Interesting impedance reading for Agilent U1252B

[giosif]

Hi,

I was playing with a couple of insulation testers today (one from Fluke and one from Gossen) and, as a test, I wanted to check the input impedance of my Agilent U1252B meter against the various voltages the insulation testers can output.
And, to my surprise, both insulation testers showed only 5 MOhms impedance for the U1252B (set to DCV) for all testing voltages (i.e. 50V, 100V, 250V, 500V and 1,000V).
At the same time, if I measure the impedance of the same U1252B (again, in DCV mode) with a regular multimeter, it shows the expected 10/11 MOhms value.
Anyone has seen this before?
I am tempted to rule out leakage in one of the input protection components of the U1252B (e.g. MOV), given the impedance stays exactly the same across the entire range of measurement voltages from 50V to 1,000V.
But, then, what else could it be?

[shakalnokturn]

No Agilents at hand due to their second hand prices being overrated here.
I do have some similar Escort DMM's that I could try this on out of curiosity.

How does the insulation tester reading change if you cycle the range on U1252B manually while testing?

[giosif]

How does the insulation tester reading change if you cycle the range on U1252B manually while testing?

Hmm... Didn't think of trying that in the first place.
I checked now and the reading hovers around 5 MOhms no matter the DCV range set on the U1252B.
This was checked with a 50V output on the insulation tester side.

[shakalnokturn]

No such behaviour here with my two Escort 97's insulation tested with Fluke 1587.

Have you tried throwing an Ammeter in the loop to double check everything makes sense?

[bdunham7]

See note (1) on page 153 of the manual. 

https://docs.rs-online.com/071f/0900766b80dc904a.pdf

Depending on how you are operating the meter, this could be normal or there could be some issue causing the low (parallel) impedance when it shouldn't be there.

If you manually select the 1000V range and then test with another DMM, what do you get?  What do the insulation testers indicate in the AC and AC+DC ranges?

[electr_peter]

What is resistance accuracy of used insulation testers? 5-10MOhm is considered low resistance for such meters.
What is the input resistance of other DMM tested with insulation testers?

[giosif]

Thanks, guys!
Please see my response below.

Have you tried throwing an Ammeter in the loop to double check everything makes sense?

No, but the same insulation testers show 10/11 MOhms input impedance for other meters I have, including a "blind" (i.e. non-working OLED display) Agilent U1253A.
So, I doubt it is the insulation testers at fault here.

See note (1) on page 153 of the manual. 
https://docs.rs-online.com/071f/0900766b80dc904a.pdf

I am not sure what to make of that note.
What exactly do they mean?
In any case, testing with a very similat DMM, Agilent U1253A, the input impedance measured by the insulation testers is 10/11 MOhms.
Not excluded, but I think it is unlikely that Agilent/Keysight have changed much on the input between the U125xA and the U125xB models.

If you manually select the 1000V range and then test with another DMM, what do you get?

I get 10 MOhms impedance.

What do the insulation testers indicate in the AC and AC+DC ranges?

In AC mode, the measured input impedance is OL for all ranges, which makes sense given the capacitor in the circuit for that measurement mode.
In AC+DC mode, the measured input impedance is 5 MOhms for all ranges.
So, it must be something in the DC mode path.

What is resistance accuracy of used insulation testers? 5-10MOhm is considered low resistance for such meters.

I don't have the specs at hand, but it's not the resolution or accuracy of the insulation testers.

What is the input resistance of other DMM tested with insulation testers?

With other DMMs but same insulation testers, the measured input impedance is the typical 10/11 MOhms.

[bdunham7]

I am not sure what to make of that note.
What exactly do they mean?
In any case, testing with a very similat DMM, Agilent U1253A, the input impedance measured by the insulation testers is 10/11 MOhms.
Not excluded, but I think it is unlikely that Agilent/Keysight have changed much on the input between the U125xA and the U125xB models.

Is the U1253A dual-display?  If so, check the manual and see if it has the same note.  I'm going to guess that at least one of those answers is "no".

On the U1252B, set it for the DC volts and then check the impedances when you select a secondary display of Hz or VAC (try both).  Do this with both your insulation tester and another DMM.  I'm going to guess that both the IT and the DMM will show 5M for the Hz secondary measurement.

It may be that the secondary measurement effectively adds another meter in parallel.  Perhaps in single-measurement mode, whatever switches that secondary meter input breaks down above a certain voltage.  If so, I don't know if that reflects an internal failure or an undocumented "feature".  It does seem very odd, but not the sort of thing 99% of users would ever notice.  One of my Fluke meters (116) has a 5M input on VAC (specified).  Nobody would know until they try to use a HV divider probe and ??

[electr_peter]

According to input impedance table and notes in manual, 1252B has 10-11MOhm nominal input impedance, but in 5-1000V range additional parallel 10MOhm impedance is added for dual display (effectively giving 2 DMMs in parallel).
Thus 5MOhm is normal and expected.

Manual could be a bit more clear about this, though.

[giosif]

According to input impedance table and notes in manual, 1252B has 10-11MOhm nominal input impedance, but in 5-1000V range additional parallel 10MOhm impedance is added for dual display (effectively giving 2 DMMs in parallel).
Thus 5MOhm is normal and expected.

Manual could be a bit more clear about this, though.

I understand now. Thanks for the explanation!
Still, the impedance measurements I took up to my previous post were with the U1252B in single display mode.
So, not sure if it is that normal / expected.

Is the U1253A dual-display?  If so, check the manual and see if it has the same note.  I'm going to guess that at least one of those answers is "no".

I've just checked and the U1253A has a dual mode as well and has the exact same note included in the manual.

On the U1252B, set it for the DC volts and then check the impedances when you select a secondary display of Hz or VAC (try both).  Do this with both your insulation tester and another DMM.  I'm going to guess that both the IT and the DMM will show 5M for the Hz secondary measurement.

I tested this and the IT and DMM indeed showed the same 5 MOhms impedance reading with the U1252B in dual display mode, but only for DCV (primary display) and ACV (secondary display).
For the other dual display combinations, the DMM displayed 10/11 MOhms impedance.

It may be that the secondary measurement effectively adds another meter in parallel.  Perhaps in single-measurement mode, whatever switches that secondary meter input breaks down above a certain voltage.  If so, I don't know if that reflects an internal failure or an undocumented "feature".  It does seem very odd, but not the sort of thing 99% of users would ever notice.

That would make sense.
Although, if this is the case, then I would not consider it as an "undocumented feature", because the input impedance for DCV would not be 10 MOhms under any circumstances (i.e. not conform with its datasheet specs).
I think it is now time for me to open up the meter and investigate the input section.

One of my Fluke meters (116) has a 5M input on VAC (specified).  Nobody would know until they try to use a HV divider probe and ??

Ah, interesting.
That reminds me that I also saw the same thing with a Gossen DMM, the MetraHit 25S, but for both DCV and ACV.
I was surprised at the time and was wondering what the purpose of having a lower impedance on the meter might be.

[bdunham7]

I think it is now time for me to open up the meter and investigate the input section.

Maybe you could first characterize the impedance change by supplying a varying voltage (0-50V) to the meter through a 5M resistor.  Take measurements every 5V (input) or so and chart it out.  For example, if you give it 15V and you see 10V displayed, you know the impedance is still 10M, OTOH if you see 7.5V you know the (whatever it is) has broken down/started conducting/turned on and the impedance has dropped to 5M.  I'll be curious to see what you find with this test and when you crack it open for a look.  I'm assuming schematics are not available?

[giosif]

Maybe you could first characterize the impedance change by supplying a varying voltage (0-50V) to the meter through a 5M resistor.  Take measurements every 5V (input) or so and chart it out.  For example, if you give it 15V and you see 10V displayed, you know the impedance is still 10M, OTOH if you see 7.5V you know the (whatever it is) has broken down/started conducting/turned on and the impedance has dropped to 5M.  I'll be curious to see what you find with this test and when you crack it open for a look.  I'm assuming schematics are not available?

That was a great idea! Thank you!

I did just that with the U1252B, but then with the U1253A as well (for this one, I got the readings using an IR-to-Bluetooth adapter, since the OLED display is dead).
And the readings and determined input impedance are very interesting - please see attached graph.
And, yes, the impedance for the U1253A in dual display mode is indeed 1 MOhm, also confirmed with a regular multimeter measuring resistance. 

[electr_peter]

Interesting results - 5MOhm in dual display, but for some reason approaching 5MOhm with higher input voltage in single display mode as well. Schematic would be helpful to understand what is going on, maybe switch is damaged as OP suggested.

1253A behaves with 1MOhm in dual display mode as it says in the manual:

Input impedance: > 1 GΩ for 50 mV to 100 mV ranges. Input impedance is 1.1 MΩ (nominal) in parallel with 1.1 MΩ at dual display.

[giosif]

1253A behaves with 1MOhm in dual display mode as it says in the manual:

Input impedance: > 1 GΩ for 50 mV to 100 mV ranges. Input impedance is 1.1 MΩ (nominal) in parallel with 1.1 MΩ at dual display.

Thanks, but that is both strange and incorrect:

• Strange, because the 2013 version of the user manual for the U1253A specifies an impedance of 10 MOhms nominal + 10 MOhms in parallel for dual display (on par with U125xB specs), while an older version (e.g. from 2008) is specifying exactly what you quote.
• Incorrect, because 1.1 MOhms + 1.1 MOhms in parallel would give 0.55 MOhms total, while we are clearly measuring around 1 MOhms in dual display mode.

I think the old version of the manual is simply wrong, as the input impedance is never more than 11 MOhms for any range, plus there isn't even a 100 mV range for this meter.

[electr_peter]

IMO manual should have read "11 MΩ (nominal) in parallel with 1.1 MΩ at dual display" instead of "1.1 MΩ (nominal) in parallel with 1.1 MΩ at dual display", then everything makes sense for 1253A. ~10x difference in resistances, so 1.1 MΩ dominates in parallel.
I bet on mistake in a manual instead of wild changes in HW design. This is subtle point in a scheme of things, easy to miss.

[giosif]

Actually, you have a point.
Also, the > 1 GOhms impedance for 50 mV to 100 mV ranges statement/note should actually read "50 mV to 1,000V mv ranges".
And I was wrong with my earlier statement about the input impedance never being more than 11 MOhms for any range; I had not realized that, for the 50 mV to 1,000 mV ranges, I needed to switch to mV measurement mode. 

So, all in all, it turns out the U1253A is working as expected in terms of its input impedance.

[giosif]

Coming back to the U1252B, it appears this one is also working as intended.
Doing a search on the net, someone has already detailed the exact same problem (https://blog.brianhoskins.uk/agilent-u1253b-input-impedance-problem/).
And they are pointing to another note in the manual for the U125xB multimeters:
"For 5 V to 1000 V range, the specified input impedance is in parallel with 10 MΩ when the input voltage is >+3 V or <−2 V"
So, in actual fact, when they specify 10 MOhms input impedance in DCV single display mode, they only mean it for voltages between -2V and 3V. 
Am I the only one finding that a bit "misleading"?

Anyway, I learned a few important lessons with this thread and I think I know my multimeters a little better now.
Thank you for the help with that!

[bdunham7]

So, in actual fact, when they specify 10 MOhms input impedance in DCV single display mode, they only mean it for voltages between -2V and 3V. 
Am I the only one finding that a bit "misleading"?

I was annoyed when I found out that most handheld DMMs aren't 10M in the lower ranges--they go 11M, 10.1M and so on.  That makes using HV probes and the like a challenge, but it is fairly easily fixed by just manually ranging up a bit.  This behavior would make the meter useless with any sort of divider and manual ranging apparently won't help.  And for the U1253A, I know bench meters are usually 1M/AC coupled but 1M DC coupled in a handheld DMM seems a bit much.  That makes circuit loading the same or worse than my ancient Simpson 270 for voltages over 10V.