Author Topic: Fluctuating AC/DC readings on my new digital meter  (Read 19751 times)

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Offline gioliciousTopic starter

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Fluctuating AC/DC readings on my new digital meter
« on: December 02, 2011, 10:20:43 am »
I got a new digital multimeter today and wanted to test some AA batteries and our mains. But before i would even test anything, i already see fluctuating readings on my meter when i turn to AC or DC. Is this normal? although i get accurate readings when testing. i've got an auto range digital multimeter.
 

Offline mobbarley

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Re: Fluctuating AC/DC readings on my new digital meter
« Reply #1 on: December 02, 2011, 10:22:38 am »
You mean that the display is changing without anything connected? How much does it vary by?
 

Offline grenert

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Re: Fluctuating AC/DC readings on my new digital meter
« Reply #2 on: December 02, 2011, 11:06:19 am »
This is a recurring question here.  You need to short the leads together to get a zero reading on your meter.  You will get nonsense readings like yours on a DMM if you are on ACV or DCV with nothing connected, or the leads connected and not shorted together.  It is a consequence of the high input impedance of DMMs, which is generally a good thing.
 

Offline gioliciousTopic starter

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Re: Fluctuating AC/DC readings on my new digital meter
« Reply #3 on: December 02, 2011, 11:12:50 am »
Quote
You mean that the display is changing without anything connected?

Yes that's what i meant.

Please see attached pictures.

Quote
This is a recurring question here.  You need to short the leads together to get a zero reading on your meter.  You will get nonsense readings like yours on a DMM if you are on ACV or DCV with nothing connected, or the leads connected and not shorted together.  It is a consequence of the high input impedance of DMMs, which is generally a good thing.

I tried shorting the leads and it went down to zero but otherwise it's displaying fluctuating readings still. So I assume this is normal? Please see attached pictures.
« Last Edit: December 02, 2011, 11:15:02 am by giolicious »
 

Offline mobbarley

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Re: Fluctuating AC/DC readings on my new digital meter
« Reply #4 on: December 02, 2011, 01:01:43 pm »
As above - nothing to be concerned about, that reading is very small. Assuming that it drops to practically zero when the leads are shorted.
 

Offline gioliciousTopic starter

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Re: Fluctuating AC/DC readings on my new digital meter
« Reply #5 on: December 02, 2011, 01:59:29 pm »
Quote
As above - nothing to be concerned about, that reading is very small. Assuming that it drops to practically zero when the leads are shorted.

yes it drops to zero when shorted. ok thank you guys.
 

alm

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Re: Fluctuating AC/DC readings on my new digital meter
« Reply #6 on: December 02, 2011, 09:46:32 pm »
One of your pictures shows a 50 mV reading. With a 10 Mohm input impedance, it takes about 5 nA to induce this voltage. This can easily be picked up by the leads. This problem is worse on some meters with an even higher input impedance. Even some cheap DMMs have a FET-input (Gohm impedance or so) mode on the lowest mV ranges.

This can actually be a real issue, and is the reason why some of the products aimed at electricians have a lower input impedance, or the ability to switch from 10 Mohm to a few kohm or so. If you're measuring the voltage between two wires, you may not have any way to check if they're connected. Capacitive coupling with other wires can easily give high (eg. 80V) voltage readings with 10 Mohm DMMs.
 


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