Basics of old-school High Voltage probes and modern DMMs - Page 1
Forum section: electronics / beginners

author: w2aew
author_rank: Super Contributor
timestamp: April 24, 2014, 10:55:48 pm
content: This video gives an overview of old-school high voltage probes, how they're made, and how they work. Included in the video are a few subtle aspects of your modern DMM that you might not have known about. For example, did you know that the input resistance of your DMM may change depending on the voltage scale you're on? This can affect the accuracy when using a typical HV probe.

author: Wytnucls
author_rank: Super Contributor
timestamp: April 24, 2014, 11:29:27 pm
content: Thanks Alan, nice concise lecture about an instrument extension which could be dangerous if not handled properly. Was a bit disappointed you didn't measure your local substation with it.

author: robrenz
author_rank: Super Contributor
timestamp: April 25, 2014, 01:29:48 am
content: AC input impedance can be vastly different than the DC also. (along with differences among ranges)

author: N2IXK
author_rank: Frequent Contributor
timestamp: April 25, 2014, 02:09:35 am
content: Almost all of those accessory HV probes are intended for use only on low energy circuits, like CRT anode supplies, etc. They should NOT be used for measuring stuff like pole transformers.Not all of those probes are a single resistor, either. I have a Fluke 80K-40, which incorporates both resistors of the voltage divider into the probe itself. This is actually safer, as it prevents HV from being present at the meter output leads if HV is applied without a meter connected, as long as the ground lead is connected to the DUT. And they mention the 11M vs 10 M impedance issue on various ranges of Fluke meters in the manual here:http://assets.fluke.com/manuals/80k40___iseng0900.pdf

author: IanB
author_rank: Super Contributor
timestamp: April 25, 2014, 02:39:46 am
content: Quote from: N2IXK on April 25, 2014, 02:09:35 amOK, but a quick calculation suggests that 40 kV across a 990 MOhm resistor would produce a current of about 40 µA. Is that really going to present a danger?

author: w2aew
author_rank: Super Contributor
timestamp: April 25, 2014, 04:32:32 am
content: Quote from: N2IXK on April 25, 2014, 02:09:35 amReally good info, thanks!

author: Vgkid
author_rank: Super Contributor
timestamp: April 25, 2014, 04:45:52 am
content: Thanks for the video.

author: robrenz
author_rank: Super Contributor
timestamp: April 25, 2014, 03:01:06 pm
content: I just measured my 8846A on DC ranges using my 87 and I get the expected 10M. I measure it with the 87 on AC ranges and get 10M but the specs say1M on ACinputs. I measure the AC ranges using my LCR meter so it is aAC excitationinstead of DC and I get the expected 1M. AFIK this makes sense since the 8846A can measure AC and DC simultaneously.

author: katzohki
author_rank: Frequent Contributor
timestamp: April 25, 2014, 03:15:53 pm
content: Here's a trap:Let's say you're using something like a 10Meg resistor and 1k resistor for a roughly 1000:1 voltage divider. Suddenly that 1Meg (or whatever it may be) input impedance on your DMM that you can usually ignore becomes a significant part of your voltage divider! Whoops!

author: w2aew
author_rank: Super Contributor
timestamp: April 25, 2014, 03:18:21 pm
content: Quote from: robrenz on April 25, 2014, 03:01:06 pmGood info. Most people don't have the ability to make the AC resistance/impedance measurement like that. I suspect, as you said, that the AC load impedance will be different from the DC load impedance on most DMMs.

author: switcher
author_rank: Regular Contributor
timestamp: April 25, 2014, 09:33:42 pm
content: Great video as always.But you didn't do the bit we were all waiting for; to hook up your probe to the HV of one of your CRT scopes.

author: w2aew
author_rank: Super Contributor
timestamp: April 26, 2014, 12:55:02 am
content: Quote from: switcher on April 25, 2014, 09:33:42 pmI was saving that for a future video. Planning on doing a scope calibration video at some point...

author: Tube_Dude
author_rank: Regular Contributor
timestamp: April 26, 2014, 04:16:21 am
content: Quote from: w2aew on April 26, 2014, 12:55:02 amWow, that will be great! Looking forward with big expectation...

author: switcher
author_rank: Regular Contributor
timestamp: April 26, 2014, 07:17:13 am
content: Quote from: w2aew on April 26, 2014, 12:55:02 amFantastic news! May I humbly request you use one of your older scopes; ISTR you've got a 465 and a 485?The 485 would be great!

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 11, 2014, 12:39:36 am
content: I picked a couple used ones at ham fest's a Heathkit 336 W - that one looks like a Flukeresistor marked RPC BP-6 1.09 G 2% Heath 2-47 7218Then the other is Pomona md. 2900 , with meter 0-30 KV D.C.resistor rpc HP-6 600M 2%Both the resistors are open , looking for replacement resistors ?

author: w2aew
author_rank: Super Contributor
timestamp: May 11, 2014, 12:04:40 pm
content: Quote from: KD0CAC John on May 11, 2014, 12:39:36 amAre you sure they're open? 990Mohm or so is higher than most DMMs can measure.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 11, 2014, 12:52:12 pm
content: Hay Allen , if got the correct spelling ?I thought about that , will have to check the spec.s , have a few meters , the one I use just before the post was a Fluke , dang now I forget , my 87 got stollen and a friend gave me a 27-? when his company let him buy another .Also have the Mastech 5308 .

author: electronics man
author_rank: Frequent Contributor
timestamp: May 11, 2014, 02:51:01 pm
content: it would be great if some DMMs had a setting for a high voltage probe that does multiplication for you internally so you dont need to bother with multiplying it up in your head.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 11, 2014, 03:17:19 pm
content: Oh shucks , I need another tool .Between the Fluke 23 III , about 300m & the Mastech 5308 200m .I have some more LCR meters , the same one in your most recent video .Any suggestions on a meter that will reach that range , and not break the bank ?The Pomona add a meter on it not math

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2014, 07:16:09 pm
content: Quote from: w2aew on May 11, 2014, 12:04:40 pmAssuming a 10 MOhm DMM input resistance, the high value resistor can be placed in series with the input to form a voltage divider allowing the DMM to measure high value resistors. I have a 999M high voltage resistor that I checked this way.