High voltage probes, from 2k$ Tek to 100$ noname, any advices - Page 1
Forum section: electronics / testgear

author: boumboauto
author_rank: Contributor
timestamp: September 15, 2017, 04:01:30 am
content: Hello,I'm working on a project where we need to check out the waveform at the output of a high power motor drive. The output voltage is 3500Vpp to gnd, switching frequency of the drive is 2kHz and I want to measure the overshot on each pulse.I have these different options:- Tek P6015A for 2000€http://fr.rs-online.com/web/p/sondes-pour-oscilloscope/7966702/- Pintek TT-HVP15 for 300€http://fr.rs-online.com/web/p/sondes-pour-oscilloscope/8917599/I do not need differential probes as I only want to measure phase to gnd waveforms. Any recommendations concerning the build quality, precision safety of these?Any other products that may be DIN rack mounted ?RegardsPaul

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2017, 05:24:34 am
content: Looking at a very similar solution for a customer today.Check the other HV scope probes that Pintek make in these links:Selection guidehttp://www.pintek.com.tw/files/pintek/HVP-provided-Fig.pdfSize comparisonhttp://www.pintek.com.tw/files/pintek/HVP-provided.pdfNothing wrong with Pintek, they've been making this sort of stuff for years.Despite notneedingdifferential probes for this requirement I'm of the mind that in the long run they might prove to be more versatile.Check some of their HV differential probes:http://www.pintek.com.tw/product_classify/landersound-tail/index.php?Product_Site_Classify_SN=17072&Company_SN=6002

author: woody
author_rank: Frequent Contributor
timestamp: September 15, 2017, 07:09:11 am
content: On the other hand, I got a Pintek DP-50 differential probe from China for about the same amount two weeks ago.https://nl.aliexpress.com/item/DP-40LV-50MHz-6500V-free-delovery-oscilloscope-differential-probe/32612135522.html?spm=a2g0s.9042311.0.0.Lmt6CEPaid €284,-, got an additional bill from Fedex for VAT (€61,46) and handling (€12,50). Ordered it on August 26th, got it delivered in the Netherlands on September 4.All in all I am very pleased with both the probe and the transaction.But maybe you need the 15kV, the DP-50 goes up to only 6500Vpp.Paul

author: HighVoltage
author_rank: Super Contributor
timestamp: September 15, 2017, 09:41:14 am
content: If money is a limiting factor, just use a 3:1 resistor divider at the motor drive and then use a TESTEC 1.5 kV 100:1 probe.https://www.reichelt.com/gb/en/Test-probes-and-BNC-adapters/TESTEC-HV-150/3/index.html?ACTION=3&LA=3&ARTICLE=32423&GROUPID=7230

author: Fungus
author_rank: Super Contributor
timestamp: September 15, 2017, 10:14:13 am
content: If you're not going to be holding it in your hand then a simple resistor divider will do. Just divide the voltage down to something your 'scope can handle easily (eg. 100:1).You can hook it up with the motor powered down then attach the scope and power everything on.For extra points: Put several resistors in the chain even though you don't need to, use physically large resistors, eg 2W, and put it in a box where you can't touch it.It will cost under $5.

author: nctnico
author_rank: Super Contributor
timestamp: September 15, 2017, 10:47:42 am
content: I wouldn't use resistors because they will create a low pass filter together with input capacitance of the probe.

author: Fungus
author_rank: Super Contributor
timestamp: September 15, 2017, 11:02:57 am
content: Quote from: nctnico on September 15, 2017, 10:47:42 ama) The motor isn't spinning at 100MHzb) You don't have to use 100MOhm resistors, you can allow 1mA through and that will be plenty to offset any capacitance in the probe without affecting the readings much (I assume the motors are quite large if they need 3500V so a 1mA 'tap' won't affect the ringing much, YMMV).(I could have mentioned point (b) I guess... let a little bit of current through)

author: boumboauto
author_rank: Contributor
timestamp: September 15, 2017, 11:03:14 am
content: Thanks,Quote from: HighVoltage on September 15, 2017, 09:41:14 amI'd say money is a limiting factor when probes cost 2000$ and above. I'm not a big fan of the resistors soldered together without caps because of reflexions and filtering. On the other hand, it would be nice to build 3 boxes with nicely done voltage divider, seal it on some insulating gel and have a measurement point easily accessible from these 3 boxes.This would need to be properly made and certified as this is a proffesional product that will be used in an industrial environment with lots of dust/grease arround.Thanks for the idea though; I'll look into making/buining or having made voltage divider. We do not need a high bandwith, just enough to see the overshots on a 2kHz square signal.Quote from: Fungus on September 15, 2017, 11:02:57 amYou're correct, in my case, the motor is spinning very slowly, we're takling a handful of RPMs no more, but I still want to see the transients on the PWM at the output of the drives: raise time, overshot, ringing and oscillations. So the probes must have a relatively good bandwith to observe these fast phenomenoms, I'd say 1MHz bandwith is fine, maybe less. It also need to be tuned with the capacitance of the cable and the scope so I do not add measurements errors on the observed signal;

author: Hydron
author_rank: Super Contributor
timestamp: September 15, 2017, 11:10:12 am
content: Quick question about the voltage, by "3500Vpp" do you mean +/- 1750V relative to GND? or +/- 3500V? If the waveform is 3500Vpp centered around GND then you're only actually looking at a 1750V rating and are in the region of cheap 100x probes. I've got a few Testec TT-HV250 probes which are very well priced (though the sprung hook attachment comes off a bit too easily):http://uk.rs-online.com/web/p/oscilloscope-probes/4376581/Edit: also note that this type of probe _MUST_ be used in DC coupling mode on most scopes if there is more than a few hundred volts of DC offset, otherwise you can blow up the coupling capacitor in the scope (DC voltage is not divided by 100x when the scope is in AC coupling mode, and the full voltage ends up across the coupling cap). Probes with an input impedance (in Mohms) that is different to their attenuation ratio probably won't have this issue - this will include most >100x probes.

author: boumboauto
author_rank: Contributor
timestamp: September 15, 2017, 11:12:36 am
content: Quote from: Hydron on September 15, 2017, 11:10:12 amI mean +/-1750V relative to GND, thanks for the recommendation, I could theroically stay in that range. But I'd be a bit nervous to use that though as I want more protection, I'd better stay in the range of 10kV for these meseurements. With loads of capacitors and large inductors I want to be sure of my probe. The power flowing though that system in the MW scale...

author: Fungus
author_rank: Super Contributor
timestamp: September 15, 2017, 11:12:54 am
content: Quote from: Hydron on September 15, 2017, 11:10:12 amAnything with large coils of wire can produce voltage spikesfarin excess of the supply voltage. I'm not sure I'd want to hold a little plastic thing in my hand and poke it into a huge 3500V inductor.

author: Fungus
author_rank: Super Contributor
timestamp: September 15, 2017, 11:15:51 am
content: Quote from: boumboauto on September 15, 2017, 11:03:14 amI think it's the simplest/cheapest solution. If it's a fixed installation then a box with test points attached to the motor could come in handy and would work with any test gear that happens to be around.

author: HighVoltage
author_rank: Super Contributor
timestamp: September 15, 2017, 11:18:15 am
content: Quote from: boumboauto on September 15, 2017, 11:03:14 amEven better.You can make a really nice and high quality divider, compensate it properly and then pot it with some silicone gel. Since you will not have a problem in loading your signal, select the resistors so you get some good current flow. If you use some good quality resistors it will works very well and will be safe and will not cost much.I have done something similar in the past and used some high quality poly carbonate boxes for housing the parts and resistors and used a BNC cable from the box to the scope. So I did not need a probe at all.

author: Hydron
author_rank: Super Contributor
timestamp: September 15, 2017, 11:23:31 am
content: I added a note about not using AC coupling (see edit to my earlier post), and also yes, I agree with Fungus in that you have to be careful when measuring this sort of thing with an unknown overshoot - 2500V rating measuring a 1750V signal may not be enough safety margin, and I certainly would be standing back just in case when first turning it on!

author: nctnico
author_rank: Super Contributor
timestamp: September 15, 2017, 11:37:12 am
content: Quote from: Fungus on September 15, 2017, 11:02:57 amBetter run some numbers first before posting. 3500V/1mA means a 3.5M Ohm resistor. Together with a (typical) 15pf probe tip capacitance that gives a 3kHz -3dB point but you will start to see the effect at frequencies over 300Hz.

author: boumboauto
author_rank: Contributor
timestamp: September 15, 2017, 11:59:28 am
content: Quote from: nctnico on September 15, 2017, 11:37:12 amThanks, 300Hz would not be enougth

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2017, 04:28:10 pm
content: Quote from: Hydron on September 15, 2017, 11:10:12 amI might have given this warning if you had not. The Tektronix P6015A high voltage probe is one of the few examples where this is not a problem and any high voltage probe which includes an internal shunt resistance is going to cost at least as much as a cheap high voltage differential probe.The Tektronix P6015A is overkill for this application. Even though a differential measurement is not required, a high voltage differential probe would be a more economical option and more versatile.If I had to do this on the cheap, then I would design and build a compensated voltage divider which could easily perform as well as a high voltage differential probe in this application.

author: boumboauto
author_rank: Contributor
timestamp: September 15, 2017, 06:22:37 pm
content: Quote from: David Hess on September 15, 2017, 04:28:10 pmThanks for the advice, i don't have much experience in high voltage measurements and would have not though about that!I will let you know what solution will be chosen for these measure, they likely need to be done in the upcomoing months but I need to get equipped first.And yes there is a quite large DC offset of about 250VDC on that signal.Thank you all for the advices

author: HighVoltage
author_rank: Super Contributor
timestamp: September 16, 2017, 12:18:00 pm
content: You also could consider a real high voltage test probe from TESTEC (Made in Germany)They have theTT-HVP 8- 1000:1- 40 MHz- 8000 V DC- 6000 V AC- for about 180 Euro

author: mk_
author_rank: Regular Contributor
timestamp: September 16, 2017, 12:54:50 pm
content: Quote from: HighVoltage on September 16, 2017, 12:18:00 pmThis is very likely an OEM from Pintekhttp://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=127265&Company_SN=6002&Product_Site_Classify_SN=17071For US-Customer it could be easier to buy directly from Pintek...

author: joeqsmith
author_rank: Super Contributor
timestamp: September 16, 2017, 03:28:39 pm
content: I am imagining some large industrial motor with some huge energy driver. Any specs on the motor? Just curious.