Why my multimeter keeps showing volts even when I am not testing with it?

Thread Starter

jobayer

Joined Nov 4, 2020
2
Hello, I am a beginner in electronics. I have UNI-T UT61A auto ranging multimeter. But I am having somw issues with it. As soon as I turn it on, it starts showing some volts reading in millimeters! But the probes are not connected with each another nor I am testing anything with it.

20210610_155936.jpg


And slowly it starts reducing the voltage. But it never stops at zero when the range is in millivolts which is the most surprising thing to me.. Because yeah when we turn it on, there might be some voltage going through it. But why it will never be stable! It stables at zero when I manually set the range to volts, not millivolts.

20210610_160148.jpg

What might be explanations or possible reasons??
Again I am sorry if it is a too silly question. As I said I am a beginner. So excuse me any way. Also thanks in advance.
 

rsjsouza

Joined Apr 21, 2014
312
This is one of the most asked questions for the new multimeter users, so you are not alone in your amazement.

The reason is that around us there are many sources of electrical and electromagnetic noise (Radio/TV, fluorescent lightbulbs, switching power supplies, Wi-Fi, Bluetooth, etc.) and these are picked up by sensitive equipment such as a multimeter. The leads and the jacks act as antennas that act as receiver of this energy, which is then measured and shown in its display.

Notice the magnitude of these voltages is quite small, as you pointed out (mV).

If the probes are shorted, you should read zero or somewhere close to it.
 

Thread Starter

jobayer

Joined Nov 4, 2020
2
This is one of the most asked questions for the new multimeter users, so you are not alone in your amazement.

The reason is that around us there are many sources of electrical and electromagnetic noise (Radio/TV, fluorescent lightbulbs, switching power supplies, Wi-Fi, Bluetooth, etc.) and these are picked up by sensitive equipment such as a multimeter. The leads and the jacks act as antennas that act as receiver of this energy, which is then measured and shown in its display.

Notice the magnitude of these voltages is quite small, as you pointed out (mV).

If the probes are shorted, you should read zero or somewhere close to it.
Yes, when I short the probes, it stables at 2.1 mV.
Also thanks for your answer. Your answer is very much logical!
 

anniel747

Joined Oct 18, 2020
1,063
Hello, I am a beginner in electronics. I have UNI-T UT61A auto ranging multimeter. But I am having somw issues with it. As soon as I turn it on, it starts showing some volts reading in millimeters! But the probes are not connected with each another nor I am testing anything with it.

View attachment 240881


And slowly it starts reducing the voltage. But it never stops at zero when the range is in millivolts which is the most surprising thing to me.. Because yeah when we turn it on, there might be some voltage going through it. But why it will never be stable! It stables at zero when I manually set the range to volts, not millivolts.

View attachment 240882

What might be explanations or possible reasons??
Again I am sorry if it is a too silly question. As I said I am a beginner. So excuse me any way. Also thanks in advance.
Stray voltage.

https://en.wikipedia.org/wiki/Stray_voltage
 

BobaMosfet

Joined Jul 1, 2009
1,776
Hello, I am a beginner in electronics. I have UNI-T UT61A auto ranging multimeter. But I am having somw issues with it. As soon as I turn it on, it starts showing some volts reading in millimeters! But the probes are not connected with each another nor I am testing anything with it.

View attachment 240881


And slowly it starts reducing the voltage. But it never stops at zero when the range is in millivolts which is the most surprising thing to me.. Because yeah when we turn it on, there might be some voltage going through it. But why it will never be stable! It stables at zero when I manually set the range to volts, not millivolts.

View attachment 240882

What might be explanations or possible reasons??
Again I am sorry if it is a too silly question. As I said I am a beginner. So excuse me any way. Also thanks in advance.
This is normal- with any meter. This is why you touch the probes together to zero (completes a circuit) before you use it.
 

MaxHeadRoom

Joined Jul 18, 2013
23,111
The old moving coil (Avo) meters were not so prone as the higher impedance electronic versions are.
BTW, a good practice to get into with any meter is to test the leads first on the ohms scale, particularly before measuring HV DC & AC.
 

rsjsouza

Joined Apr 21, 2014
312
@MrChips , the fuses on modern DMMs are not in the same path as the V/Ω, so this would not necessarily tell anything about the status of the fuses. Even the ones that have V/Ω/mA/μA in the same input.

You could potentially put the meter in ohms and then insert the red probe into the A/mA/μA jack to see if its fuse is working (although some meters do isolate these inputs as well).
 

MrChips

Joined Oct 2, 2009
23,528
@MrChips , the fuses on modern DMMs are not in the same path as the V/Ω, so this would not necessarily tell anything about the status of the fuses.
You are correct.

In our student lab I had to test every DMM at the end of each lab because a fuse would be blown and the next group of students would not realize it. I made a simple tester with an LED, resistor, and battery for this specific task.

Thanks for correcting me and reminding me.
 
In our student lab I had to test every DMM at the end of each lab because a fuse would be blown and the next group of students would not realize it. I made a simple tester with an LED, resistor, and battery for this specific task.
These "fit for purpose" test jigs are always useful - I lost count of how many I designed and built in the past.
 
The DMM is set to measure DC, not AC and it can never show 0VDC even with its leads shorted so it has a defect or it is so cheap that is how it is made.
My Fluke multimeter measures 0.000VDC even when the autorange sets it to measure millivolts and the leads are antennas.
Set to AC it shows up to 50mVAC interference when the leads are antennas, but when the leads are shorted together it quickly shows 1mVAC then very slowly drops to 0.2mVAC and less.
 
I've seen many multimeters do that, especially in the mV scale, which typically presents an even higher impedance and thus it is more sensitive. Nothing to worry about.

Both of my Aneng multimeters tend to drift when floating, but they always read correctly nevertheless. The values they tend to drift to are in the vicinity of 100mV. I suspect that they pickup voltage from their own circuit. This kind of drift is typical of circuits that:
- Are in a PCB;
- Have a very high input impedance;
- Don't have a guard ring around the sensitive traces.

The above conditions will suffice in order to pick up readings from adjacent traces, internally.
 
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