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
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?
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
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.
Good. Means my devices meet my ohms spec. Now, for my MM ...
If I have to place the decimal point, the reported number is -not-
the "actual resistance in numbers".
The reported # is a scaled #. Scaled by 1000. When the scale setting
on the MM device is 2000.
The next highest setting is 200k ohms. If I measured resistance on the
same circuit, precisely what would it report? I'd test it but the
circuits are now installed where I can't get to 'em.
The scale setting and display are completely consistent. You have the
meter set to the 2k ohms scale, and the largest (non-overrange)
displayed value will be 1.999 k ohms. If you want the result in ohms,
and not k ohms, you need to do the conversion by yourself.
Similarly, if it had a 200 ohm scale, the decimal place would be one
from the right, so the largest display would be 199.9, and the result is
read as ohms directly. If it had a 2 Mohm scale, the decimal place
would be one in from the left, and the largest reading is 1.999 M ohms.
Again, you have to convert from Mohms to ohms if you want that.
The only difference with really expensive meters is they may have a
scale indicator in the display, so you can read whether the units are
ohms or K ohms or M ohms on the display. That's necessary for an
auto-ranging meter, but for a manual meter you ought to know what range
you set it to.
The reported number is in k ohms - as you'd expect from the scale you
Now the decimal place has moved 2 positions to the right to make the
largest display 199.9 kohms. 500 ohms is still 0.5 kohms, so your meter
would display 0.5 on this range - correct, but not much precision.
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.
It's pretty much the same for a digital meter. The readings will be
accurate, just not precise.
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.
Measuring a 470 ohm resistor on my $4 digital multi-meter with the
following scales shows:
200 shows 1 .
2000 shows 487
20k shows 0.48
200k shows 00.4
2000k shows 0.00
The self ranging meter doesn't make sense when it puts the 00 up but I
know what it means.
On Fri, 05 Sep 2008 14:26:13 -0500, Peetie Wheatstraw
They are *both* properly scaled. His meter calls that range "2000
ohms", and displays the result properly as ohms. Your meter calls that
range "2 kohms", and displays the result properly as kohms.
The only "problem" is you failing to apply the scale factor implied by
the knob setting to the displayed result.
But even if the knob was marked incorrectly (e.g. the scale is labelled
2000 but the display decimal is placed so the largest reading is 1.999),
you really ought to be able to figure out how to scale the result
yourself. You know what the largest reading is; everything else is
proportional. You don't even need the decimal point; it's just a
What are you measuring? Remember that with things such as light bulb or
heating elements the 'cold' resistance reading is much lower than the
'hot' resistance. For example A 90 Watt bulb I have here should be 160
ohms at 120V however measuring the bulb 'cold' with an ohm meter reads
11.5 ohms. The resistance increases at the temp goes up.
More expensive units (such as those made by Fluke) are auto-ranging and thus
show exact values. They are $150+ though.
Take a resistor of known value and hook it up to your meter, then find out
what it shows. You have just trained yourself.
El cheapo DVMs are not as accurate (uncalibrated when it left factory).
I bought a good used Fluke and it'll last my life time with reliable
service. I still have old Simpson 260 which I am taking with me to our
cabin to trouble shoot electric hot water tank problem. There are times
analog meter is more useful. Oh, I am taking digital clamp amprobe as
well with built in temperature probe. No matter what tool one uses, s/he
has to know how to use it and what s/he is doing with it.
Anyone remember old RCA Ohmyst VTVM? I got one of that too in perfect
working order. Good for RF measurement. I have some tektronic scopes as
well. But digital Fluke scope is fine for most works.
You can get packets of 5 of whatever ohms you want at Radio Shack for about
$2. Likely they are 10% tolerance, if it matters this much you can look for
5% or maybe even <2% tolerance, but you'll pay more.
Bottom line, folks.
Duff has a $4 digital multi-meter which, with the range properly set to
2k ohms, reads the actual integer ohms. Such behavior is not
I got a Chinee Crapsman MM that, in the same circumstance, reports a
fraction that needs to be applied to a scale factor of 1k.
My manual sez "the display will indicate the proper decimal point
It lies. There is no mention of the scale factor of 1k. I do not
find that it is implied by the upper limit of the range (2k).
Some guys *like* esoteric crypticity in measurement devices. I
got no use for it.
This is Chinee BS. I've got a broken analog meter on the workbench.
If I can't fix it, I'll likely buy another analog MM. Best to have
one of each.
On Fri, 05 Sep 2008 14:26:13 -0500, Peetie Wheatstraw
On Sun, 07 Sep 2008 14:37:08 -0500, Peetie Wheatstraw
Buy as many as you like. They are worthless if you don't know how to
What were you needing a 500 ohm resistor for? What project could you
be working on that is more complicated than reading a digital meter?
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