OHMMETERS
altavoz
John Graham:
This test was supposed to be done with an analog ohm meter. Can someone
explain why?
I borrowed one with an analog display..... It seemed to read basic
continuity,
but it's resistance measurements differed a good bit from my digital
meter.
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Jack Stands:
You need an analogue meter becasue it has a lower internal resistance
and the transistor needs sufficient current drawn to " pull up ",a
digital
meter has a very high impedance so not enough current is drawn to "
pull up " the transistor gate.
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MP says
Bullshit. The low input impedance of an analog meter is one reason why
you can't use one to measure O2 sensor voltage, for instance, while a
Hi-Z (that's high-impedance in case you didn't know Al) JFET-input DVM
works fine, you idiot.
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altavoz: It dpnds on the scale you are on and not analog/digital .
The current sourced thru the unknown R is related to the battery and
resistor in the meter, not
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MP:
Bullshit again Alternatingvoz. The front ends of analog VOMs are
completely different than the front ends of DVMs. Get your facts
straight you idiotic charlatan.
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altavoz: You are wrong . Both analog/digital need the same
LOW IMPEDANCE source as they are sourcing a lot of current
on the low range .
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dnult:
Uhhh, your argument about impedance is only valid if your talking about
voltage measurements. The guy your calling a DUMB SHIT specifically
said resistance (ohm) measurements. He is correct - analog and digital
ohm meters are the same. Now don't you feel silly.
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altavoz: No , cause i didn't make that statement
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Larry Smith:
Here is another case of a lot of smoke and BS in the air.
When you are speaking of voltmeters, high impedance is best for most
accurate measurements across the board.
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altavoz: No it's not . Thats why i have a 100k,100,.1 ohm tacked on
to the inputs of my DVM . It allows me to read the source Z . If it
says
117vac and i switch to 100 ohms and it says 0.01vac , i have a very
high impedance source , something that is not likely to drive any load
nor electrocute anyone . FLUTE made a misake on some of their early
DMM and one needed to put a 10 meg across the inputs in order to make
the DMM read 0.00 for no input ! It would read over 2 volts sometimes
with no input .
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Larry:
The ohmmeter situation is a little less clear.
If you want to measure the resistance of very sensitive devices - even
those of low resistance - you cannot just blast the current to them.
You can damage sensitive devices if you use these killer ohmmeters.
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altavoz: Less clear to you, not me . If they are low resistance ,
they are not sensitive . Power is what destroys devices , not voltage.
Name a 1 ohm device that can't take 100 mills ?
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Larry:
The ideal ohmmeter would apply very low voltage to the circuit to
be measured, and would also pass currents approaching zero. (Do you
remember, perchance, the classic Wheatstone bridge that is still used
to measure critical resistances and voltages. When the bridge is
balanced, no current at all flows in the galvanometer.)
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altavoz: Poor Larry , can't figure that the curents in the arms don't
get
small like the meter /device under test !
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Larry:
Doesn't essentially make a damn whether the unit is analog or digital-
there are extremely sensitive instruments available of both types.
True enough, cheap analog voltmeters are usually lower in impedance
than digital meters. High quality laboratory type instruments of both
types are available.
Cheap VOM's may have ratings of 1000 ohms per volt, or 20,000 ohms per
volt - in that range. Laboratory instruments may go ten billion or even
higher.
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altavoz: How many times must i remind you ! were talking ohmmeters ,not
voltmeters . VM need high Z , ohmmeters need LOW Z !!!!!
MSgt WLong
Any of you remember the Vacume tube volt meter of years gone by?? (VTVM)
Had an analog meter movement and the High input impedance of a modern
digital. and the multi purpose models (VTVOM) did an fine job of checking
the early Transistor devices. One had to be careful not to use the High
ohms scale though as source voltage was a bit high and could damage some of
the lower current devices. Great for checking coax though.
Ross Bernheim
MSgt WLong <msgt...@aol.com> wrote:
I remember them well. Though I would prefer TDR for checking coax.
Though a network analyzer will work for reasonably short lengths in a
pinch if you have a proper termination for the other end of the coax.
--
Ross Bernehim
ross.b...@pieinc.com