Author Topic: DMM problem in DC millivolts range  (Read 2841 times)

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Offline amir.razzaqiTopic starter

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DMM problem in DC millivolts range
« on: July 17, 2023, 07:52:42 am »
Hello guys
I have a Victor VC97 DMM than has an odd behavior in millivolts range. when the millivolts(DC) range selected and while the probes "OPEN ON THE AIR" then numbers showed on LCD start continuously increasing automatically. It sets to Zero when probes contact each other and measure millivolts correctly as well.
does this annoying behavior related to any problem?
Thanks for reading 
« Last Edit: July 19, 2023, 11:16:37 am by amir.razzaqi »
 

Offline EEEnthusiast

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Re: DMM problem in DC millivolts range
« Reply #1 on: July 17, 2023, 08:40:40 am »
This is prefectly Ok. This happens due to the noise pickup by the leads. Try to twist the red and black cables together and the problem will reduce. The best way to test the residual voltage is to short the two cables together and then measure the voltage. Remember that an open circuits is not the same as Zero voltage.
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Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #2 on: July 17, 2023, 09:46:17 am »
Thank you for reply
by the way twisting probes to each other did not change anything. how ever touching probes head by two hands change the numbers and speed of changing.
 

Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #3 on: July 17, 2023, 09:56:46 am »
 

Offline 2N2222A

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Re: DMM problem in DC millivolts range
« Reply #4 on: July 17, 2023, 11:17:24 am »
There is some very small leakage current that is causing the Voltage to slowly rise. In the higher ranges there is a resistor divider that guarantees an impedance of usually 10M ohms. In lowest mV range it may be connected to the ADC through a 1M ohm resistor with no other resistor to give it a stable impedance.

You can measure the impedance of your meter using another meter at the 20M ohm test range.
 
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Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #5 on: July 17, 2023, 11:28:50 am »
Thank you for reply
Does it mean that there is some conflict in this multimeter? Or can it be repaired?
 

Offline Kleinstein

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Re: DMM problem in DC millivolts range
« Reply #6 on: July 17, 2023, 12:40:19 pm »
A few handheld meters have a mV range with a very high impedance (e.g. > 1 GOhm) and in this case minute leakage and bias currents from the amplifier can charge up an open input. In this case it is absolutely normal than the voltage drifts and wanders if left open. No need for a repair.

One gets a similar effect with high end meters, where a high impedance input is standard.

If one want one could check the biasing current, by connecting something 10 M or 100 M to the input and than read the final voltage.
If not sensitive enough one could also use a low leakage capacitor (e.g. 10-100 nF PP) and use the speed at which the votage goes up to estimate the current.
 
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Offline Gyro

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Re: DMM problem in DC millivolts range
« Reply #7 on: July 17, 2023, 12:50:46 pm »
Checking the small print in the VC97 manual, it indicates input resistance of >40MOhm on the 400mV DC range. It is probably much higher than that in practice (basically indicates that the 10MOhm voltage range divider isn't in circuit on that range). As long as the meter reads approximately zero with the probes shorted together then it is fine and normal.
Best Regards, Chris
 
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Offline Xena E

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Re: DMM problem in DC millivolts range
« Reply #8 on: July 17, 2023, 01:22:27 pm »
If this is a new phenomenon it may be there is some contamination causing the leakage somewhere in or on your instrument, doesn't sound like a big deal though.
 
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Offline tom66

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Re: DMM problem in DC millivolts range
« Reply #9 on: July 17, 2023, 01:44:55 pm »
Thank you for reply
Does it mean that there is some conflict in this multimeter? Or can it be repaired?

There is no fault with the meter if when connected to a known voltage source on mV it reads correctly.

For instance connect to 0.5V source and if it reads 500mV on the mV then it is ok.

I have two multimeters which do this, very normal behaviour due to very high impedance input in mV range.
 
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Offline Gyro

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Re: DMM problem in DC millivolts range
« Reply #10 on: July 17, 2023, 08:14:14 pm »
...
For instance connect to 0.5V source and if it reads 500mV on the mV then it is ok.
...

In this case (the VC97), the high impedance mV range is 400mV (399.9mV) full scale. Above that it defaults to 10MOhm. The OP confirmed that it reads mV and shorted input zero correctly.
« Last Edit: July 17, 2023, 08:16:52 pm by Gyro »
Best Regards, Chris
 
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Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #11 on: July 18, 2023, 08:36:24 am »
If this is a new phenomenon it may be there is some contamination causing the leakage somewhere in or on your instrument, doesn't sound like a big deal though.

Thank you for reply
yes, at first the DMM showed numbers bounce approximately 0.5-1.0 millivolts around 0 (I mean fluctuate) but after reading the multimeter's EEPROM (using Arduino) this phenomenon happened.
« Last Edit: July 18, 2023, 08:52:27 am by amir.razzaqi »
 

Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #12 on: July 18, 2023, 08:48:27 am »
Thank you for reply
Does it mean that there is some conflict in this multimeter? Or can it be repaired?

There is no fault with the meter if when connected to a known voltage source on mV it reads correctly.

For instance connect to 0.5V source and if it reads 500mV on the mV then it is ok.

I have two multimeters which do this, very normal behaviour due to very high impedance input in mV range.

Thank you for reply
But this happened after reading multimeter's EEPROM contents and before that every thing looks good.
 

Offline dobsonr741

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Re: DMM problem in DC millivolts range
« Reply #13 on: July 18, 2023, 09:09:39 am »
Did you use a 3.3V level translator for the Arduino when reading the EEPROM? I assume you did it without desoldering.
 
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Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #14 on: July 18, 2023, 10:28:55 am »
Did you use a 3.3V level translator for the Arduino when reading the EEPROM? I assume you did it without desoldering.

 Thank you for reply
I didnt use any voltage level translator also I'm in doubt if I use Arduino 5.0V output pin for the first first time as vcc  |O :-// But when I wire up EEPROM and Arduino (in circuit reading) I used Arduino 3.3V output pin as EEPROM's VCC.
The DMM's main chip (DTM1106) working voltage is 2.4~3.6V according it's datasheet.
« Last Edit: July 18, 2023, 12:04:00 pm by amir.razzaqi »
 

Offline dobsonr741

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Re: DMM problem in DC millivolts range
« Reply #15 on: July 18, 2023, 03:37:13 pm »
The higher than normal voltage applied to the chip can explain larger than expected leakage currents. Think it as a small damage, not enough to create visible impact, other then the leakage current slowly charging up the input capacitance.
« Last Edit: July 19, 2023, 04:51:42 pm by dobsonr741 »
 
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Offline EPAIII

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Re: DMM problem in DC millivolts range
« Reply #16 on: July 19, 2023, 10:07:24 am »
That is an undocumented feature: an RF signal strength meter. Just add a proper, calibrated dipole antenna and a resonant circuit and you have an expensive instrument.

 8)
Paul A.  -   SE Texas
And if you look REAL close at an analog signal,
You will find that it has discrete steps.
 
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Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #17 on: July 19, 2023, 10:50:32 am »
That is an undocumented feature: an RF signal strength meter. Just add a proper, calibrated dipole antenna and a resonant circuit and you have an expensive instrument.

 8)

 :palm: :palm: :palm:
 

Offline tooki

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Re: DMM problem in DC millivolts range
« Reply #18 on: July 19, 2023, 03:25:02 pm »
Did you actually get anything out of the EEPROM?

I wonder if it’s possible that you inadvertently erased or damaged the EEPROM, and that the behavior you’re seeing is because the main chip is not finding the correct configuration in the EEPROM.
 

Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #19 on: July 19, 2023, 09:27:42 pm »
Did you actually get anything out of the EEPROM?

I wonder if it’s possible that you inadvertently erased or damaged the EEPROM, and that the behavior you’re seeing is because the main chip is not finding the correct configuration in the EEPROM.
I already read and learn about the data structure of this DMM's EEPROM. If there was a big problem in EEPROM then the multimeter must showed  "Err E" in addition I active write protect futures of EEPROM while reading it.
 

Offline tooki

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Re: DMM problem in DC millivolts range
« Reply #20 on: July 19, 2023, 11:10:33 pm »
Yeah but it could be a minor error, like a configuration bit for input impedance. (In theory; I don't know what the chip can and cannot do.)
 
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Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #21 on: July 20, 2023, 06:50:45 am »
Yeah but it could be a minor error, like a configuration bit for input impedance. (In theory; I don't know what the chip can and cannot do.)

But as I mentioned I active write protected mode of EEPROM before reading it.
 

Offline tooki

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Re: DMM problem in DC millivolts range
« Reply #22 on: July 20, 2023, 08:31:21 am »
Yes, but if wrong voltages were applied, as you suggested may have happened, it could have caused unpredictable behavior or damage.
 

Offline amir.razzaqiTopic starter

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Re: DMM problem in DC millivolts range
« Reply #23 on: July 20, 2023, 07:43:58 pm »
Yes, but if wrong voltages were applied, as you suggested may have happened, it could have caused unpredictable behavior or damage.
That EEPROM is "atmel 24c02" and can handle up to 5.5 volt
 
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Offline Gyro

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Re: DMM problem in DC millivolts range
« Reply #24 on: July 20, 2023, 09:26:52 pm »
Yes, the 24C02 has a supply range of 1.7 - 5.5V, it will be on the same supply rail as the DTM0660 DMM chip though. This has a supply voltage range of 2.4 - 3.6V (Abs max 4V). This will have created some (over)stress.
« Last Edit: July 20, 2023, 09:30:33 pm by Gyro »
Best Regards, Chris
 
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