Is a Differential probe -a la micsig 1300v- going to give the accuracy i need? - Page 1
Forum section: electronics / testgear

author: Kracin
author_rank: Newbie
timestamp: October 05, 2021, 03:37:04 am
content: So, I'm not new to electrical and testing, but i generally work with higher voltage than just mains and 24v. Although i do a lot of 24v because most industrial control circuits are either 24vdc or 120vac (USA).But, I am getting more into the nitty gritty when it comes to troubleshooting, and I want to have in my arsenal the ability to accurately read out 3 phase VFD outputs that are 480v leg to leg (or 676vdc on the bus). Measuring a smaller output VFD, I can get a good looking waveform and signal that i can see and measure with standard 10x probes on a rigol 1104, but this is on a single phase to 3 phase output that only puts out 230v 3 phase PWM. I am NOT going to hook up this scope to try to read a leg of 480 which normally not being PWM would be at 277v without isolated differential probes for sure even though it says 300v capable, I'm just unsure as to whether it could handle it or not, and I'm not willing to fry a scope yet.I want to know whether or not a differential probe is going to lower the quality of the reading to the point that it won't be useful to look at a high frequency PWM signal coming from an inverter at higher voltages, meaning it would have to be on 500x mode.currently I just use the Rigol I have to verify quadrature encoder signals, Resolver signals, noise on lines, feedback noise and coil spike before/after suppression is added.Thanks in advance.

author: bob91343
author_rank: Super Contributor
timestamp: October 05, 2021, 04:13:27 am
content: Maybe you can fabricate an attenuator to bring the voltage down to manageable levels. This may be a bit of work but if you seriously need some of these measurements I wonder if it isn't worth it.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2021, 04:20:28 am
content: Yeah don't trust 10:1 probes on mains or really any elevated voltages.100:1 probes are not expensive and you can probe away at high energy sources with a much higher level of safety although like any probe take note of frequency derating.

author: Kracin
author_rank: Newbie
timestamp: October 05, 2021, 04:25:52 am
content: how exactly does that higher probe affect the incoming voltage to the scope itself though, I was under the impression that using a 10:1 would show a 120v at 12v levels and then setting the scope to read 10:1 allows you to then see it with the right divisions and numbers on the screen itself.is using a 100:1 going to bring a 600vdc source down to 6v at the scope itself? or will it still have way too much voltage running into it?

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 05, 2021, 04:47:02 am
content: It doesn’t matter if it’s only a 120V system, you cannot measure line to line signals on a motor with a 10:1 probe because the ground clip of the probe is at AC main grounds potential. You’ll blow stuff up at a minimum, kill yourself at worst. That’s why the differential probes exist, they allow you to measure things floating above ground without blowing up the probe, scope or you.You are correct about the reduction in signal levels. Most modern scopes go down to 1 mV of vertical resolution so if you have 500:1 probe, the 1400 volt levels you might encounter in a 480V VFD system would be presentable on a scope using the 200 and 500 mV per division vertical ranges.

author: bdunham7
author_rank: Super Contributor
timestamp: October 05, 2021, 05:02:12 am
content: Quote from: Kracin on October 05, 2021, 03:37:04 amI can't comment specifically on this situation nor the Micsig probes because I haven't used them. In general, I think you want a differential probe made for high energy circuits, even though this may not technically be a CAT III environment. You probably shouldn't be anywhere near this with a regular scope and probes. The Micsig DP20003 is CAT III/1000V, and the 200X range might work the best. Again, in general, DPs have a lot susceptibility to common mode noise and in your use case, this may be an issue. If you do try it, I'd be interested to know what you think. Of course, the proper solution is something like a Fluke ScopeMeter, but obviously they're $$$.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2021, 05:10:48 am
content: Quote from: Kracin on October 05, 2021, 04:25:52 amSimply probes are resistive dividers and the division ratio is what protects your instrument although the max voltage rating of the probe determines what it is safe to use on.Typically 10x probes max out at some 600V although much less as the frequency goes up however 100x and 1000x probes have a much higher voltage rating which offers both the operator and the instrument a higher level of protection.Still, irrespective of what you use , proper understanding, safety and correct probing technique overrides everything else.Differential probes OTOH and despite their additional cost permit connections to measurement points that might be elevated above mains ground.With limited knowledge and understanding they are the safest option.Dave has a quite good one although there are cheaper available:https://www.eevblog.com/product/hvp70/

author: JohanH
author_rank: Frequent Contributor
timestamp: October 05, 2021, 06:01:03 am
content: Quote from: Kracin on October 05, 2021, 04:25:52 amAs long as you use DC mode. Switching to AC mode on the oscilloscope, will receive the full voltage on the scope's input capacitor, which may or may not be rated for 600V. This with normal probes, whatever the ratio. Differential probes are safer.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2021, 06:13:57 am
content: Quote from: Kracin on October 05, 2021, 04:25:52 amYes. There's resistors inside the probe to divide the voltage down.Those Micsig probes have 500x or 2000x division depending on the model and are rated for 1300V/5600V respectively.You should have no problem.Quote from: WattsThat on October 05, 2021, 04:47:02 amHence the differential probe.Or you could get a Micsig 'scope to go with it.

author: Kracin
author_rank: Newbie
timestamp: October 05, 2021, 12:22:38 pm
content: Quote from: WattsThat on October 05, 2021, 04:47:02 amNo worries, I'm not connecting any line to ground, as I stated I've used my current setup to deal with voltages within the specified range of the scope itself. I was only worried about not knowing if the scope could take higher voltages even with a higher attenuation probe attached. I work in Controls and regularly work with 480v 3 phase from entry in cabinets to PLC output control.Quote from: jukk on October 05, 2021, 06:01:03 amI haven't heard this one before, does this stay true if you use a scope that has an AC/DC in addition to DC only?Quote from: tautech on October 05, 2021, 05:10:48 amThanks, that was the idea. So even if the scope itself says 300vpp Max, using a higher attenuation probe will be fine to measure beyond that, as it should take that voltage level down to the proper level, and then when setting in the scope itself it then adjusts divisions.the only variable left that i'm still wondering is how much resolution is lost going from 10x to 100x to 500x on a signal with pulses between 10khz to 1mhz?

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 05, 2021, 02:24:43 pm
content: QuoteResolution? As in vertical sensitivity? None. We’ve already explained that. As for bandwidth, if you’ve got a 1 MHz max signal and you’re using a 100 MHz capable scope and probe, why would something NOT be present in your waveform under examination?Personally, I cannot imagine a scenario in the field that requires a scope for looking at the motor side waveforms. There’s no useful information in anything you’d see on a scope.

author: Kracin
author_rank: Newbie
timestamp: October 05, 2021, 03:02:16 pm
content: Quote from: WattsThat on October 05, 2021, 02:24:43 pmmostly because some customers want definitive proof as to what their issue is before spending thousands of dollars on a fix. so if someone has excessive long motor leads, it's helpful to show them the reflected wave voltage that's causing them to lose motors faster than normal. This way they know when they spend 15,000 to re-route a cable, it will settle well with accounting and their morning meeting.I don't have access to worry free test equipment like a fluke 190 or higher that is built for using on large loads.sorry for coming off as not understanding what I'm looking at, I just wanted to make sure I understood what the tradeoff was for going with either a high attenuation probe, or a very high attenuation differential probe. there had to be a tradeoff somewhere in my mind.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2021, 03:05:51 pm
content: Quote from: Kracin on October 05, 2021, 12:22:38 pmYou have to adjust the channel input attenuations for each type of probe yourself for any scope measurement to be correct. This is the power of a DSO, not only providing measurements but also being able to set the correct input attenuation therefore not having to do the mental calc/div to get a measurement.As some example of 4 different passive probes in use an old post from 2018 where all 4 are connected to the 3V probe compensation output. Each channel is set correctly for each probe and all are at 2V/div where of course the 1000x probe will be providing just 3mV to the scope which is why we see a little more noise on ch4.

author: Kracin
author_rank: Newbie
timestamp: October 05, 2021, 03:20:08 pm
content: Quote from: tautech on October 05, 2021, 03:05:51 pmThanks, the visual helps a bit. I imagine like the others said, whether its 100x or 500x it should be perfectly readable.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2021, 03:29:47 pm
content: Quote from: Kracin on October 05, 2021, 03:20:08 pmOnly if your instrument supports the correct probe attenuation ratio or has a user definable one. Normally most scopes support all the common ratios and some support custom probe ratios.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2021, 03:46:06 pm
content: Quote from: Kracin on October 05, 2021, 04:25:52 amNope. AC divides down just as well as DC does and enabling AC mode on the 'scope doesn't change the voltage arriving at the input BNC in any way.The only way to overload that capacitor is to have a massive difference between the scope ground and the AC supply's ground. This has nothing to do with the 'scope's setting and will overload the 'scope in DC mode too.

author: Ian.M
author_rank: Super Contributor
timestamp: October 05, 2021, 04:17:04 pm
content: Quote from: Fungus on October 05, 2021, 03:46:06 pmYou'd think so, as it would be logical to maintain the same input impedance, with 1 Meg to ground in both AC and DC coupled modes, but that's not the way its usually done.The coupling switch also grounds the input, so is almost invariably immediately after the BNC, before the Y Volts/div attenuator. Therefore, as most passive probes with attenuators use the 1 Meg scope input resistance as the lower arm of the attenuator, if the DC level of the signal exceeds the coupling capacitor rating you risk it breaking down. This limitation is quite clearly called out in most scope manuals.

author: bdunham7
author_rank: Super Contributor
timestamp: October 05, 2021, 04:51:04 pm
content: Quote from: Fungus on October 05, 2021, 03:46:06 pmUm, no? Some scopes use a plain blocking capacitor on the input and when AC-coupled, their DC input resistance is infinite, less the capacitor leakage. Others maintain a 1M DC input resistance in all modes, including GND. It varies by make and model. If you have the first kind and you use a 100X probe that is essentially a 99M resistor, the blocking capacitor will eventually charge up to the full DC bias at the probe tip.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2021, 05:37:53 pm
content: Quote from: bdunham7 on October 05, 2021, 04:51:04 pmSurely any 'scope with a switchable 1x/10x input must do the latter. The little switch on the probe won't work in AC mode otherwise.Quote from: Ian.M on October 05, 2021, 04:17:04 pmYep, that was what I meant when I said: "The only way to overload that capacitor is to have a massive difference between the 'scope ground and the AC supply's ground."(ie. the DC offset of the AC signal being probed, maybe I could have worded it better)

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2021, 06:26:28 pm
content: Quote from: Kracin on October 05, 2021, 03:37:04 amA high voltage differential probe is exactly what you need to look at line side PWM. Only an isolated probe would be better.Quote from: Kracin on October 05, 2021, 04:25:52 amThat is right, although old timers had practice in doing the conversion themselves before oscilloscopes did it for them. The readout factor on the oscilloscope does not actually affect anything and is only for convenience.QuoteThe probe itself attenuates the voltage before it reaches the oscilloscope.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2021, 06:35:19 pm
content: Quote from: Kracin on October 05, 2021, 03:37:04 amGetting back to the point: Differential is the correct way to go. The bandwidth of those Micsig probes is 100Mhz and the gain accuracy is 2%, ie. better than most 'scopes input gain amplifier accuracy.You don't say exactly what "high frequency" is but I don't think you'll have any problems.

author: Kracin
author_rank: Newbie
timestamp: October 05, 2021, 08:12:28 pm
content: Quote from: Fungus on October 05, 2021, 06:35:19 pmyou're right, "high frequency" for the output of a vfd isn't going to reach near even 20mhz. in my world the highest frequency I deal with is a radio knob on the drive home.

author: wraper
author_rank: Unknown
timestamp: October 05, 2021, 08:28:58 pm
content: Quote from: Fungus on October 05, 2021, 03:46:06 pmI checked my GWinstek GDS-2204E and input becomes open instead 1M when I switch to AC coupling. It will change the voltage arriving, since probes use 1M resistor in the scope as a part of the divider. If you apply DC to the probe, scope will see a full input voltage of the probe. On the other hand DC input resistance of my Rigol DS2072 does not change and remains 1M regardless of the coupling setting.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2021, 09:51:28 pm
content: Quote from: wraper on October 05, 2021, 08:28:58 pmDo you have a switchable 1x/10x probe?Connect it to the test signal with AC coupling and flick the switch. Does the amplitude change on screen?

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2021, 10:21:33 pm
content: Quote from: Fungus on October 05, 2021, 06:35:19 pmOn differential probes, common mode rejection is usually more of a limitation than frequency response. It does not matter when the probe displays high frequency content when it is meaningless.Quote from: Fungus on October 05, 2021, 05:37:53 pmEven oscilloscopes intended to operate with switchable probes used that configuration just fine. That there is no ground return in AC coupled mode does not matter to the switchable probe when DC is being removed anyway.The complication is that a x10 or x100 probe will *not* attenuate DC in this situation when AC coupling is used, which can lead to the DC voltage at the oscilloscope's input exceeding its specification possibly damaging it. This is not much of a problem with x10 probes but it is definitely a hazard with common x100 probes.Some x100 probes include the dividing shunt resistor internally and do not suffer from this problem. They can be identified by having an input resistance lower than 100 megohms; 50 megohms is common. There used to be some x10 probes with this feature as well, but I am not aware of any currently produced ones.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2021, 10:43:31 pm
content: Quote from: David Hess on October 05, 2021, 10:21:33 pmDepending on the model, the Micsig probes seems to fare quite well.There's a thread about it:https://www.eevblog.com/forum/testgear/new-eevblog-hv-probe/