Author Topic: Random values in MegOhmmeter  (Read 366 times)

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Offline Jasonbit

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Random values in MegOhmmeter
« on: January 30, 2019, 12:25:47 am »
Hello,

I am trying measuring the insulation resistance of this transformer trough this MegOhmmeter (https://uk.farnell.com/tenma/72-9400/tester-insulation-250v-500v-1kv/dp/2064096). But I receive the weird values. In this images you can see three diferrent value during resistance test. The values are always changing!

Image 1: https://postimg.cc/Hjy1Zyzc
Image 2: https://postimg.cc/4YwD7bfM
Image 3: https://postimg.cc/kVpLwJtv

What could be happening?
I already checked with a multimeter and got the correct voltage values (250VDC, 500VDC and 1000VDC). And the test applied to a multimeter has a value of 9.9M Ohm

Thanks
 

Online RoGeorge

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Re: Random values in MegOhmmeter
« Reply #1 on: January 30, 2019, 12:34:44 am »
Try measuring without the 2 spool of wires, using shorter leads (probes) instead.

My best guess is those 2 spool of wires are nothing but huge coils, acting here as antennas, thus altering the measured value.

Online Andy Watson

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Re: Random values in MegOhmmeter
« Reply #2 on: January 30, 2019, 12:51:14 am »
Can you power the meter from a battery? If not, can you ground one of the power supply leads? I suspect that your power supply is coupling a large mains component into your meg-ohm meter - which is probably saturating the front-end of the meter. And as RoGeorge points out, this will not be helped by those long antennas.
 

Offline graeme.c.payne

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Re: Random values in MegOhmmeter
« Reply #3 on: January 30, 2019, 01:18:47 am »
I agree with the above. As a general rule, the connection between the unit under test and the test/measurement equipment should always be as short as practical.


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Offline Jasonbit

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Re: Random values in MegOhmmeter
« Reply #4 on: January 30, 2019, 01:49:58 am »
Thanks to your help.
Yes, you are right. I did this test:

image1: https://postimg.cc/7bs2xbTN
image2: https://postimg.cc/LnJZhRtF

When I tested the transformed in the moment I press the button "Test" the value 685MOhms appear but disappear in the next second. Then it remains with the value1 MOhm. I repeat the process and I get another value.

Thanks
 


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