The evil thing about Voltmeters with very high impedance is they will read induced voltages that analog meters wouldn't. It makes a voltmeter useless for checking for live circuits in a crowded panel.
Arlowe wrote in news: snipped-for-privacy@gmail.com:
Why? If the source has a low resistance/impedance then you're ok as soon as you have good probe contact. If not, then you could always twist the probe leads together a bit to try to make a balanced line to cancel induced noise. Crude, but it would help if the signal to be read was DC or audio AC.
Capacitive coupling between live and open circuits will show close to line voltage no matter how many twists you put into the leads.
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"Michael A. Terrell" wrote in news:dfKdnZsd8vZ- qu7UnZ2dnUVZ snipped-for-privacy@earthlink.com:
Ok. I just took 'induced' to literally mean induction there. Which would normally mean some low resistance loop, I know, but I assumed that if twisted pairs can reduce noise into high impedance inputs, as is standard use, then a crude emulation of that would help a bit. Got to ask, why would the capacitative coupling not also cancel if it's affecting both leads equally?
Even if the test leads were as long as the electrical circuit, it would reduce it less than half, because there are usually bundles of AC lines grouped together, or runs in conduit with a higher capacitance. At that point it becomes a capacitve voltage divider.
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A craftsman never blames tools for his failures. Hackers, on the other hand...
"Michael A. Terrell" wrote in news:OsmdnXmGwI2lFe7UnZ2dnUVZ snipped-for-privacy@earthlink.com:
Ok, but then isn't the meter just telling it like it is? These are external conditions. I was branching off from someone's assertion that the meter would be no good for measuring voltages on a board, but in that case there's usually a collection of interlinked and closed circuits. In a case like that the twists would help a bit if running the two probe leads separately was causing bother. Not that I think it's significant, if running separate causes that much bother, something more significant needs to be done. I guess many people don't notice because most meters aren't that high in impedance, and they'll be measuring circuits that aren't either. I think in most circuits I've seen that do need a high impedance point to give a signal out, it's locally buffered to avoid this sort of problem.
Also, I seem to have got the original quote interpreted differently: "It makes a voltmeter useless for checking for live circuits in a crowded panel." I took that to be live traces on a crowded PCB.
I agree that in a larger system of conduits and cables there's enough coupling to be far more awkward.
Because it can give false readings when bad (or no) contact is made. That's a BAD thing when dealing with high voltage/high energy gear. That is why proper "electrical" DMM's have LOW input impedance modes. e.g. Fluke 113 or 110series.
Best to just get the right tool for the job.
Dave.
Because a false reading due to a bad (or no) contact can be a BAD thing when dealing with high voltage/high energy circuits.
That's why good purpose designed "electrical" meters have a LOW input impedance mode. e.g. Fluke 113 or other 110 series meters.
Much better idea to simply use the right tool for the job.
Dave.
Because if you get a reading when there is bad or no contact that can be a BAD thing in high voltage/high energy circuits. That's why proper meters for that job have a LOW impedance mode (Kohms). e.g. Fluke 113, the other 110 series, and the Fluke 289 etc.
Much better to use the right tool for the job.
Dave.
krw explained on 19/01/2009 :
If you work with electricity you had better know the limitations of your tools or you will find them...the hard way.
Apples and oranges.
"A craftsman" works the medium he ends up selling directly and irrecoverably with the tool or tools involved so his 'toolwork' is his work product, and one can never blame the tool for the finished product, only the tool pusher.
"A hacker" doesn't use a 'tool', they hack. When they fail, it is because they didn't hack long enough or correctly enough. They *ARE* the tool that failed, and ALL failures are their fault.
In article ,=20 snipped-for-privacy@gmail.com says...>=20
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That is certainly true (though perhaps your heirs are the ones who=20 will find you), but doesn't modify my statement. In this case, the=20 tool *can* be used as long as the one using it knows what he's=20 doing.
krw used his keyboard to write :
Ok....everything seems to pass right over your head.... I pointed out a limitation of a DMM and you seem to be inferring that I am somehow a hack who blames his tools... BTW> I am not a "craftsman" I am an electrcian.. You don't do what I do for as many years as I have without knowing what the f*ck you are doing.
In article ,=20 snipped-for-privacy@gmail.com says...>=20
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No, your incompetence didn't pass by anyone.
=20
It is not a limitation of the tool, only the user. Lower the=20 impedance if it's too high. =20
I'll take your word that you don't do "craftsman like" work. You=20 clearly don't know how.
=20
*I* know what I'm doing. You've clearly demonstrated that you=20 don't.
Then pick the right voltmeter for the job, duh.
Hope This Helps! Rich
Late at night, by candle light, Arlowe penned this immortal opus:
What's wrong with using a hi Z voltmeter in a live panel? I do it more often than I really care for, and never have a problem.
- YD.
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"HI-Z"
It's not the right tool for the job. A meter which gives you a false reading with bad (or no) contact is a BAD idea in dangerous high voltage/high energy circuits. That is why proper meters with low impedance (Kohms) modes exist for the job, e.g. Fluke 113, 110 series, 289 etc.
Oh dear...
Dave.
There isn't anything wrong with it. Analog meters are dead. High impedance digital meters are only a problem if there is a loose nut inside the panel, holding the leads.
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