Author Topic: wrong Input impedance of latest dmms?  (Read 958 times)

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Offline 001

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wrong Input impedance of latest dmms?
« on: September 07, 2018, 11:46:47 am »
 

Offline MK14

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Re: wrong Input impedance of latest dmms?
« Reply #1 on: September 07, 2018, 12:10:41 pm »
In general, ICs when powered off, present a diode drop to ground and the power rails. This is often a side effect of how ICs are made (unintended stray PN junctions all over the place, depends on technology used to make the IC as well) and intentionally, usually for ESD protection.
Since most DMMs use a few volts in resistance checking mode, these diode drops (0.6/0.7 volts), plus resistors all over the place. Are probably creating that "strange" meter reading of around 1.1 million ohms (the picture is too blurry for me to be sure).

I.e. It is probably a perfectly normal effect for switched off (unbiased) electronics.
« Last Edit: September 07, 2018, 12:14:05 pm by MK14 »
 
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Offline threephase

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Re: wrong Input impedance of latest dmms?
« Reply #2 on: September 07, 2018, 12:23:26 pm »
Wouldn't you want the meter to be completely open circuit when it is switched off? i.e all electronic components should be out of circuit.

I guess the concern would be what would happen if you were measuring a voltage and turned the instrument off prior to disconnecting the leads, what current would be drawn? Probably bad practice, but it does happen.

They allude that with 20V applied, 10mA flowed. If it is linear, then it shouldn't be too much.

Kind regards
 
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Offline HKJ

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Re: wrong Input impedance of latest dmms?
« Reply #3 on: September 07, 2018, 03:10:37 pm »
Somebody tells https://forum.banggood.com/forum-topic-400344.html  :wtf:
WTF

I sort of doubt that. The first meter I found was the AN8009 and it do not have it:


2kOhm sounds like a PTC and some resistance on the board.
« Last Edit: September 07, 2018, 03:36:36 pm by HKJ »
 
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Offline HKJ

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Re: wrong Input impedance of latest dmms?
« Reply #4 on: September 07, 2018, 03:34:31 pm »
I was too fast, now I found the AN8008 and it has switched the PTC in in off mode.

AN8002:


AN8008:
 
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Online Mechatrommer

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Re: wrong Input impedance of latest dmms?
« Reply #5 on: September 07, 2018, 03:38:21 pm »
Somebody tells https://forum.banggood.com/forum-topic-400344.html  :wtf:
WTF
the lesson is...
1) dont leave the meter unattended, or connected to some potential while turned off.
2) otherwise... buy a not $20 meter.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 
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Offline 001

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Re: wrong Input impedance of latest dmms?
« Reply #6 on: September 07, 2018, 05:06:52 pm »
Thanx a lot!
It is horrible news  :rant:
 

Offline HKJ

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Re: wrong Input impedance of latest dmms?
« Reply #7 on: September 07, 2018, 05:29:16 pm »
It is horrible news  :rant:

Why? The normal recommendation for multimeters is to select the correct range before connecting it?

The PTC (and DMM) will probably survive mains voltage. It is not worse than having the meter in ohms range.

Of course I would prefer a disconnected input or at least 10Mohm, but I do not see it as a big problem. I doubt I would ever encounter it, I always turns the meter on before connecting it.
 

Offline David Hess

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Re: wrong Input impedance of latest dmms?
« Reply #8 on: September 07, 2018, 06:05:32 pm »
The meter's design is bad or the meter is broken.  That should never happen.
 

Offline 001

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Re: wrong Input impedance of latest dmms?
« Reply #9 on: September 07, 2018, 07:22:58 pm »
Again,
Few DMM have 1M input impedance
Is it problem?
 

Offline MK14

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Re: wrong Input impedance of latest dmms?
« Reply #10 on: September 07, 2018, 08:07:27 pm »
Again,
Few DMM have 1M input impedance
Is it problem?

Such a problem, could well be minor, compared to the (probable) lack of safety features on many very cheap multimeters. Which could lead to them exploding and injuring or killing the operator, if accidentally connected to a powerful enough, high voltage supply.
I've seen teardown video(s) of that DMM, and it seems to have these crazily tiny fuse(s). Which I suspect, may not have anything like the safety of proper full sized HRC fuses.

tl;dr
You are worrying about one specific fault of these multimeters. Apart from the (probable) lack of real safety capabilities, they probably have many issues and limitations, compared to proper, expensive quality branded multimeters.

But I haven't got any such DMM (AN8008), so could be wrong and or biased. (partial Fluke fan-boyism etc).
 

Online joeqsmith

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Re: wrong Input impedance of latest dmms?
« Reply #11 on: September 08, 2018, 12:10:39 am »
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 
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Offline David Hess

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Re: wrong Input impedance of latest dmms?
« Reply #12 on: September 08, 2018, 06:14:54 am »
Again,
Few DMM have 1M input impedance
Is it problem?

The input impedance does not matter for low impedance measurements like batteries, power supplies, and the AC line.  Even variable impedance with different ranges can be handled if documented like on old analog meters where the impedance was typically 20,000 ohms per volt.

Where you can get into trouble however is medium and high impedance circuits and high voltage probes where changing the meter range alters the reading.  Most or all automatic ranging handheld digital multimeters are not suitable for use with high voltage probes but for obvious reasons, this is not widely advertised or documented.

 
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