Author Topic: UT61E+ mv Range Floats and displays OL  (Read 836 times)

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Offline AlgomaTopic starter

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UT61E+ mv Range Floats and displays OL
« on: February 24, 2023, 03:01:36 pm »
Picked up a UNI-T UT61E+ on sale as an extra meter to toss around on the bench.

Trouble is the mV DC/AC Range on the Dial gradually Floats up to an OL state when not connected to anything. Anyone else with this notice this behavior? Trying to figure out if this behavior is something defective, or by design of this meter's front end.  Compared to my Brymen BM786 that shows proper low mV values when disconnected that respond to waving my hand anywhere in the general direction of the meter.
 

Offline myf

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Re: UT61E+ mv Range Floats and displays OL
« Reply #1 on: February 24, 2023, 04:21:40 pm »
Hi,

perhaps this input has a very high impedance, directly (or almost directly) connected to the operational amplifier at xxx G.Ohm (in 10^9 Ohm - 10^12Ohm). Such an input looks more like an antenna.

This is not the case with the usual multimeter inputs at about 10MOhm (=10^7 Ohm)

More knowledgeable people will fill in the blanks...

F. (from France).

 

Offline AVGresponding

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Re: UT61E+ mv Range Floats and displays OL
« Reply #2 on: February 24, 2023, 04:44:00 pm »
Even my old Micronta 22-195 will do this, with its 10MΩ DC input impedance. More expensive (better designed, often) meters have auto-zeroing which negates this, though when you get into the GΩ impedances, they struggle to cope with the stray voltages that EMI and the Seebeck effect (thermocouples use it to their advantage) generate in their front ends.

EDIT: Your UT61E+ isn't faulty, no need to worry.
nuqDaq yuch Dapol?
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Offline Fungus

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Re: UT61E+ mv Range Floats and displays OL
« Reply #3 on: February 24, 2023, 05:09:30 pm »
So long as it measures properly and goes to zero when you short the leads then probably nothing to worry about.

 

Offline bdunham7

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Re: UT61E+ mv Range Floats and displays OL
« Reply #4 on: February 24, 2023, 05:38:58 pm »
The UT61E is Hi-Z (high input impedance) on the lowest voltage range, which is a bit unusual but not unheard of.  What is very unusual though is that it is also Hi-Z in the 220 mVAC range.  When a meter has a nearly-infinite input impedance instead of 1M or 10M, it will do that drifting up or down thing as small random voltages and its own internal bias current charge up whatever parasitic capacitance is across the inputs.  Once you connect it to anything those small charges will dissipate and it will work fine.  Strange, yes but broken, no.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline shakalnokturn

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Re: UT61E+ mv Range Floats and displays OL
« Reply #5 on: February 24, 2023, 11:09:26 pm »
Put something like 100M Ohm across the input, if it still drifts away consider you may have some PCB contamination.
 


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