34401A 4 wires measurment behavior? - Page 1
Forum section: electronics / repair

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 10:55:35 pm
content: So when I turn on 4 wire measurement mode on a 34401A, and you don't have anything plugged in, what should it do?I have a meter that if I leave it alone in 4 wires mode, it eventually reads a short circuit when there is nothing plugged in. It clicks for a while and then reads 0.000 ohms, until you reset it.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 23, 2022, 11:28:54 pm
content: I don't have that model, but others I've used (HP, Agilent, fluke) give either some random readings or show open circuit.It's one of my pet peeves about many commercial 4-wire ohm meters: If you have the sense wires connected properly, and one or both of the source leads is not connected, the instrument doesn't give you a warning but instead gives you an erroneous reading.

author: Whales
author_rank: Super Contributor
timestamp: January 23, 2022, 11:32:59 pm
content: I would have thought there might be a high-value resistor between each input and its sense terminal, but it doesn't look like it(page 155)https://xdevs.com/doc/HP_Agilent_Keysight/service/34401A/34401A%20service.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: January 24, 2022, 12:11:34 am
content: I'm not gonna lie for some reason whenever I try to understand sense circuits that people implement (not the way I would do it), its very confusing. Especially power supplies.

author: TimFox
author_rank: Super Contributor
timestamp: January 24, 2022, 12:30:03 am
content: I don't have a 34401A, but many power supplies with remote sense option have diodes between the source and sense terminals so that the source doesn't run away with disconnected sense leads. In two-terminal mode, there are jumpers on a screw terminal strip to connect the source and sense leads.

author: xrunner
author_rank: Super Contributor
timestamp: January 24, 2022, 12:33:25 am
content: Quote from: coppercone2 on January 23, 2022, 10:55:35 pmI just tried mine. It starts out at ~20 kOhms and goes up for a while to ~85 kOhms, then settles back down to 0.005 ohms.

author: coppercone2
author_rank: Super Contributor
timestamp: January 24, 2022, 01:39:27 am
content: Quote from: xrunner on January 24, 2022, 12:33:25 amI tested it, both of mine do this.When it gets to 0.005 ohms, does it get 'locked"? I need to reset my meter after it gets to 0.005 ohms because it seems to not care if you plug something into it at that point. Kind of a big over sight for a feature...

author: Kleinstein
author_rank: Super Contributor
timestamp: January 24, 2022, 08:41:55 am
content: The behavior is pretty undefined. As the sense inputs are floating, they can give relatively random readings. A tendency to finally settle to a low values is plausingle from charge carry over between the inputs that are measured alternating. It is not sure if all units behave the same - depending on the bias currents there may be a few groups: like some reading near zero, some showing an overflow and some maybe read negative.

author: J-R
author_rank: Super Contributor
timestamp: January 24, 2022, 08:54:25 am
content: My 34401A clicks for a while then drifts around the k-Ohms and M-Ohms for a bit. Going up and down the ranges manually and I get some pretty high non-zero values in the upper ranges and basically all zero for the lower ranges. Attaching Kelvin leads and switching to auto at that point shows overload. No lockups.

author: electr_peter
author_rank: Unknown
timestamp: January 24, 2022, 10:25:52 am
content: Unconnected 2W voltage mode pickups stray voltages, etc., so I would not be surprised if unconnected 4W resistance mode showed nonsense values due to high input impedances.Current 34401A setup: 4W plug adapter/gripers on back panel connectors, 2W DMM leads on front panel connectors. 4 wire adapter is shorted (clipped together) if not used. This allows quick change between 4W and 2W measurement with terminal change button.I have not noticed drift to 0 Ohm in 4W mode, but then sense and force terminals are almost always shorted together.

author: NaxFM
author_rank: Regular Contributor
timestamp: January 24, 2022, 01:46:57 pm
content: I just used my 34401A for 4W resistance measurement something like 5 minutes ago.Mine will usually show a short circuit when nothing is connected, but sometimes it tends to drift a bit.It's normal. Since force and sense are now indipendent, it just tries to read the voltage at the sense terminals. What happens when you do a DC voltage measurement and don't connect anything? Sometimes it shows 0V, sometimes it drifts.It doesn't do that in two wires because the internal current source shares the same output as the sense voltage input, so in this case it reads the high voltage produced by the constant current source, which is desperately trying to raise the voltage to output its current into thin air.

author: xrunner
author_rank: Super Contributor
timestamp: January 24, 2022, 02:05:06 pm
content: Quote from: coppercone2 on January 24, 2022, 01:39:27 amLocked? Mine doesn't get "locked". I tried it several times this morning. Left the leads off and switched to 4 wire. After it get to 0.005 ohms I put in the leads and it measures normally ...

author: mawyatt
author_rank: Super Contributor
timestamp: January 24, 2022, 03:15:10 pm
content: Interesting, both of our 34401A (AG and HP) don't lock-up under the open circuit 4 Wire Ohms mode, the read correctly as soon as a 4 wire proper resistive connection is made. They do read 0.003 and 0.007 ohms respectively when open-circuited after the input capacitance discharges, tho.Best,

author: 555
author_rank: Newbie
timestamp: January 25, 2022, 06:56:09 pm
content: Mine (Rev 11-05-02) comes down to 0.004 Ohms from 0.2 M-Ohms in about 30 sec without leads connected. When i then connect the leads readout is normal, no lockups..

author: coppercone2
author_rank: Super Contributor
timestamp: January 25, 2022, 09:33:14 pm
content: Now mine is disengaging.. but once I think I had it on 0.005 ohms or so and when I plugged in the kelvin clips it would not do anything even when I opened and closed both sides or touched them together. But I can't seem to repeat it.

author: chekhov
author_rank: Regular Contributor
timestamp: January 31, 2022, 09:11:00 am
content: And if you manually go into megaohms range, behavior seems to be different ...

author: Kleinstein
author_rank: Super Contributor
timestamp: January 31, 2022, 10:54:09 am
content: With many meter the highest ohms ranges are no longer 4 wire mode and they switch to 2 wire mode. This is because the 4 wire mode usually adds leakage currents and is thus less accurate about some 100 K or 1 M.In 2 wire mode the open circuit case is easy.

author: TimFox
author_rank: Super Contributor
timestamp: January 31, 2022, 02:44:31 pm
content: Also, note which two jacks are used in 2-wire mode. Connecting to the wrong pair when the meter is in 2-wire mode will give strange results.