Author Topic: UNI-T UT61E Multimeter teardown photos.  (Read 503987 times)

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Offline honeybadger

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1050 on: April 01, 2019, 10:52:37 am »
I guess when it is meant from full range only percentage value is stated - like for an analog meter.

For this DMM it is from reading:


I will confront Bangood with this issue.
 

Offline joeqsmith

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1051 on: April 01, 2019, 11:06:52 am »
Always read the manual first.   :-DD  Looks like you have a bad one. 
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline TheNewLab

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1052 on: April 01, 2019, 11:21:12 am »
Hi, my new UT61e just arrived and I must say I am confused about voltage ranges.

at "mV" DC range it says "OL" when nothing is connected.
at "mV" DC range it measure "0.20mV" even with terminals shorted with thick wire with gold plated banana plugs.
at "V" DC range it masure "0.0020V" with shorted terminals.


This shouldn't be happening I guess. With all my others DMMs when voltage is selected and nothing is connected it shows something but definitely not "OL" and when I short the terminals it shows something really close to zero - definitely not 20 digits above zero.

edit: accuracy is suppose to be:
220mV +/- (0.1%+5)
2.2V / 22V / 220V +/- (0.1%+2)

So for 220mV range it is suppose to show 0.05mV in worst case scenario -> offset is 400% off.
For 2.2 range it is suppose to show 0.0002 in worst case scenario -> offset is 1000% off.
how is this possible with a new multi-meter ?  5USD noname is more accurate.

The "OL" doesn't make sense to me. I learned from an electrician who says he only gets DMMs that when nothing is connected with or without probes attached the voltage numbers drift all over the place. He calls it ghost voltage and it tells him if an electrical circuit is completely open. Some of my DMMs do that others don't they just show 0.0000 when nothing is connected with or without probes attached.

whoa wait! I had tested a meter in voltage mode probes attached not connected to anything just measuring the air in different places and different rooms. I could swear the voltage changed in the air differently from room-to--room..I, we were drunk as hell though!! :popcorn::-DD
« Last Edit: April 01, 2019, 11:30:28 am by TheNewLab »
 

Offline TheNewLab

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1053 on: April 01, 2019, 11:39:03 am »
Joe, That photo seems within specs, but any drift at all I would think should not happen at all.
I have noticed that when manufacturers include in their calibration sheet both temp and RH% when tested. Never have been sure about that. i mean you buy a multimeter calibrated in Cupertino and you live in Denver. both altitude and humidity is way different.




Mine has been modified and may not represent how a stock meter would behave.   With no leads attached, selecting the mV function, it will display OL and slowly drop.   Of the meters I have looked at, this meter was one of the worst for temperature drift.  One of the things I had done was compensate mine.  I would have no way of knowing the tolerance of your 30V supply but I have been tracking the drift.  I have attached today's data.  The voltages are checked against a Fluke 731B reference standard.   The 100ohm is a 0.01% 2ppm Caddock part.   Capacitor is a COG.   

Attached picture shows how badly this meter has been treated.
[/quote]

I have watched your videos on modding multimeters.  I wondered about is, if changing out MOVs, didn't the calibration change?
 

Offline honeybadger

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1054 on: April 01, 2019, 12:18:32 pm »
 TheNewLab: the "OL" at mV range might be some residual voltage in the input circuitry capacitance. When it discharge to zero - that is fine with me. My meter shows "OL" indefinitely.

"Ghosts voltages" are also acceptable - there is a high impedance input... it can change by itself. But there is no place for these ghosts when leads are shorted.

The "OL" at mV range gave me an idea. There must be some current leaking into the ADC input - probably after the 10M resistor.
This would clarify:
- "OL" at mV range (current is leaking into ADC and overloading it, it is "grounded" when anything is connected)
- high zero offset
- asymmetry of measurement when switching polarity

Do I understand the schematic correctly (picture attached)?
Am I dreaming or there is no external voltage clamping after the 10M resistor?!


 

Offline joeqsmith

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1055 on: April 01, 2019, 09:17:31 pm »
Joe, That photo seems within specs, but any drift at all I would think should not happen at all.
....
I have watched your videos on modding multimeters.  I wondered about is, if changing out MOVs, didn't the calibration change?

If only zero drift were even possible!   I'm not suggesting you couldn't buy a really low end meter that never changes one place with temp and time...  A meter that can only resolve down to a volt can't display nV of drift.   Check a manual for some higher end equipment and see what they list for temperature requirements.

If you watched those videos, I would imagine there was some data collected before adding the MOVs.   I would assume the MOVs would change the baseline capacitance but I would null that out before making a measurement.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 
The following users thanked this post: Marco1971

Offline joeqsmith

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1056 on: April 01, 2019, 09:23:01 pm »
Am I dreaming or there is no external voltage clamping after the 10M resistor?!
If you download the datasheet for the controller, you may gain some insight as to why this is. 

Be aware that the schematic you show may not match your meter. 
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline honeybadger

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1057 on: April 01, 2019, 10:21:31 pm »
joeqsmith: in ES51922 datasheet there is:
Most of pins are protected by the ESD protection circuits. However pins, V+, V-, AGND, DGND and
VR1 are not protected enough because the parasitic effect must be decrease. Therefore enough external
protection is needed for assembling, carrying and keeping. In addition, components connecting to these
unprotected pins have to be soldered on board before the IC is soldered.


I just wonder. There is no PTC... nothing. When the 10M resistor fails (or more probably any of the capacitors around) the current will go straight to the IC.



 

Offline joeqsmith

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Re: UNI-T UT61E Multimeter teardown photos.
« Reply #1058 on: April 01, 2019, 11:02:28 pm »
joeqsmith: in ES51922 datasheet there is:
Most of pins are protected by the ESD protection circuits. However pins, V+, V-, AGND, DGND and
VR1 are not protected enough because the parasitic effect must be decrease. Therefore enough external
protection is needed for assembling, carrying and keeping. In addition, components connecting to these
unprotected pins have to be soldered on board before the IC is soldered.


I just wonder. There is no PTC... nothing. When the 10M resistor fails (or more probably any of the capacitors around) the current will go straight to the IC.

That's a VERY good point.  Consider that for DC the capacitors are an open.  Apply a fast transient, like ESD, and they will pass it right on through.   In this case, adding the MOVs won't help.  They may have a fair amount of capacitance but the inductance seems to dominate at these higher frequencies.  No real surprise.   

The PTCs and surge rated resistors are there to limit the current through the clamps.  The problem I see with the 5mm parts is they will break down.   

I made a video some time ago showing all of this on the 61E which includes the part of the document you posted.   I basically walked through the problems and explained a few ways to attempt to improve it.   Sadly, most viewers had little interest in the theory.   I've added a link.

https://youtu.be/cMutvk_6xhY?list=PLZSS2ajxhiQDrk4o1Y45auwK7LomjnNBU
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 
The following users thanked this post: TheNewLab, Marco1971


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