Measuring ~ 400V on oscope - 10:1 OK or differential probes ? - Page 1
Forum section: electronics / testgear

author: TomKatt
author_rank: Frequent Contributor
timestamp: October 31, 2023, 05:10:00 pm
content: I need to work on an old tube based scope and want to use my modern Siglent SDS1104X-E to view some signals. The tube scope has +400VDC and -300VDC rails. My measurements would be from a rail to ground (0V / chassis ground that is earthed) and not rail to rail (potential 700VDC). So the max signal amplitude should not exceed 400VDC.Question - I note the SDS1104X-E specs maximum input at 400Vpk. Would using 10:1 probes be within spec, or should I use a my Micsig DP10007 10X/100X that is rated to 700Vpk ? I'd be looking for distortion in my readings and I know the Micsig probe doesn't have the greatest CMRR, so I'd probably get cleaner measurements using a normal 10:1 probe.Wouldn't the 10:1 probe reduce a 400VDC signal to 40VDC, which is safely within spec?Thanks!Edit - My 10X probes are ProbeMaster 4900 series, rated at 600VDC

author: KungFuJosh
author_rank: Super Contributor
timestamp: October 31, 2023, 05:16:42 pm
content: Your probes are rated 50% higher than your target voltage, you'll be fine. You only need to worry about the scope's voltage rating when your probes aren't voltage rated (for example, using coax cable instead of a probe).You can also get higher rated passive probes, they don't have to be differential probes.

author: TomKatt
author_rank: Frequent Contributor
timestamp: October 31, 2023, 05:21:05 pm
content: Thanks.I get the probe capability part, just want to ensure I don't blow up my nice Siglent by overloading the front end or something.Edit - technically, 10:1 probes are simple voltage dividers so the scope should only see 40VDC when probing 400VDC... If the voltage was a bit less I'd just give it a whirl but since I'm at the edge of the scopes rated input I wanted to ask more experienced folks here.

author: KungFuJosh
author_rank: Super Contributor
timestamp: October 31, 2023, 05:44:34 pm
content: Unless the probe has some kind of weird catastrophic failure, you only need to pay attention to the probe's voltage.I use ProbeMaster probes rated at 600V, and probe stuff in the 400V range with no issue (tube guitar amps mostly, and my tube tester/tracer). I also have a PR2000B 2kV probe, and a DP10013 diff probe, but those almost never get used.

author: bdunham7
author_rank: Super Contributor
timestamp: October 31, 2023, 05:54:48 pm
content: Two thoughts:1.) A 10X probe will just barely work, I say that because the real limitation isn't the rated input voltage on the front of the scope, it's the lowest vertical gain setting (most V/div) which IIRC is 10 V/div. So a 40 volt signal is 4 divisions, anything larger and you'll have issues keeping the whole signal on the screen. I use a 100X probe in these situations, and depending on which 100X probe you use, this may have the additional benefit of reducing circuit loading.2.) The voltage rating is one thing, but you have to watch for the frequency derating as well--and this may or may not be explicitly stated. At those voltages I'd stay under 100kHz unless you have a reliable derating graph or table.

author: BrokenYugo
author_rank: Super Contributor
timestamp: October 31, 2023, 06:54:35 pm
content: Fixed 10:1 is best for probing higher voltage as you can't bump it to 1x accidentally and send 10x the intended voltage into the front end. Or go to a 100:1 as bdunham7 suggested.Other than that and high frequency derating (see probe datasheet for graph) yeah, that's how it works, e.g. probe 1kV with a suitable 100:1 probe and it will only put 10V across the 1 megohm scope input, same for 10kV on a 1000:1 probe.

author: Fungus
author_rank: Super Contributor
timestamp: October 31, 2023, 07:46:09 pm
content: x10 should be fine in theory butmake 100% surethat the fiddly little switch is in the x10 position.Me? I'd buy a fixed x10 probe for doing this sort of work.(...or even a fixed x100 probe)

author: Gyro
author_rank: Super Contributor
timestamp: October 31, 2023, 07:47:58 pm
content: Just one thing to maybe watch. Be careful if you have the scope on AC coupled input setting. I'm not sure how the inputs on the Siglent are implemented, but there is thepossibilityof input overload. If you quickly switch from probing a +400V node to a -300V one, you possibly have an [Edit: transient] 70V swing from 0V on the scope active input circuitry (after the x10 probe) due to the existing +40V charge on the input coupling cap. I haven't completely worked it through, but there is thepotentialfor an issue there.EDIT: Sorry, looking at your OP again and checking the SDS1104X-E spec. I see that it does have a 400V (DC+AC Pk), not 40V as I assumed, so not a problem... Err, it might still be if the 1M input resistor is after the input cap as is traditional on analogue scopes, you may have a transient 700V swing.

author: bdunham7
author_rank: Super Contributor
timestamp: October 31, 2023, 08:59:27 pm
content: Quote from: Gyro on October 31, 2023, 07:47:58 pmOn this scope the input is always 1M regardless of the coupling selection, so the input capacitor issue doesn't arise. Even if it did--say on a Tek 2465B--you'd just be applying 700V momentarily across the 9M resistance of the probe plus the 1M of the scope if you switched from +400 to -300VDC.

author: Gyro
author_rank: Super Contributor
timestamp: October 31, 2023, 09:14:50 pm
content: Yes, you're right (I did say I hadn't thought it through), The 10M total resistance would limit the current to something non-damaging - another benefit for fixed x10 probes though.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 31, 2023, 09:18:00 pm
content: Quote from: Fungus on October 31, 2023, 07:46:09 pmWhen measuring voltages that could damage the input, I take no risks, I fix it in place with 2 turns of tape!It's easy to overlook that detail, also some probes have weak switches, too easy to move while handling it.Like trying to insert the clip... "click" -Ahhh!Better safe than sorry!

author: KungFuJosh
author_rank: Super Contributor
timestamp: October 31, 2023, 09:28:17 pm
content: He edited his comment out, but he previously mentioned using fixed 10X 600V ProbeMaster probes. He's fine.

author: TomKatt
author_rank: Frequent Contributor
timestamp: November 01, 2023, 12:22:34 pm
content: Actually, my ProbeMaster set is 1X/10X. I'm probably one of the minority that frequently uses 1X for low impedance power measurements.I do pay special attention to that, and fortunately the ProbeMaster design is such that the small switch is quite stiff and not easily moved by mistake - you really need to intentionally move it. The switch also has a center ground position between 1X and 10X, so if it does move it's most likely to just provide no signal at all.Thanks everyone for the input!

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2023, 12:50:56 pm
content: Quote from: TomKatt on November 01, 2023, 12:22:34 pm... until you forget to check, and bugger your nice new shiny toy. If you never forget to check, you are a better technician than I am.... until something slips, etc.You don't mention the frequency and risetime of the signals that will be on the high voltages. Beverycareful to verify that they won't exceed theprobe'srating, and melt the probe. If the probe datasheet doesn't have a V-vs-f derating graph, be suspicious.Personally I'd invest in a HV differential probe, so I could have peace of mind.

author: Fungus
author_rank: Super Contributor
timestamp: November 01, 2023, 12:54:56 pm
content: Quote from: TomKatt on November 01, 2023, 12:22:34 pmRe-check it after you've had visitors. Or every time you pick it up if there's children around.

author: Fungus
author_rank: Super Contributor
timestamp: November 01, 2023, 01:01:57 pm
content: Quote from: tggzzz on November 01, 2023, 12:50:56 pmIt's worth it if you do this a lot. The Micsig ones aren't expensive for what you get.Speaking of Micsig: My STO1104C came with four fixed 10x probes. Smart decision, IMHO.

author: TomKatt
author_rank: Frequent Contributor
timestamp: November 01, 2023, 01:11:28 pm
content: As mentioned, I have a Micsig DP10007 10X/100X differential probe - it's just that I understand it to be noisier than conventional probes, and noise/distortion is exactly what I'm investigating.I'll probably pick up a set of fixed 10X ProbeMasters at some point - they are really nice and have the best set of optional adapters I've seen for probes. But it's not often I measure voltages approaching my scopes limit.

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2023, 01:27:55 pm
content: Quote from: Fungus on November 01, 2023, 12:54:56 pmOr there are cats around.Or - perhaps especially - if there is you around. Don't ask me how I know; you would get an essay rather than a bullet point

author: KungFuJosh
author_rank: Super Contributor
timestamp: November 01, 2023, 03:00:00 pm
content: Quote from: TomKatt on November 01, 2023, 12:22:34 pmOh, you only mentioned the 10X, so I assumed. My bad. I have 4 fixed PM 10X probes. I'm going to get a fixed 1X probe or two soon. I don't like the switches, not because of their quality, but because I don't want to be able to forget what position it's in. 😉

author: TomKatt
author_rank: Frequent Contributor
timestamp: November 01, 2023, 03:01:42 pm
content: Quote from: KungFuJosh on November 01, 2023, 03:00:00 pmYeah, if I had it to do over again I'd probably have got 2 pairs each fixed. Oh well.

author: KungFuJosh
author_rank: Super Contributor
timestamp: November 01, 2023, 03:08:06 pm
content: Quote from: TomKatt on November 01, 2023, 03:01:42 pmDon't worry, more probes will eventually call out to you. 🤣