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Author Topic: open circuit reading of multimeter in mV range  (Read 6225 times)

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Offline d3javuTopic starter

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open circuit reading of multimeter in mV range
« on: October 12, 2010, 10:24:58 am »
Hi folks,
          I have just bought a Uni-T UT61C multimeter and while playing with it, i notice that the voltage reading in mV range is increasing constantly and sometime when i turn on the multimeter and straight away set it to mV range, it shows overload. Previously, i have been using cheap multimeter which never had such a behaviour in 200mV range.

« Last Edit: October 12, 2010, 10:37:09 am by d3javu »
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Offline scrat

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Re: open circuit reading of multimeter in mV range
« Reply #1 on: October 12, 2010, 10:54:24 am »
Couldn't it be that it catches some noise? They have a very high input impedance, and signal is rectified for DC measurement, so I suppose noise rectified charges input capacitance and makes voltage grow up.
Have you tried to go, for example, outdoors? Does the problem increase if you touch a terminal?
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Offline d3javuTopic starter

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Re: open circuit reading of multimeter in mV range
« Reply #2 on: October 12, 2010, 11:09:47 am »


here is an example, another thing is while on auto range, after 60mV, the value jumped to 200+, obviously the range changed automatically but the value jumps from 60mV to 200mV is kinda weird to me. the probe is not in contact with anything. there is only a fan running next to me, i did move out and try, it is the same. it is normal??
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Offline Kiriakos-GR

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Re: open circuit reading of multimeter in mV range
« Reply #3 on: October 12, 2010, 11:26:53 am »
As long ,when you sorting the leads , the reading becomes 0000  your multimeter is healthy.
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Offline d3javuTopic starter

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Re: open circuit reading of multimeter in mV range
« Reply #4 on: October 12, 2010, 12:24:01 pm »
Nah, i'm just curious about the reason behind this behaviour. The voltage actually decreases after it reaches about 200-250mV at very very slow rate.
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Offline saturation

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Re: open circuit reading of multimeter in mV range
« Reply #5 on: October 12, 2010, 03:08:20 pm »
Sensitive DMM will pick up every stray EMF out there.  To eliminate it, remove the impedance between the probes tips so there is no potential difference between them, this is done by shorting it, as you did, it then stops.

If you had an oscilloscope, you can actually see the waveform of the EMF, usually it picks up line voltage radiation at 60 Hz, in the USA.
« Last Edit: October 12, 2010, 03:18:29 pm by saturation »
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Offline Mechatrommer

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Re: open circuit reading of multimeter in mV range
« Reply #6 on: October 12, 2010, 03:26:08 pm »
short the leads, some caps charging up and down in there. you know... very sensitive to "the power of the force"
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Offline Time

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Re: open circuit reading of multimeter in mV range
« Reply #7 on: October 12, 2010, 04:46:22 pm »
Its just your leads detecting basic background radiation or EMI.  Things of this nature are very stochastic and difficult to pinpoint but its probably from your mains or something.
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Offline JohnS_AZ

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Re: open circuit reading of multimeter in mV range
« Reply #8 on: October 12, 2010, 06:04:00 pm »
Quote
Things of this nature are very stochastic

You win the Proper Use of Uncommon Vocabulary Award for this week!!!!    :)
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Offline Time

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Re: open circuit reading of multimeter in mV range
« Reply #9 on: October 12, 2010, 06:39:24 pm »
Quote from: JohnS_AZ on October 12, 2010, 06:04:00 pm
Quote
Things of this nature are very stochastic

You win the Proper Use of Uncommon Vocabulary Award for this week!!!!    :)

Haha, sweet!
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