DMM problem in DC millivolts range  - Page 1
Forum section: electronics / general

author: amir.razzaqi
author_rank: Contributor
timestamp: July 17, 2023, 07:52:42 am
content: Hello guysI 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

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 17, 2023, 08:40:40 am
content: 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.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 17, 2023, 09:46:17 am
content: Thank you for replyby 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.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 17, 2023, 09:56:46 am
content: the video of this DMM:https://www.youtube.com/shorts/Dy-N1WPPqag

author: 2N2222A
author_rank: Regular Contributor
timestamp: July 17, 2023, 11:17:24 am
content: 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.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 17, 2023, 11:28:50 am
content: Thank you for replyDoes it mean that there is some conflict in this multimeter? Or can it be repaired?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2023, 12:40:19 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2023, 12:50:46 pm
content: 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.

author: Xena E
author_rank: Frequent Contributor
timestamp: July 17, 2023, 01:22:27 pm
content: 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.

author: tom66
author_rank: Super Contributor
timestamp: July 17, 2023, 01:44:55 pm
content: Quote from: amir.razzaqi on July 17, 2023, 11:28:50 amThere 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.

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2023, 08:14:14 pm
content: Quote from: tom66 on July 17, 2023, 01:44:55 pmIn 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.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 18, 2023, 08:36:24 am
content: Quote from: Xena E on July 17, 2023, 01:22:27 pmThank you for replyyes, 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.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 18, 2023, 08:48:27 am
content: Quote from: tom66 on July 17, 2023, 01:44:55 pmThank you for replyBut this happened after reading multimeter's EEPROM contents and before that every thing looks good.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 18, 2023, 09:09:39 am
content: Did you use a 3.3V level translator for the Arduino when reading the EEPROM? I assume you did it without desoldering.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 18, 2023, 10:28:55 am
content: Quote from: dobsonr741 on July 18, 2023, 09:09:39 amThank you for replyI 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 vccBut 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.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 18, 2023, 03:37:13 pm
content: 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.

author: EPAIII
author_rank: Super Contributor
timestamp: July 19, 2023, 10:07:24 am
content: 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.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 19, 2023, 10:50:32 am
content: Quote from: EPAIII on July 19, 2023, 10:07:24 am

author: tooki
author_rank: Super Contributor
timestamp: July 19, 2023, 03:25:02 pm
content: 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.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 19, 2023, 09:27:42 pm
content: Quote from: tooki on July 19, 2023, 03:25:02 pmI 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.

author: tooki
author_rank: Super Contributor
timestamp: July 19, 2023, 11:10:33 pm
content: 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.)

author: amir.razzaqi
author_rank: Contributor
timestamp: July 20, 2023, 06:50:45 am
content: Quote from: tooki on July 19, 2023, 11:10:33 pmBut as I mentioned I active write protected mode of EEPROM before reading it.

author: tooki
author_rank: Super Contributor
timestamp: July 20, 2023, 08:31:21 am
content: Yes, but if wrong voltages were applied, as you suggested may have happened, it could have caused unpredictable behavior or damage.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 20, 2023, 07:43:58 pm
content: Quote from: tooki on July 20, 2023, 08:31:21 amThat EEPROM is "atmel 24c02" and can handle up to 5.5 volt

author: Gyro
author_rank: Super Contributor
timestamp: July 20, 2023, 09:26:52 pm
content: 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.

author: amir.razzaqi
author_rank: Contributor
timestamp: July 23, 2023, 09:06:32 am
content: Quote from: Gyro on July 20, 2023, 09:26:52 pmBut I say this abut just EEPROM to reply for:Quote from: tooki on July 19, 2023, 03:25:02 pmnot the main chipset of DMM

author: tooki
author_rank: Super Contributor
timestamp: July 23, 2023, 11:51:44 am
content: Quote from: amir.razzaqi on July 18, 2023, 10:28:55 amQuote from: amir.razzaqi on July 20, 2023, 07:43:58 pmFYI: Note that while the data inputs are specified to +7V in the absolute maximum ratings (i.e. the values at which the chip will be damaged), they are rated to a maximum of Vcc+0.5V in the DC characteristics. So if you powered the chip with 3.3V, but applied 5V signals to the inputs, then you’re operating it out of spec, since 3.3V+0.5V=3.8V. This should not permanently damage it, but the chip is not guaranteed to operate correctly while doing so.