Has anybody used the Agilent U1272 Smart Ohms feature?

[ejeffrey]

Has anybody used the smart ohms features on this meter?  How well does it work?  The theory is that it turns the excitation current on and off to subtract off the offset voltage -- the intent is to measure ground resistance when there might be other voltages present.  I would be interested to see how it works in the presence of 30 mVAC + 10 mVDC.

[alm]

So they've found a new name for offset compensated ohms which has been present in some bench meters for at least thirty years? The specs should tell the voltage range. At least the appnote about ground resistance measurement has the information, so I expect that it's also in the manual. I wouldn't expect it to work with AC unless explicitly stated, since resistance tends to be a DC only measurement on a DMM.

[Kiriakos-GR]

I do not have adjustable AC calibrator.
But I do have at DC.

Enjoy 30 mVDC.   

[Kiriakos-GR]

I can confirm it works with AC too, 
I did one small test circuit, just to test about AC, and no about accuracy.

The circuit is simple, one 3V T/F one 2K R + the 8010 measuring mA + 3K R , and back to the T/F.
The DMM got in parallel with the 2K R .

This is what I got in the screen.  ( pictures 1+2+3)

Then removed the 8010 DMM , tested the circuitry again.
1) Disconnect the AC
2) Tested the R2K alone = 1.999
3) Reconnect the R2K with the R3K and the  T/F  ( The R2K fall to 1.18K)
4) AC on
5) Instead of mV I changed to mA ,  and I got exactly the same mA that I was measuring with the 8010A DMM,
and just a bit more, because with out it the resistance of the circuitry become lower,
and the mA got a bit high.

I can not explain why with the voltage on, the R2K looks as pictured in the picture No4.

But speaking about accuracy,  the mA are spot on, on AC. ( smart Ohm) 

 
   

[ejeffrey]

Thanks!  I know it is an old trick with a new name, but it is nice to see it working in a hand held meter.

[alm]

According to the manual, the maximum correctable voltage is +50mV/-30mV for the 30ohm range, and +1V/-0.2V for 300ohm to 300kohm. It's the data sheet only mentions it to only correct for DC voltages/currents, so I'm surprised that it works at all. You're likely outside the specified range, however, so don't rely on the accuracy.

The current measurement appears to work, even though it fails to measure the resistance correctly. I'm surprised that this doesn't influence the current measurement, since I would expect it to calculate R=V/I.

[Kiriakos-GR]

The current measurement appears to work, even though it fails to measure the resistance correctly. I'm surprised that this doesn't influence the current measurement, since I would expect it to calculate R=V/I.

The R=V/I it does work   V=0,331V / I=0,748A = R = 0.442 kohm  (very close to what the display shown)

The point is that because my test circuit is a close circle that contains and the secondary resistance,
and the resistance of the of the  t/f , I can not tell how those factors play up,  and mess up the resistor value to drop from the 2K to 440 Ohm.   

What I have here is  Rtotal= R1/2 + R2/2+R3/2        R1=2K  R2=3K  R3=776 Ohm ( the t/f)     
   

[alm]

The transformer is in series with R3, isn't it? So that would be 2k // (3k + 776), which is much more than 400ohm. Does the resistance value stay the same if you leave the transformer connected but disconnect the primary side?

[Kiriakos-GR]

The 2K ( single)  falls at 1.18K (close circuit)  , with out the primary active.
The output of the secondary is 3V.
As soon the primary gets active it drops down to 440 ohms.

I do not have study ( read the manual ) about smart-Ohm ,
if there is a limit about the 1V , then this is it.

I did try and something else,  I connect the DMM to the t/f , to measure the secondary ( 720 ohm ).
And then activated the primary.
The DMM acted like crazy, all digits was dancing LoL       

[ejeffrey]

According to the manual, the maximum correctable voltage is +50mV/-30mV for the 30ohm range, and +1V/-0.2V for 300ohm to 300kohm. It's the data sheet only mentions it to only correct for DC voltages/currents, so I'm surprised that it works at all.

I was hoping, and it looks like those hopes were justified, that the meter would ignore the AC component automatically due to its built-in averaging functionality.  If the current source is reasonably high impedance at 50/60 Hz (not a difficult task) and the volt meter's filter has good 50/60 Hz rejection, even the 'normal' ohms mode should not be strongly affected by a small AC voltage.  However, that is all theory, and there is almost nothing so trivial someone can't screw it up.  I expect the agilent guys know what they are doing, and it looks like in this instance I wasn't disappointed

[Kiriakos-GR]

Personally I wasn't disappointed, by the lightning speed that this damn box measures the resistors.
Its speedy like a devil . Lol   

[alm]

I was hoping, and it looks like those hopes were justified, that the meter would ignore the AC component automatically due to its built-in averaging functionality.  If the current source is reasonably high impedance at 50/60 Hz (not a difficult task) and the volt meter's filter has good 50/60 Hz rejection, even the 'normal' ohms mode should not be strongly affected by a small AC voltage.

I agree, NMRR should help if the frequency is very close to 50/60Hz, at least for the volt measuring part.

  However, that is all theory, and there is almost nothing so trivial someone can't screw it up.  I expect the agilent guys know what they are doing, and it looks like in this instance I wasn't disappointed

Indeed, 440ohm instead of 1.18kohm is only 63% error. That's still within one order of magnitude.