Author Topic: UNI-T UT139E negative intrinsic capacitance...  (Read 543 times)

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Online Manx

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UNI-T UT139E negative intrinsic capacitance...
« on: March 09, 2019, 05:01:28 am »
I can't measure small capacitors with my UT139E. It shows 0.000 nF. I thought it's some mysterious inability to deal with small capacitance. But when I tried to determine the lowest measurable value, it turned out the meter systematically measures about 200 pF too low. So anything less is just zero. Does anyone have any idea how to calibrate this?
 

Offline HKJ

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Re: UNI-T UT139E negative intrinsic capacitance...
« Reply #1 on: March 09, 2019, 07:15:13 am »
Can you remove it with the "REL" button?
Another way is to measure a 1nF capacitor first, then put the pF capacitor in parallel with the 1nF and measure again.
 
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Offline supercilious

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Re: UNI-T UT139E negative intrinsic capacitance...
« Reply #2 on: March 09, 2019, 12:34:59 pm »
I just checked on my UT139S (which is AFAIK identical to the UT139E except the white on black screen) and it reads a fluctuating 12-26pF with the probes shorted depending on how I hold them. I'm using the eBay cheapie gold tipped probes here rather than the included ones. The reading is noisy, but I can measure a 150pF ceramic capacitor as ~0.162nF which seems reasonable.
 
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Online Manx

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Re: UNI-T UT139E negative intrinsic capacitance...
« Reply #3 on: March 09, 2019, 04:24:12 pm »
Thanks.
I cannot do anything with the REL button. I guess that since the meter normally shows zero, and not a negative value, using REL just subtracts this zero, so nothing changes. It seems that my (particular?) device not only compensates for its intrinsic capacitance, but it also over-compensates. Adding 220 pF cap helps. Then I can measure even single digit pF caps.
So another question is: would soldering the additional cap inside the meter affect other measurements? (like V_AC or Hz, which are on the same socket?)
 

Offline supercilious

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Re: UNI-T UT139E negative intrinsic capacitance...
« Reply #4 on: March 09, 2019, 04:54:54 pm »
So another question is: would soldering the additional cap inside the meter affect other measurements? (like V_AC or Hz, which are on the same socket?)

I would strongly advocate against any modification like that, even if safety was not a concern.

I just looked up a teardown online, and there is an 8 pin SOIC chip on the board that almost certainly contains the calibration data. Looking at the part number, it is a 24C02 I2C EEPROM. I would start by poking at the values in there (after making a backup of course) to see if I can improve the results. The datasheet of the DMM chip might even tell you exactly which values you need to change to get the desired result.
 
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