Re: Multimeter Ohms


"HeyBub" <heybub@xxxxxxxxxxxxxxx> wrote in message
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Ralph Mowery wrote:
"Peetie Wheatstraw" <peetie.wheatstraw@xxxxxxxxx> wrote in message
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I've never really understood about MM readouts for ohms.

I have here a Chinee-Sears digital MM (maybe $15 on sale) and a
circuit that
should measure 450-550 ohms. I set the MM to 2k ohms, the MM reads
say .500.

Is it telling me it measures .5 x 2000 = 1000 ohms? If not, what?

Being as it's digital, shouldn't it report actual integer ohms? More
expensive MM's do this?

Thx,
Peetie

When you set the MM scale to the 2K position, that is the full scale.
It will not read a resistance of a higher value than 2000 ohms and
should show an OL, OR or some other indication you have exceeded the
range, just as an open circuit will show. You could have a 4000
ohm resistor hooked up to the leads and on the 2 K range it will show
as an open circuit. Some meters may have a slight overange and not
top out at exectally 2000 ohms on that scale.

When it shows .500 it is actually telling you that you have a 500 ohm
resistance between the leads.
In other words, the number is the actual resistance in numbers and
you have to place the decimal point in the correct place.

I don't think that's right for an analog meter. If you set the meter to
the 1 Megohm range and place a 100-ohm resistor across the terminals, the
meter will read 100 ohms - you just won't be able to tell it. The setting
is for the SENSITIVITY not the allowable range.

Conversely, the setting for reading voltage does imply a maximum range.


Analong meters are not much differant. YOu get a number where the meter
pointer stops. To this number you usually add the number of zeros from the
range switch. That is if the meter stops at the number 5 and the meter is
on the RX1 range , you have 5 ohms. If on the RX100 then it will be 500
ohms and on the RX10000 range it will be 50,000 ohms.

There have been a few analog meters made that seem to work backwards. It
has been so long ago that I saw one of these I don't recall how they work.
I think they are manily for low resistance (under 1000 ohms) and the zero is
on the left side of the meter instead of the more usual right side.

If you place a 100 ohm resistor across the analog meter and it is set on the
rx10000 range, the meter will be so far to the right (toward zero ohms) that
it will look like a short circuit.

If you place a 1 meg resistor across the analog meter and it is on the RX1
scale, the meter will move so slightly that you will still think the circuit
is open, just as the digital meter would show.

When the resistance is way out of the expected range, then the meter (either
one) will seem to show an open circuit or a short circuit.


.

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