Cheap 100x probe; good value, or good grief? - Page 1
Forum section: electronics / testgear

author: AVGresponding
author_rank: Super Contributor
timestamp: March 21, 2020, 04:30:40 pm
content: https://www.ebay.co.uk/itm/Oscilloscope-Probe-100-1-High-Voltage-Withstand-5KV-300MHz-for-Tektronix-HP/332518004351Seems very good value, if it meets its b/w specs. Don't think I'd want to risk measuring 5kV with it, I'd want something a bit more substantial, in terms of separation ribs at the least.I think I'll order one, unless anyone has had a bad experience with this particular item/seller.I can test the insulation up to 2.5kV when I get it.

author: MarkL
author_rank: Unknown
timestamp: March 21, 2020, 06:26:42 pm
content: I am not familiar with that particular probe or seller, but if your intent is to test and measure up to 2.5kV, I would not go "value" shopping for something that would be devastating to either yourself or your equipment if the insulation broke down. I'd much rather take my chances with a used Tek probe of similar specification.If you have to have that probe, before you buy ask them for the derating curve. Or what safety standards the probe is certified to comply with.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2020, 06:52:49 pm
content: The clean probe on its own may (or may not) withstand 5kV.Personally I'd look for something that was rated for much higher than you want to measure, e.g. a Tek 6013 or 6015 or equivalent.I also suggest you calculate the current through a 6.5pF capacitor with a 1kV sinewave at, say, 100MHzIf you are thinking of looking at the grid-kathode voltage on an CRT, there are indirect better ways. I used them to diagnose an open grid once upon a time, but I'm not going to mention how I did it because people would rightly say it was iffy

author: tautech
author_rank: Super Contributor
timestamp: March 21, 2020, 08:05:57 pm
content: Very similar to the PB925 10x ones supplied with SHS1000 isolated channel handheld scopes.The flying reference lead would worry me for 100x use.https://int.siglent.com/upload_file/user/siglent_probe_datasheet201908.pdf

author: gby
author_rank: Frequent Contributor
timestamp: March 21, 2020, 08:38:09 pm
content: Looking at the listing it says:5000 Working Voltage (Vp-p)Seeing this I would call this a 2500 volt probe and derate from there.Also, it lists 100 Meg as input R. This means 99 Meg in series inside the probe plus 1 Meg in your scope. When not plugged into your scope you get full voltage at the BNC end.Even more serious, some scopes when you engage ac coupling place 500 volt cap in series with the input BEFORE the 1 Meg load R. This means that the ac coupling cap sees the full dc voltage and not 100x attenuated. Very sneaky way to pop your front end if you have high voltage and click ac coupling. There are 100x probes with series and shunt R's to solve this problem. For example series 49.5 Meg at probe tip, 1 Meg shunt at BNC which goes in parallel with the scope 1 Meg input. Net effect is 100x with 50 Meg input instead of 100 Meg input loading at tip.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 21, 2020, 08:45:25 pm
content: *sighs*Why do people never read the OP properly before they post?It is not my intention to buy this item in order to test its ability to withstand 2.5kV DC, I merely intend to test it so before I use it. Should I so desire, I could test its ability to withstand 10kV AC, but that's not my purpose here.The fact the probe has no ribs/flanges to increase creep resistance should be indication enough that it's NOT a proper HV (vague definition) piece of equipment.For reference purposes, 50V-10kV is intermediate voltage. High voltage starts at 10kV and above.@tggzzz if I could provide a 1kV sinewave at 100MHz, I'd be a broadcaster, not an EE hobbyist. That is not a real world situation for this kit; if I was working on such gear I'd have expensive TE that my employer paid for.@ tau, everything about this worries me, if I was dealing with signals at or near the advertised limits!In reality, I will be using it at UK mains and rectified mains voltages, so nothing above 400VDC/250VAC or so.

author: tautech
author_rank: Super Contributor
timestamp: March 21, 2020, 08:53:43 pm
content: Quote from: AVGresponding on March 21, 2020, 08:45:25 pmYeah the form factor is not quite what you'd expect for a 100x probe nevertheless for mains and rectified mains it should be fine. Note in this form factor Siglent spec their 10x as a 600 V CAT III, 1000 V CAT II rated probe.So to answer your original question, yes these are well priced.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 21, 2020, 09:02:21 pm
content: Quote from: tautech on March 21, 2020, 08:53:43 pmThose would be my guesses at CAT ratings.I will order one, and I will post test results on the TEA thread.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2020, 09:13:18 pm
content: Quote from: AVGresponding on March 21, 2020, 08:45:25 pmMy comment was intended to make you think about physical limitations, and to indirectly indicate important specifications that you should consider - in this case the max voltage vs frequency spec.As you hadn't hadn't mentioned your limited uses (50Hz 240V), that was a reasonable concern.For your application ion, I'd seriously consider an isolated HV differential probe, since that avoids problems that will occur if you connect the probe shield to a point that isnt at 0.0V. Hint: earth/ground is a fiction.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 21, 2020, 10:01:52 pm
content: tsk tggzzz I'm sure you saw I just (accidentally) bought a PM99 II, so ground isolation isn't an issue.The physical limitations of this item are, I'm sure, well short of the *claimed* specs. No doubt these are obtained by the manufacturer under ideal conditions (if at all).

author: edavid
author_rank: Super Contributor
timestamp: March 21, 2020, 10:18:10 pm
content: Did you see this previous thread?https://www.eevblog.com/forum/testgear/high-voltage-oscilloscope-probes-55594/

author: tautech
author_rank: Super Contributor
timestamp: March 21, 2020, 10:46:19 pm
content: After a short hunt the probe in question is made by the same company as make the P6100 probes and the P2300C is rated for 1.5KV but apparently without a CAT rating although the same design in 10x have CAT II and III ratings.Link shared privately with the OP.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 21, 2020, 11:36:59 pm
content: Quote from: tautech on March 21, 2020, 10:46:19 pmTesting will be an exciting time!Let me check my will...

author: Mortymore
author_rank: Frequent Contributor
timestamp: March 21, 2020, 11:45:23 pm
content: Quote from: edavid on March 21, 2020, 10:18:10 pmOr smbaker forget to report, or theHantek T3100probes still lives. I'm hoping for the latersince I also have one T3100, but he highest voltage measured was when checking a small power supply for CCFL testing. 265Vrms@35KHzI'm curious to see the OP probe testing.

author: tggzzz
author_rank: Super Contributor
timestamp: March 22, 2020, 09:23:57 am
content: Quote from: AVGresponding on March 21, 2020, 10:01:52 pmNo, I didn't spot that, and I'll bet >99% of people reading this thread won't.That's why context can be important.

author: Electro Detective
author_rank: Super Contributor
timestamp: March 22, 2020, 09:58:10 am
content: Quote from: AVGresponding on March 21, 2020, 11:36:59 pmBEFORE you test, set up a Patreon account first, then Youtube the proceedings,so family and friends can get back some cash on the funeralHOPING that doesn't happen mateplay safe = glasses, gloves, rubber soles,and kick out the cat!

author: AVGresponding
author_rank: Super Contributor
timestamp: March 22, 2020, 12:19:08 pm
content: Haha no worries, I'll not be electrocuting myself.I've a 2.5kV bench psu, and I'll use it in series with my UT 139C on uA to see if I get any leakage as I ramp the voltage up.It's got some nice safety features, one is that it shuts the output off if it detects earth leakage above 0.6mA, so I should be ok and I'm not one of those people that likes to touch conductors to see if they're live...

author: joeqsmith
author_rank: Super Contributor
timestamp: March 22, 2020, 03:37:07 pm
content: I looked at a low cost 100X probe and picked up a couple. While testing one within the manufacture's derate curve and damaging it, I cut it apart. Inside, a 1/4W axial resistor. That ended my search for a cheap probe and, I ended up rolling my own.https://www.eevblog.com/forum/testgear/teledyne-lecroy-waverunner-64xi/msg676682/#msg676682

author: joeqsmith
author_rank: Super Contributor
timestamp: March 22, 2020, 03:54:36 pm
content: Quote from: gby on March 21, 2020, 08:38:09 pmSo? Even with 5kVDC, with 100M we are only talking about 50uA. The outlets in my lab, with nothing connected to them also have their full voltage and a much lower source impedance. I would rather touch a 5KV low energy source with a 100M limiting resistor than a 110 or 220V outlet.After watching this guys video on a Fluke meter, a few people asked that I have a look at one.I made a few videos but in this link you can see me try to break down the meter using my hand as part of the circuit.https://youtu.be/k8hhtTtWfVc?list=PLZSS2ajxhiQBz1fid9OiPNw0gVodm3eeV&t=2249

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: April 09, 2020, 12:40:40 pm
content: Being how times are so hard, I just orderedthis cheap-ass probe setfrom Amazon for $17

author: CDaniel
author_rank: Frequent Contributor
timestamp: April 09, 2020, 09:11:03 pm
content: Insulation is one issue , but cheap probes don't have the bandwidth advertised or is very non linear ... bad investment anyway .

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: April 10, 2020, 12:47:13 am
content: Yes but, for $17, I can test up to 100 MHz and 1,000V and not worry about frying my scope; mission accomplished.