Author Topic: EEVblog #252 - Multimeter Ohms Overload  (Read 2248 times)

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Offline IanB

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EEVblog #252 - Multimeter Ohms Overload
« on: March 03, 2012, 06:37:08 pm »
Love the follow up video on overload recovery with different meters. Looks like the Agilent still has some details to work on to catch up with the Fluke. Although I've never feed 240 V into the ohms range and I'm never likely to. I always check and double check the meter setup before I go anywhere near dangerous measurements.

Also I'm surprised none of the meters gave a "beep beep beep" alarm signal when exposed to an overloaded input like that. It would seem like an obvious design feature to incorporate.
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Offline Neilm

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Re: EEVblog #252 - Multimeter Ohms Overload
« Reply #1 on: March 03, 2012, 11:01:08 pm »

Also I'm surprised none of the meters gave a "beep beep beep" alarm signal when exposed to an overloaded input like that. It would seem like an obvious design feature to incorporate.

IEC61010 says that they should be safe - not necessarily report an issue. Though to my mind they should

Neil
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Offline free_electron

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Re: EEVblog #252 - Multimeter Ohms Overload
« Reply #2 on: March 04, 2012, 05:46:58 am »
i think this has something to do with a special feature in the agilent.
the agilent machine can actually measure ohms while a voltage is present !

Here is an example : you have a ground wire coming from a machine. Let's say there isa problem with this ground wire ( loos / bad connection ) and the resistance is around 40 ohms.. Due to leakage in the machine there will be a voltage  present across this wire.

Attempt to measure the 'resistance'  and it is possible that the meter says : very low. simply because the measurement is upset due to the voltage drop across the cable.

The agilent meter WILL give a correct reading . That particular model that dave has has this special feature built in. (Qik-V and Smart-Ohms)
The way it works is that they switch the ohms current source off and sample the incoming voltage, store it in a cap and then pull off sime trickery ..

by 'force-feeeding' 220 into the meter in ohms mode i suspect that there is so much residual charge in this sampler that it simply takes a long time to bleed off.

I suggest the following test :
switch the meter out of auto range and into the 400 ohms range. activate the dual display ( so the temperature goes away and a voltmeter or ampere meter appears ). now do the test again...

This is a measurement technology not present in any other manufacturer.
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 


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