Author Topic: ohm meter rant  (Read 15512 times)

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Offline Wytnucls

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Re: ohm meter rant
« Reply #25 on: October 12, 2013, 02:59:19 pm »
'Why the lack of high precision cheap ohm meters?'
I wouldn't call a 1% milliOhm DMM delivered to your door for 140$ expensive, but hey, I know some people struggle in life, even in Germany.
The OP has no idea how to turn his 60$ CAT I DMM into a super duper micro Ohmmeter with 7$ of parts and if he ever manages it, I'll eat my hat.
That mA only CEM meter has the GS TUV markings on the faceplate and if you don't believe it is legal, do me a favor since you live in Germany and report the manufacturer for counterfeiting.
 

Offline Lightages

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Re: ohm meter rant
« Reply #26 on: October 12, 2013, 03:07:32 pm »
This has been mentioned before on this forum. It is not going to be super accurate but it is cheap to make.
http://www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf
 

alm

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Re: ohm meter rant
« Reply #27 on: October 12, 2013, 03:18:45 pm »
I would agree that it seems extremely unlikely that that CEM meter meets IEC61010. The markings near the connector are enough to show that it does not comply with IEC61010-2 (750V AC on the main inputs, and 30 V on the sense leads), which calls for the maximum rated voltage to be applied on all control setting and input combinations. Not sure if this was in version 1.

If it performs up to spec without major flaws, then this seems a fine product to me. I wouldn't complain about a $130 low resistance meter that's limited to CAT I 60 V. But why did they have to stamp CAT III 1000 V on it? Is there some use case where people would use this to measure voltage on high energy circuits but also to measure low resistances on SELV low voltage circuits?
 

Offline iloveelectronics

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Re: ohm meter rant
« Reply #28 on: October 12, 2013, 04:00:22 pm »
For those who can read Chinese the following is a DIY milliohm and volt meter. The author claims 0.1%+40 LSD accuracy down to 1uOhm.

http://bbs.38hot.net/thread-40484-1-1.html

There are some schematics in English too.
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Offline sync

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Re: ohm meter rant
« Reply #29 on: October 12, 2013, 04:01:27 pm »
For the milliohms 1% accuracy is ok. But it has also 1% on the voltage ranges. That's not good at all.

About the GS TÜV markings. Ignore them. They only proof that the TÜV earned some money (if they are genuine). Here are a few examples of GS marked DMMs with unfused current range.









 

Offline sync

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Re: ohm meter rant
« Reply #30 on: October 12, 2013, 04:37:04 pm »
But why did they have to stamp CAT III 1000 V on it?
Yes, that's a general problem. I bought a UT-61E in a safer version from http://www.pollin.de. It has a big 600V HRC fuse for the 10A range and a thinner 600V ceramic fuse for the mA range. It's also derated as CAT II 600V, CAT III 300V. I think this could be right and is really no problem in practice.

At the same time it was also sold as CAT III 1000V, CAT IV 600V with the small 250V fuses. That's just a fraud. Why? The UT-61E audience should stay away from CAT III 1000, CAT IV 600V anyway.
 

Offline Matje

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Re: ohm meter rant
« Reply #31 on: October 12, 2013, 10:06:52 pm »
About the GS TÜV markings. Ignore them. They only proof that the TÜV earned some money (if they are genuine). Here are a few examples of GS marked DMMs with unfused current range.

Oh, come on.

The third one is certainly fake - the CE mark is wrong (the distance between C and E is too small).

All of them just say "CAT II" without specifying voltage - obviously garbage as I understand it.

While I also think it is wrong for the TÜV to get paid by the manufacturers the more serious problem with the GS mark is that it is routinely faked.

Why the fakers do not bother to at least get it right - e.g. for the CE mark the official shape is public information - I do not understand. Well, good for us since we can spot at least the lazy bums.
 

Offline jaxbird

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Re: ohm meter rant
« Reply #32 on: October 13, 2013, 06:46:25 am »
This has been mentioned before on this forum. It is not going to be super accurate but it is cheap to make.
http://www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf

I am experimenting with using something based on this (from AD8276 app note) as the current source:



Using a DAC for the Vref to control the output current. It will be more than $7 in parts though and no high voltage/energy protection, but precision is good :D

Should probably do a separate thread on it.
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Offline Wytnucls

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Re: ohm meter rant
« Reply #33 on: October 13, 2013, 09:23:06 am »
I would agree that it seems extremely unlikely that that CEM meter meets IEC61010. The markings near the connector are enough to show that it does not comply with IEC61010-2 (750V AC on the main inputs, and 30 V on the sense leads), which calls for the maximum rated voltage to be applied on all control setting and input combinations. Not sure if this was in version 1.

If it performs up to spec without major flaws, then this seems a fine product to me. I wouldn't complain about a $130 low resistance meter that's limited to CAT I 60 V. But why did they have to stamp CAT III 1000 V on it? Is there some use case where people would use this to measure voltage on high energy circuits but also to measure low resistances on SELV low voltage circuits?
It is rather strange, but the DT-5302 was certified by GS TUV, regardless.
Here is the listing: http://www.certipedia.com/companies/33446/certificates_by_product?additional_title=Digital+Multimeter&locale=en&title=Multimeter
« Last Edit: October 13, 2013, 09:29:48 am by Wytnucls »
 

Offline sync

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Re: ohm meter rant
« Reply #34 on: October 13, 2013, 02:49:53 pm »
The third one is certainly fake - the CE mark is wrong (the distance between C and E is too small).
The CE mark is wrong. The GS seems genuine: http://www.certipedia.com/search?utf8=%E2%9C%93&locale=en&q=dm830
 

alm

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Re: ohm meter rant
« Reply #35 on: October 13, 2013, 03:00:39 pm »
Why would they fake a CE mark if they already got certified for EMC and compliance to the low voltage directive? Does CE require more tests than that for a DMM? I wonder if GS tested the same meter that's now being sold on eBay, or if they sent them a souped-up version with better fuses and protection, or even a lower voltage rating like the UT-61E.
 

Offline dannyf

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Re: ohm meter rant
« Reply #36 on: October 13, 2013, 03:29:25 pm »
Quote
internal mosfets ground those 5 reference resistors

Those pins are energized. The meeter then reads the voltage off the SMV pin to determine Rx.
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Offline dannyf

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Re: ohm meter rant
« Reply #37 on: October 13, 2013, 03:30:23 pm »
It is not that difficult to make a CCS and then run the (known) current through a Rx to measure its voltage.
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Offline sync

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Re: ohm meter rant
« Reply #38 on: October 13, 2013, 04:00:49 pm »
Why would they fake a CE mark if they already got certified for EMC and compliance to the low voltage directive? Does CE require more tests than that for a DMM? I wonder if GS tested the same meter that's now being sold on eBay, or if they sent them a souped-up version with better fuses and protection, or even a lower voltage rating like the UT-61E.
Maybe they just don't know how the real CE mark looks like?

This is interesting: http://www.certipedia.com/certificates/50245999?locale=en. A multimeter which is sold by the German discounter Lidl.

The GS certificate is issued in Jan 2013. But it's only tested against old EN 61010-1 standards. Maybe this is the trick.
 


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