Question related to Oscilloscope HV probes  - Page 1
Forum section: electronics / testgear

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 07, 2020, 11:02:26 am
content: I have a question for the oscilloscope experts, it is about probes.I need HV probes, this one is quite cheaphttps://www.batronix.com/shop/measurement/probes/Testec-TT-HV-250.htmland this one from Rohde & Schwarz costs a small fortune.https://www.batronix.com/shop/measurement/probes/Rohde-Schwarz-RT-ZH03.htmlWhat can the one from Rohde & Schwarz do better, which justifies this high price ?Does anyone know?can the Testec-TT-HV-250 be a danger for the measuring amplifier with high voltages in switching power supplies and tube amplifiers ?Thanks,Frank

author: tggzzz
author_rank: Super Contributor
timestamp: November 07, 2020, 11:27:57 am
content: Don't forget the scope's case will be connected to protective mains earth, and the probe's shield will be connected to the case. If you connect the probe's shield to something "unfortunate", then there will be unpleasant surprises.Never ever disconnect the scope's earth lead; that can beliterally lethal.FFI, see the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ if you have a scope where that isn't the case, then you probably wouldn't be asking your question!

author: Performa01
author_rank: Super Contributor
timestamp: November 07, 2020, 12:32:04 pm
content: Here's a practical evaluation of the TesTec HV250:https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/msg1435196/#msg1435196

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 07, 2020, 12:57:50 pm
content: One caveat. If they are for power electronics work, to measure Vds switchings, usually these probes are badly compensated from factory. The only one I've seen well compensated is this one which is mid-priced and also hihger quality than the others:https://www.staubli.com/en/connectors/test-and-measurement/test-and-measurement/high-frequency-measurement/touch-protected-passive-probes/isoprobe-ii-1001/

author: joeqsmith
author_rank: Super Contributor
timestamp: November 07, 2020, 03:46:55 pm
content: I ordered a couple of Hantek 100X probes and received a substitute. Both were very low cost, but I had some experience with Hantek's products. While checking it out, running it well within their derating curves, I damaged it. Cutting the probe apart, found a small 1/4w axial part. The cost or repairing my equipment could far outweigh the price of the probe, plus down time. For me, it just wasn't worth trying to find another low cost probe and I ended up rolling my own using parts designed for HV.Manuals for both probes are here.https://www.testec.de/assets/pdf/TT-HV/TT-HV-250_Datasheet_DE.pdfhttps://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_common_library/dl_manuals/gb_1/r/rt_zh/RT-ZH03_UserManual_de_en_01.pdf

author: David Hess
author_rank: Super Contributor
timestamp: November 07, 2020, 03:49:39 pm
content: The probes are functionally identical and must be used with DC coupling.The TT-HVP-15HF shown at the first link and RT-ZH10 and RT-ZH11 shown at your second link are examples of probes that use shunt attenuation making them safe, or at least safer, for AC coupled applications but most people do not care about the difference. You can tell because their input resistance is a fraction of the oscilloscope input resistance multiplied by the probe attenuation factor.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 07, 2020, 06:40:47 pm
content: david, thank you.But I didn't quite get what you meant

author: _Wim_
author_rank: Super Contributor
timestamp: November 07, 2020, 07:17:38 pm
content: Quote from: Frank_MV on November 07, 2020, 06:40:47 pmInput impedance of the probe is 100Mohm instead of the 1Mohm of your scope => large series resistor installed (99Mohm) in the probe => 1/100 attenuation by voltage divider created by the probe resistor and input impedance of the scope. As stated above, compensating a probe like this is what is difficult (any one can install a large series resistor), and this could be a difference between an R&S probe and the Testec probe, however it is equally possible to Testec probe performs just as good.

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2020, 11:46:38 am
content: Quote from: Frank_MV on November 07, 2020, 06:40:47 pmThe two probes you mention are functionally identical; they both use a 99 megohm series resistor which when combined with the 1 megohm oscilloscope input makes a 100:1 probe with a 100 megohm input resistance.If the oscilloscope is configured for AC coupling, then the full DC voltage at the probe tip is applied to the oscilloscope input through the 99 megohm input series resistor. These probes must not be used with AC coupling at high voltages.The 1000:1 TT-HVP-15HF shown at the first link is very different. It has the same 100 megohm input resistance however there is a parallel shunt resistance so DC is also attenuated even when AC coupling is used. This probe could be used up to about 4000 volts DC with AC coupling.They do make 100:1 high voltage probes with an input resistance lower than 100 megohms which can be safely used in AC coupled applications however they are very rare now.Some oscilloscopes maintain a 1 megohm DC input resistance even when AC coupling is used so are safer to use with common 100:1 probes, but it is difficult to know which do this without testing.

author: mawyatt
author_rank: Super Contributor
timestamp: November 08, 2020, 03:11:17 pm
content: Quote from: David Hess on November 08, 2020, 11:46:38 amMy old Tektronix 2445 does not maintain a 1M input resistance when AC coupled, while my SDS2102X Plus does, I prefer the later.Best,

author: Hydron
author_rank: Super Contributor
timestamp: November 08, 2020, 06:05:44 pm
content: The testec 250 compensates decently (though not perfectly, see pics attached) on my RTB2004. YMMV depending on scope - the RTB2k has a rather low input C (9pF) that can actually cause issues with some probes.There is only one adjustment unfortunately - it could do with a HF trimmer for the first 10us or so to tame the residual overshoot.Note that the insulator on the probe barrel can be removed to access the ground connection much closer than the earth lead clip - by using this low-L connection is how I got the pictured waveform on the scope's <1ns risetime calibration output. There should also be a BNC adapter included which will give a nice clean connection, albeit not at a 2.5kV rating!I will second the comments about AC coupling - don't do it unless one of the following applies:- The probe input resistance is less than the division ratio, in meg-ohms (i.e. <100M for a 100x probe)- The scope input measures 1 megohms or less DC input resistance in AC coupling mode (I think some of the rigols do?)- The DC voltage you're trying to get rid of is less than the rated input voltage of the scope

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 08, 2020, 06:13:10 pm
content: First of all thank you for all your answers and linksThis all has helped me a lot

author: switchabl
author_rank: Frequent Contributor
timestamp: November 08, 2020, 09:38:36 pm
content: I don't know about this one specifically, but Testec probes are generally not bad. I would expect them to work fine and definitely would expect them to meet their safety specs.Keysight/Tektronix/Lecroy/R&S probes are usually quite a bit more expensive, even when they are sometimes exactly the same probes. E.g. mostdifferentialHV probes are actually made by Sapphire. Some of the reasons I can think of:- you DO pay for the name- probe interfaces: so the scope can automatically identify the probe, set the correct attenuation/units (don't think this one actually has that though)- support and specified system performance: if the probe won't compensate on your (officially supported) scope, you know who to callAlso, the price listed by Batronix may not be what peopleactuallypay, particularly if you are a big customer, a university or are buying a couple of expensive scopes at the same time.The Stäubli probes mentioned by MasterTech do look pretty nice as well (with a nice range of accessories) and are suprisingly affordable. They may actually be preferable to both the Testec and the R&S. I might have to get one for myself. They also offer one with a built-in high-pass filter. The corner frequency on that is 1kHz, so not quite the same as AC coupling, but looks interesting.Finally, while these passive HV probes definitely have their place, I think they may look more useful at a first glance than they actually are. Apart from the obvious problem that often you DON'T really want to measure relative to earth ground, they also do not play well with isolation transformers: assuming you have your DUT connected to the mains through an isolation transformer (NOT the scope, NEVER the scope), as soon as you connect the ground lead of the probe, you defeat the safety gain from the transformer (AND your GFI won't trip either!). Worse, if you hook up the ground lead to something else than DUT ground (by accident or on purpose), the DUT chassis will be live.To be fair, they are cheaper than a differential HV probe and they usually go to higher frequencies (but getting a good ground connection for higher frequencies may be a challenge).

author: bdunham7
author_rank: Super Contributor
timestamp: November 08, 2020, 10:07:16 pm
content: Quote from: Frank_MV on November 07, 2020, 11:02:26 amWhat voltages and frequencies are you looking to measure? And what energy/power levels? SMPS covers a lot of ground.

author: bdunham7
author_rank: Super Contributor
timestamp: November 08, 2020, 10:14:05 pm
content: Quote from: joeqsmith on November 07, 2020, 03:46:55 pmMany of these probes come with minimal manuals and a derating chart that can bequestionableridiculous. In this case, the Testec derating chart seems to indicate it would be able to handle 5kVp-p @ 1MHz. I'm betting it can't.

author: switchabl
author_rank: Frequent Contributor
timestamp: November 08, 2020, 11:02:41 pm
content: For work on mains-powered SMPS, I would say, no, it is not safe unless you have additional measures in place to protect you from touching live parts when it is powered on. An isolation transformer and differential probes with the proper safety rating (or an isolated-channel oscilloscope) is likely the way to go. For high energy stuff, things may be a bit different. For one, you will probably have an additional safety enclosure for blast protection anyway. But that is outside my comfort zone.At the risk of stating the blatantly obvious: the voltage rating of the probe only means that it won't blow up and it provides sufficient isolation so you won't get shocked when you touch it. Whether it is safe will depend very much on how you use it and what you hook it up to.

author: David Hess
author_rank: Super Contributor
timestamp: November 09, 2020, 02:00:04 am
content: Quote from: mawyatt on November 08, 2020, 03:11:17 pmUsually this "feature" has nothing to do with supporting common high voltage probes with AC coupling. There is a way to implement AC coupling with only low voltage analog switching in low cost designs but I think it has performance disadvantages.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 10, 2020, 04:35:05 pm
content: the Stäubli SET Isoprobe III-HP probe has 8pF input capacity, the Testec 4pF.Does the Stäubli fit better with an RTB2000 with 9pF input ?And:I will only measure on DC/DC switch power supplies ( step up converter ) - this is safer for meThanks again for all answers

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2020, 05:21:38 pm
content: Quote from: Frank_MV on November 10, 2020, 04:35:05 pmWhat matters is the compensation range of the probe which must include the oscilloscope's input capacitance. The probe's tip capacitance is irrelevant.

author: switchabl
author_rank: Frequent Contributor
timestamp: November 10, 2020, 05:31:20 pm
content: The Isoprobe HP is a rather specialized probe with a built-in high-pass filter (3dB @ 1kHz). See the Stäubli catalog for details on the different models:https://ec.staubli.com/AcroFiles/Catalogues/TM_HF-Main-11014113_(en)_hi.pdfAll the probes discussed here, the Testec, the regular Isoprobe and even the Rohde & Schwarz have a specified compensation range starting at 10pF. If your scope really has a 9pF input capacitance, depending on the tolerances, that may or may not work. I would definitely test if my particular probe can be properly compensated on all channels. And make sure I can return it if it doesn't.

author: Hydron
author_rank: Super Contributor
timestamp: November 10, 2020, 06:55:33 pm
content: Quote from: switchabl on November 10, 2020, 05:31:20 pmMy VNA tells me the same as the datasheet - close to 9pF. Most probes I have tried do compensate fine, albeit often near the end of the adjustment range.

author: switchabl
author_rank: Frequent Contributor
timestamp: November 10, 2020, 07:18:07 pm
content: Yes, you are right, the datasheet indeed states 9±2 pF and I would be surprised if it wasn't true (but always good to check). It is just a bit unusual (not that it is a bad thing).If I had to guess, I would expect all three probes to work just fine. R&S lists theirs as an official accessory for the RTB, would be curious what they have to say about it. Anyway, the compensation range is the guaranteed range and I doubt the trimming cap has tight tolerances, so in practice there will have to be a significant margin. BUT unfortunately there is no way to be sure without trying the individual probe with the individual scope.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 12, 2020, 07:53:35 pm
content: I was able to compensate my Testec TT-HV-250 successfully on all 4 channels of RTB2004

author: Hydron
author_rank: Super Contributor
timestamp: November 12, 2020, 10:22:25 pm
content: Glad to hear that, didn't realise you had the same scope as I do. Did your calibration waveforms look similar?

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 13, 2020, 07:22:22 am
content: I just adjusted the probe to the rectangular signal via the menu of the RTB - nothing more ;-)

author: MathWizard
author_rank: Super Contributor
timestamp: November 13, 2020, 08:13:00 am
content: I have the Hantek 100x probes. Now that I have a good functino gen, I should compare all my probes, but I don't have real coax, or accesories like terminators yet......I should order some next week.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 27, 2020, 07:09:49 pm
content: Hello,Today I adjusted my new Testec TT-HV250 probes.At NF I can adjust a good square wave signal, but at 1MHz you can see clear overshoots with all my 3 probes.Now I think about whether I should keep them or exchange them.Maybe a Staeubli probe does better here ?What do you think ?It has been mentioned here before, but I have now also noticed that when I touch the red BNC connector of the probe, I can see clear interference that overlays the measurement signal.Br,Frank

author: David Hess
author_rank: Super Contributor
timestamp: November 27, 2020, 09:09:03 pm
content: Quote from: Frank_MV on November 27, 2020, 07:09:49 pmThat is the result I would expect if the ground lead is too long.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2020, 09:26:13 pm
content: Quote from: Frank_MV on November 27, 2020, 07:09:49 pmOnce you connect test leads, probes, whatever, they become part of your circuit. Here you have some ~150MHz ringing. It doesn't mean you probes are 'bad', it just means that your new circuit has some ringing--perhaps for the reason David Hess mentions. Once you start getting up into these frequencies, things start getting trickier. As for touching the BNC, I'm not sure why it would be that touchy, but perhaps there is some issue in your whole ground setup. Post a photo maybe?

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 28, 2020, 04:38:50 pm
content: Enclosed is my measurement setup. It is only the calibration of my HV probe at 1MHZ at my RTB2004.Am I possibly doing something wrong ?

author: bdunham7
author_rank: Super Contributor
timestamp: November 28, 2020, 05:14:18 pm
content: Quote from: Frank_MV on November 28, 2020, 04:38:50 pmDo the probes included retractable test hooks? I would try using a test hook and removing the ground lead entirely, as in not attached to the probe or the scope.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 28, 2020, 05:42:16 pm
content: Yes, retractable test hooks is included but nothing changes.The overshoot becomes smaller when I touch the test cable with my hand.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 28, 2020, 05:46:23 pm
content: My Rohde & Schwarz probes still have 2 additional adjustment trimmers for RF compensation but the Testec has this trimmers not.I still have a week to replace these probes at my distributeur - but which ones ?I need HV probes for my tube amps

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2020, 06:12:04 pm
content: Quote from: Frank_MV on November 28, 2020, 04:38:50 pmThe only thing that matters is the transition time; the period is completely irrelevant.https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/If the probe has, say, a 15pF tip capacitance (C), and your ground lead is 150mm long (i.e. ~120nH inductance (L)), then the LC will resonate/ring at ~100MHz.https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/The solution is to either reduce the tip capacitance (difficult!) or have a shorter ground lead (easy).https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/but there are other techniques.Also work out the impedance of 15pF at 100MHz, and you will see your "high impedance" probe is not high impedance

author: tggzzz
author_rank: Super Contributor
timestamp: November 29, 2020, 10:18:03 am
content: Quote from: PeDre on November 29, 2020, 09:36:14 amYes, that ringing is to be expected, for the reason I gave in my previous post.

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2020, 03:38:29 pm
content: Quote from: Frank_MV on November 28, 2020, 05:42:16 pmIs that with no ground lead installed? OK, then if you have the little groundspring attachment, try that. Otherwise, if you can hold the probe so that the tip is on the calibrator output and the exposed ground shield part is on the ground, you should be able to eliminate that ringing. If not, then IDK what's going on.

author: Frank_MV
author_rank: Regular Contributor
timestamp: November 29, 2020, 06:24:35 pm
content: There are new findings. I took a look at the accessory bag of the HV250 - unfortunately it doesn't say anywhere what it is needed for.So I just combined and tried it. The protective cover of the probe can be removed and I pushed the metal sleeve over it, so I don't need a ground cable anymore - so the ground connection is shorter.And what should I say, there are no more overshoots at 1MHz rectangle - it's really the ground cable.But is this still a real use case ?For measuring in circuits I always need a ground wire with clamp.