Maximum Scope Voltage - Page 1
Forum section: electronics / beginners

author: DigitalAura
author_rank: Contributor
timestamp: October 09, 2018, 02:45:21 pm
content: I have recently changed my scope from a Rigol DS1070z to a fully unlocked Keysight DSOX1102G and I've noticed the maximum input voltage is 150v, whereas on the Rigol it was 300v.Sorry if this is a really dumb question but am I correct in assuming that only applies when using a probe attentuation of 1x? I only ask as I have a 220v inverter I wish to test and figured it best I ask on here first before turning my expensive new toy into garbage. Would it be okay just switching the probe into 10x or would I need to use a high voltage probe for this?Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: October 09, 2018, 03:01:56 pm
content: I wouldn't trust a switchable probe for that - one moment's inattention or a buildup of conductive dust in the switch and its bye-bye expensive new toy.Also scope probes have to be derated for voltage with frequency - usually their rating is flat up to a few KHz or tens of Khz then drops off exponentially with respect to log(freq). This is especially nasty in a HV chopper circuit, where there may be high frequency high energy ringing on edges, so if the inverter boosts to a HV bus then chops that, its almost certainly unsafe to scope the switching nodes of the boost section or output H-bridge. If it uses a line frequency output transformer, its a lot safer - but personally I wouldn't probe the secondary side with anything less than a CAT II rated differential HV probe.

author: DigitalAura
author_rank: Contributor
timestamp: October 09, 2018, 03:12:08 pm
content: Thanks for that, I knew I'd find a use for my differential probe sometime.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2018, 03:13:44 pm
content: Quote from: DigitalAura on October 09, 2018, 02:45:21 pmFor either scope you need to use an isolated differential HV probe. Realise that the BNC connector's shield (and hence probe ground) is connected directly to the protective mains earth.Never ever disconnect the scope's protective mains earth; "floating the scope" can belethaland can (more or less) subtly damage the scope. And also give unreliable measurements due to the high capacitance!Apart from that...Sooner or later a *1/*10 probewillbe in the *1 position.Check your scope probe carefully: the maximum voltage is a function of frequency, and often falls to 30V at high frequencies.

author: DigitalAura
author_rank: Contributor
timestamp: October 09, 2018, 03:19:37 pm
content: I'd never float the scopes earth as I have an isolation transformer. Thanks anyway, that's a lesson I made sure I understood quite some time ago. I was planning on powering the inverter from either a battery or a bench supply so I figured earth wouldn't be a problem.

author: drussell
author_rank: Super Contributor
timestamp: October 09, 2018, 03:29:58 pm
content: Yes, check the probe's rated voltage, regardless of whether it is X10 or even X100 the rated voltage on a typical probe is not going to magically make it suitable for 1500 or 15000 volts on your 150 volt rated input.A high voltage differential probe intended for this kind of work is best when working on higher voltage power circuits. If youmustwork with just resistive dividers, be really, really careful where you put that ground!

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 09, 2018, 03:30:40 pm
content: Quote from: tggzzz on October 09, 2018, 03:13:44 pmThat is exactly what happened to a tech where I work. And the damage was so extensive, that the scope was totaled.Fortunately, it was an older, well worn scope which was slated for replacement.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2018, 03:44:36 pm
content: Quote from: DigitalAura on October 09, 2018, 03:19:37 pmI thinkyouhave got the message (unlike others elsewhere!), but any isolation transformer should be on the DUT.Even with everything isolated or battery powered, mistakes are easy and common-mode capacitance/interference can be a problem. An isolated HV differential probe provides considerable scope (ho ho ho) for safety and better measurements. There's good information inhttp://in.tek.com/dl/3AW_19134_2_MR_Letter.pdf

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2018, 05:18:17 pm
content: Wow, that is really low. General purpose oscilloscope inputs are usually designed to handle at least a peak voltage from 240VAC or 120VAC with a voltage doubler which is 340 volts so usually the rating is 400 volts peak.A common x10 probe should *not* be relied on raise the maximum input voltage rating because when AC coupling is used, the entire DC voltage at the probe tip is applied to the oscilloscope input. Neither should a common x100 probe be used for the same reason.A high voltage differential probe is probably the best thing to use to avoid the input voltage limitation. There are some very rare suitable x10 "high voltage" probes but I do not know of any currently manufactured ones and they would cost almost as much as a high voltage differential probe anyway. These can be identified by having an input resistance lower than 10 megohms which indicates that they have an internal shunt resistance.

author: BravoV
author_rank: Super Contributor
timestamp: October 09, 2018, 05:28:20 pm
content: Rather than risking your life, your new scope and also your equipment, just a HV differential probes as its not very expensive these days, and for sure its much safer than using an isolation transformer.Since can afford a Keysight DSOX1102G, pretty sure a HV diff probe is within your budget .An example on how convenient and relatively safe to use at HV diff probe in action, the scope was measuring directly at the mains lines at 220 VAC, at the same power sockets where it was powered and plugged.

author: tooki
author_rank: Super Contributor
timestamp: October 09, 2018, 05:53:32 pm
content: Quote from: Ian.M on October 09, 2018, 03:01:56 pmQuote from: tggzzz on October 09, 2018, 03:13:44 pmHonest question: why do we even use switchable probes? In what situations is a x1 probe preferable to a x10? It seems to me that the x1 position is used so spectacularly rarely — and is a clear source of unreliability and/or user error — that I wonder why we don’t just use fixed x10 probes, and let the few who need x1 buy them separately (it’s not as though they cost much!).

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 09, 2018, 06:06:21 pm
content: For Me:Quote from: tooki on October 09, 2018, 05:53:32 pmThey came free. I would never buy one.Quote from: tooki on October 09, 2018, 05:53:32 pmFor low impedance, low voltage sources running at low frequencies. By low voltage I mean too low to resolve well with a x10 probe.Quote from: tooki on October 09, 2018, 05:53:32 pmPut them in 10x mode and tape over the switch. Tape it over in such a way you can verify the switch is in x10 mode. Use Kapton tape for the pro look.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2018, 06:46:09 pm
content: Quote from: tooki on October 09, 2018, 05:53:32 pmX1 probes are suitable where maximum sensitivity is required at the expense of bandwidth and loading. AC coupled noise measurements of power supplies are a good example.

author: tggzzz
author_rank: Super Contributor
timestamp: October 09, 2018, 06:48:03 pm
content: Quote from: tooki on October 09, 2018, 05:53:32 pmThe only unique use case for *1 probes is low voltages where the extra input capacitance and lower frequency response is tolerable.So-called "high impedance *10 probes" aren't high impedance. Work out the impedance of their 15pF capacitance at 1, 10 and 100MHzHint: at 100MHz the "low impedance resistive divider probes" have ahigherinput impedance.

author: DigitalAura
author_rank: Contributor
timestamp: October 11, 2018, 01:12:21 pm
content: I already have a HV diff probe, and always use that in situations where it is required. As this situation is slightly different I figured I'd ask just to be on the safe side.

author: tautech
author_rank: Super Contributor
timestamp: October 11, 2018, 09:02:04 pm
content: Quote from: tooki on October 09, 2018, 05:53:32 pmMany might never need to use a probe on 1x setting but when you need the extra resolution, you need it !The most common 1x probe requirement is checking ripple on power supply rails. For a linear PSU a DMM is often sufficient but not all DMM's can display the AC component at SMPS frequencies.With sensitive scopes, those that might have some uV/div input selection the need for 1x probes is much reduced.QuoteUser error and unfamiliarity with their equipment is the biggest risk..........know your tools and how to properly use them ! Thebesttool on the bench is the one between your ears !Some years back I decided to focus on scopes only with 400V rated inputs as all but the mid and high level units come with switchable probes and as much equipment/appliances have SMPS and with the extent of which they fail the EE is often needing to repair them and here in NZ rectified mains is ~325V so it's obvious a 300V rated scope is looking down the barrel of possible damage. I don't need that nor do my customers !The best investment for those working with higher voltages where probe voltage vs frequency spec might be challenged is some 100x probes as their addition cost is little compared to popping the inputs of your scope.

author: tooki
author_rank: Super Contributor
timestamp: October 13, 2018, 06:28:11 pm
content: I understand what you are saying. It just seems to me that x1 (like higher ratios, like x100) are a fairly rare use case compared to x10, and that the vast majority of users — at least of a low-end scope like a beginner will get — stand very little to gain from a switchable probe. I think it’d make far more sense to ship with fixed x10 probes and then let the handful of folks who need x1 or x100 or whatever buy those as accessories. Ditto for the handful of people who routinely switch between x1 and x10, let them buy switchable as an accessory. It’s not as though they cost very much.I just know that I find the switch to be an annoyance that I wish were not there.