There is chance that I can connect ground from osc probeto some line which is oposite from ground (example on power phase) and in this case it is short circuitid ground from probe is same as ground from oscilloscope wire to power network.This can be case with voltage regulator with transformer 220V/3.3V so no high voltage.So, question is what will be if I put oscilloscope on UPS so osc power will be separated (of course UPS disconnected from powernetwork) from voltage regulator connected on power network.Is there also change to bur something?Thanks in advance!Tomy

author: Simon
author_rank: Unknown
timestamp: October 17, 2013, 04:35:38 pm
content: you scopes ground terminal on the probe is connected to earth, so if connecting it to any point will cause a short you can't and need to use the method I described. classical example is trying to look at the mains, connecting your scopes earth side to one of the mains wires could cause a big bang and kill you if you touch it.

author: crnazvijezda
author_rank: Contributor
timestamp: October 17, 2013, 07:11:28 pm
content: I get it and understand!Thank you Simon! Thank you alm!All the best!Tomy