Seen that one.
Low IR with Multimeter.
- Thread starter conny
- Start date
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But if the leads were connected wouldn't the current be 1mA whichever voltage rating you are using but
when measuring the resistance of the object it calculates it by measuring the resulting current.
Do you mean 'shorting' the two leads together? If so, yes, that is my understanding. As I said, taking 500V as an example, if the resistance being measured is 0.5 MΩ or less (all the way down to a zero resistance 'short'), the meter limits the current to 1 mA, no matter how low the resistance gets. Then, as you go on to say...
Quite. Once (with 500V) the resistance being measured gets higher than 0.5 MΩ, then the current starts falling below 1 mA, all the way down to the the limit of the range, when current will be 1 μA when the resistance being measured is 500 MΩ. If (with 500V) the current gets below 1 μA, then the meter displays the resistance as ">500 MΩ".
Kind Regards, John
Mine doesn't give up showing an actual value until it gets to >1GΩ .
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As I said, the 1652 gives up when it gets to a 1μA current. Having just checked, there is one slight twist. At 1000V it does, indeed, give up at >1000 MΩ. At 500V, it does, indeed, give up at >500 MΩ. However, for reasons better known to itself, at 250V it gives up at >200 MΩ (i.e. 1.25μA), rather than the 'expected' 250 MΩ. As they say, "go figure"
Kind Regards, John
Yes IR does in almost all cases reduce when the applied voltage increases.
Only if the IR was a pure resistance would it remain constant irrespective of the applied voltage.
IR is almost always a mixture of various elements some of which are voltage sensitive.
Indeed - and as EFLI and I have illustrated, the degree of voltage sensitivity can be dramatic.
Certainly in the case of my experiment, where the conductor was almost entirely water and a few electrolytes, I would presume that many/most of the "elements ... which are voltage-sensitive" were electro-chemical in nature. In the case of solid materials, I would imagine that the most common scenario would probably be for there to be a sudden decrease in resistance when an increasing voltage exceeds a threshold which 'does things' to the material (quite possibly permanently), but not much is the way of a general voltage/resistance relationship over most ranges of voltages - but, of course, I could be wrong.
I'm going to do some bench experiments (can't do them with the MFT, which doesn't go below 250V) to determine if there is a voltage above which the reduction in resistance 'takes off' - and, more generally, what the V/R (which I would normally call 'E/R'!) curve for my bit of wet card actually looks like. Watch this space.
Kind Regards, John
Don't forget corroded copper creating diodes between the copper and the other item. Also thermocouple effects can create enough voltage to confuse a low voltage resistance measurement. I recall the look of disbelieve on a students face when the multi-meter gave a negative resistance value.
Indeed - all possible factors. Such issues may well have been operative in relation to EFLI's tests on an 'old oven element' (which I presume was 'dry'), but obviously not in terms of my 'wet card' experiments - for which, as I said, I presume that electro-chemical issues are the explanation.
Kind Regards, John
It was dry - but has not been used for a long time so could have absorbed some moisture.
The resistance was rising when tested with the 1652.
However, if that was due to the voltage it would make the difference between that and multimeter even more pronounced.
The resistance was rising when tested with the 1652.
However, if that was due to the voltage it would make the difference between that and multimeter even more pronounced.
Yes, possibly - although, as bernard said, corrosion alone might be enough to upset low-voltage resistance measurements, even in the absence of moisture.
Indeed. As I said, when I have a moment I'm going to do some more bench experiments. I rather suspect that it does not need all that much voltage (nothing like as much as 250V) to get beyond the realm of the very misleading 'low voltage resistance measurements). Watch this space.
Kind Regards, John
You did - and I didn't even notice
However, given the very large difference, I wouldn't imagine that the 'rising' you are talking about would have been having much impact on the size of the difference, was it? What amount of change did you observe?Kind Regards, John
I didn't take much notice - it was just going up slowly.
I could do it again after dinner if you wish.
I could do it again after dinner if you wish.
Only if you felt it would be helpful - I'm not really sure what it would mean. In my case, with the wet card, I re-measured (both with low and high voltage) over quite a period of time, and saw no appreciable changes.
Kind Regards, John
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