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| UNI-T UT61E Multimeter teardown photos. |
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| TheNewLab:
--- Quote from: honeybadger on March 27, 2019, 01:21:19 pm ---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. --- End quote --- 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 |
| TheNewLab:
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. --- Quote from: joeqsmith on March 31, 2019, 03:40:18 pm --- --- End quote --- 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? |
| honeybadger:
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?! |
| joeqsmith:
--- Quote from: TheNewLab on April 01, 2019, 12:39:03 am ---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? --- End quote --- 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. |
| joeqsmith:
--- Quote from: honeybadger on April 01, 2019, 01:18:32 am ---Am I dreaming or there is no external voltage clamping after the 10M resistor?! --- End quote --- 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. |
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