34401A 4 wires measurment behavior?

[coppercone2]

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.

[Kim Christensen]

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.

[Whales]

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

[coppercone2]

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.

[TimFox]

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.

[xrunner]

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.

I 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.

[coppercone2]

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.

I 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.

I 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...

[Kleinstein]

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.

[J-R]

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.

[electr_peter]

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.

[NaxFM]

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.

[xrunner]

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.

I 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.

I 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...

Locked? 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 ...

[mawyatt]

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,

[555]

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..

[coppercone2]

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.

[chekhov]

And if you manually go into megaohms range, behavior seems to be different  ... 

[Kleinstein]

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.

[TimFox]

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.