meter with continuity test and diode test on same setting

[redbelly]

I don't think so. We simply disagree. I think
what you posted is wrong, and apparently you
think what I posted is wrong.

or are you just one those that


Nope. I dislike the posts where the issue
is lost and ad hominum junk takes its place.


Probably not relevant whether it is auto ranging or not,
but the op did not say he has an Auto range meter.
He said he had an unbranded meter he got from Webtronics
and the only marking on it at all is "CCL " followed by
a 9 digit number. What he said makes the next paragraph
relevant.

I have what is probably the same meter, which I got
from them the same way he did - as a freebie when he
ordered a bunch of parts. The one in my hand right
now has CCL031017886 on it, which I assume is the serial
number. I have at least one more from them, also
received for free when I ordered a bunch of stuff.


What I think of your reason for texting that
way is not relevant.


On the Webtronics meter, assuming it is the same as
his, the reading has no decimal point. It shows
up as 566. When you use a second meter in parallel
with the diode testing meter and set the second
meter to read voltage, it reads .566, or very close
to it. My readings were 570 on the Webtronics, and
.575 volts on the second meter set to volts. Incidentally,
the second meter in diode test reads .572 with the
decimal point, and puts a V on the screen to indicate
it is displaying voltage.




Probably true. Considering the information value of the
reading, however, is not something that should be so
easily dismissed. On the one hand, we have information
that says, if you treat it as resistance, this diode has
566 ohms resistance. On the other hand, we have information
that says, if you treat it as voltage, this diode drops .566
volts. In both cases, the value is dependent on the current
in the circuit - an unknown. But .566 volt drop is much
closer to the actual drop that will appear when the diode is
in most circuits, than 566 ohms. For example, look at a 12
volt circuit where you need 100 mA through a diode. The math
says the damn thing would drop 56.6 volts if you use the 566
ohm figure, which is clearly impossible. A drop of .566 is a
lot closer to the truth.

Your disclaimer "you just got to be aware of the component
you're testing and not assume ohms" is right on target.


That wasn't my intention, but I don't see any reason or
need for you to apologize.

Ed

Ed,

A simple test to decide whether the meter is reading resistance or
voltage would be to run the diode test on a couple of resistors.
Something in the ballpark of 1k, but use two different values in case
one resistor gives identical numbers for voltage and resistance at
your particular test current.

Mark

 

[redbelly]

Ed,

A simple test to decide whether the meter is reading resistance or
voltage would be to run the diode test on a couple of resistors.
Something in the ballpark of 1k, but use two different values in case
one resistor gives identical numbers for voltage and resistance at
your particular test current.

Mark

Whoops, I shouldn't post before having morning coffee.
As long as the test current is not exactly 1 mA, you can do the test I
described with a single resistor. The problem with 1 mA is, you'll
get the same reading whether it's giving ohms or mV (or, k-ohms and
Volts).

Mark

 

[ehsjr]

You have gotten me to doubt my experience and assumptions on this. I
have two cheap digital multi-meters, a GE2524 and a DT-830B.

I used each one to measure the open circuit voltage and short circuit
current from the other on its diode test range. I then calculated a
possible internal current limiting resistance.

GE2524 2.64 V 1.31 mA 2.02 kohms
DT-830B 2.92 V 1.18 mA 2.47 kohms

Then I measured a diode junction with each meter while measuring the
test current and voltage drop with the other meter. Here is the meter
reading (with no decimal point showing), junction voltage and junction
current and calculated diode resistance at that operating point.

GE2524 487 .518 V 1.06 mA 488 ohms
DT-830B 526 .514 V 0.97 mA 529 ohms

So it looks like both these meters read out whole ohms, not millivolts,
as I have been assuming, on the diode test scale.

The main difference between the diode test scale and the 2000 ohm range
is that that scale supplies a much lower open circuit voltage:

GE2524 295 mV
DT-830B 141.5 mV

Either of those ranges will allow pretty good resistance measurements
without forward biasing silicon PN junctions very much.

Thank you for inspiring me to get to know some of my test equipment better.

John, the test is flawed. When you add a meter in series to
measure current, the added meter changes the current supplied
by the diode testing meter, due to the internal resistance of
the added meter (relatively large on the low mA scales). That
automatically upsets the "calibration" such as it is, of the
reading on the diode testing meter. You may happen to get
a reading on the diode testing meter that agrees, but you
would need to make multiple readings across a range of
resistances that always agreed to make a proper conclusion.

Also, it sounds like you used only 2 meters. You would need
3 to be sure your setup is measuring what you think it is.
From your description: "Then I measured a diode junction with
each meter while measuring the test current and voltage drop
with the other meter."
You can't measure both current and voltage simultaneously
with the other meter. When you remove it from a series
connection for measuring current, to a parallel connection
for measuring voltage, you change the current in the circuit.


There are two ways you can test that come to mind that do
not upset that calibration:
1) Diode test normally and measure the voltage
across the diode with a second meter.
2) Diode test, except use a low value resistor,
like 150 ohms.


I've performed test #1 a number of times in the
past, and the reading on the second meter (which
is voltage drop) has always been close to the
reading on the first.

I just performed test #2. My 150 ohm resistor
read 220 on the Webtronics meter in diode test.
(I believe that is the same meter the op has.)
On the ohmmeter setting, it reads 149.2

I then tested the same way with an Alfa 2360 meter.
It reads 149.8 ohms on the ohmmeter setting, and
..154 volts on the diode setting.

Surprisingly, when I performed the test on the
cheap $3.00 meter from Harbor Freight, it read
149 on both diode test and ohms.

Ed

 

[John Popelish]

John, the test is flawed. When you add a meter in series to
measure current, the added meter changes the current supplied
by the diode testing meter, due to the internal resistance of
the added meter (relatively large on the low mA scales). That
automatically upsets the "calibration" such as it is, of the
reading on the diode testing meter.

I have no argument with your analysis. My only defense is
that adding the milliamp meter across the diode check meter
leads or in series with the diode junction changed only the
least significant digit of the diode check reading. I was
not trying to check the accuracy so much as determine
whether these two meters were displaying voltage or
resistance. I think the voltage check proves that they are
not displaying voltage, and the combination of voltage and
current measurement confirms, pretty closely that these
meters are displaying something close to resistance. Am I
wrong about these conclusions?

My tests say nothing about what other meters display, and I
am sure that there are many variations. I have to repeat
this test for every meter I use and put a label on the back
that reminds me what that meter displays on the diode check
scale.

You may happen to get
a reading on the diode testing meter that agrees, but you
would need to make multiple readings across a range of
resistances that always agreed to make a proper conclusion.

That would certainly increase my confidence.

Also, it sounds like you used only 2 meters. You would need
3 to be sure your setup is measuring what you think it is.
From your description: "Then I measured a diode junction with
each meter while measuring the test current and voltage drop
with the other meter."
You can't measure both current and voltage simultaneously
with the other meter. When you remove it from a series
connection for measuring current, to a parallel connection
for measuring voltage, you change the current in the circuit.

Agreed, if accuracy were important. Also measuring the
resistance of each meter on its 20 mA scale would be a good
idea, so I knew what resistance they added when measuring
the current through a test resistor.

There are two ways you can test that come to mind that do
not upset that calibration:
1) Diode test normally and measure the voltage
across the diode with a second meter.
2) Diode test, except use a low value resistor,
like 150 ohms.

I've performed test #1 a number of times in the
past, and the reading on the second meter (which
is voltage drop) has always been close to the
reading on the first.

I just performed test #2. My 150 ohm resistor
read 220 on the Webtronics meter in diode test.
(I believe that is the same meter the op has.)
On the ohmmeter setting, it reads 149.2

I then tested the same way with an Alfa 2360 meter.
It reads 149.8 ohms on the ohmmeter setting, and
.154 volts on the diode setting.

Surprisingly, when I performed the test on the
cheap $3.00 meter from Harbor Freight, it read
149 on both diode test and ohms.

The two I posted results for are like your cheap one. It is
clear that there is no standard for what is displayed on the
diode test scale.

 

[John Popelish]

...Also measuring the resistance of
each meter on its 20 mA scale would be a good idea, so I knew what
resistance they added when measuring the current through a test resistor.

Resistance of 20 mA scale on each measured with the 200 ohm
scale of the other meter is:

GE2524 13.2 ohms
DT-830B 10.1 ohms

 

[ehsjr]

Ed,

A simple test to decide whether the meter is reading resistance or
voltage would be to run the diode test on a couple of resistors.
Something in the ballpark of 1k, but use two different values in case
one resistor gives identical numbers for voltage and resistance at
your particular test current.

Mark

I just posted much the same to John Popelish,
before reading your post. I tested with a 150 ohm
resistor - it read 220 on diode test and 149.2 on
the ohmmeter (200 ohms) setting. The same meter
reads 570 when testing a diode. Clearly, that
meter is not indicating a valid resistance when in
the diode testing position. Actually, a valid test
would have to include a number of different
resistances, all agreeing with the meter indication,
to warrant thinking that it was indicating resistance.

But aside from that, think how absurd it is to treat
a diode as having a 570 ohm equivalent resistance.
When inserting that diode into a circuit, you are not
inserting a ~570 ohm resistance - you're inserting a
voltage drop of at least ~.570 volts. (Forward
assumed for discussion.)

Ed

 

[John Popelish]

But aside from that, think how absurd it is to treat
a diode as having a 570 ohm equivalent resistance.
When inserting that diode into a circuit, you are not
inserting a ~570 ohm resistance - you're inserting a
voltage drop of at least ~.570 volts. (Forward
assumed for discussion.)

Unfortunately, meter manufacturers are not constrained by
what is absurd.

 

[ehsjr]

I have no argument with your analysis. My only defense is that adding
the milliamp meter across the diode check meter leads or in series with
the diode junction changed only the least significant digit of the diode
check reading. I was not trying to check the accuracy so much as
determine whether these two meters were displaying voltage or
resistance. I think the voltage check proves that they are not
displaying voltage, and the combination of voltage and current
measurement confirms, pretty closely that these meters are displaying
something close to resistance. Am I wrong about these conclusions?

I don't know if you are wrong. But what I have done with
3 meters so far indicates that the reading is displaying
voltage, not resistance. I took a number of resistors -
47, 75, 120, 150, 390, 500 and 1K through each in diode
test, with a second meter measuring voltage across each
resistor. In every case, the reading on the diode testing
meter agreed closely with the reading on the voltage
measuring meter. I regret now that I didn't record the
values, but the greatest difference as I recall was 10
mV. But treating the reading on the diode meter as
resistance, gave a much wider variation. For example,
my 500 ohm resistor read 600 ohms or 600 mV, depending
on what you want to call it, on the diode meter, and
600 mV on the voltage meter.

Ed

 

[redbelly]

I just posted much the same to John Popelish,
before reading your post. I tested with a 150 ohm
resistor - it read 220 on diode test and 149.2 on
the ohmmeter (200 ohms) setting. The same meter
reads 570 when testing a diode. Clearly, that
meter is not indicating a valid resistance when in
the diode testing position. Actually, a valid test
would have to include a number of different
resistances, all agreeing with the meter indication,
to warrant thinking that it was indicating resistance.

But aside from that, think how absurd it is to treat
a diode as having a 570 ohm equivalent resistance.
When inserting that diode into a circuit, you are not
inserting a ~570 ohm resistance - you're inserting a
voltage drop of at least ~.570 volts. (Forward
assumed for discussion.)

Ed

I agree that it is an absurd thing to do, but disagree that nobody
would ever make a meter that does it that way.

Actually, I wouldn't have entertained the possibility of a resistance
display if not for John's measurements. I read your objection to his
measurement, and will see if I can figure out how his meter resistance
of 10-13 ohms might affect his results.

Mark

p.s. for my two meters, the meaning of the diode-test numbers are
clear owing to the "V" that is displayed along with the numbers.

 

[redbelly]

I have two cheap digital multi-meters, a GE2524 and a
DT-830B.

I used each one to measure the open circuit voltage and
short circuit current from the other on its diode test
range. I then calculated a possible internal current
limiting resistance.

GE2524 2.64 V 1.31 mA 2.02 kohms
DT-830B 2.92 V 1.18 mA 2.47 kohms

Then I measured a diode junction with each meter while
measuring the test current and voltage drop with the other
meter. Here is the meter reading (with no decimal point
showing), junction voltage and junction current and
calculated diode resistance at that operating point.

GE2524 487 .518 V 1.06 mA 488 ohms
DT-830B 526 .514 V 0.97 mA 529 ohms

So it looks like both these meters read out whole ohms, not
millivolts, as I have been assuming, on the diode test scale.

The main difference between the diode test scale and the
2000 ohm range is that that scale supplies a much lower open
circuit voltage:

GE2524 295 mV
DT-830B 141.5 mV

Either of those ranges will allow pretty good resistance
measurements without forward biasing silicon PN junctions
very much.

Resistance of 20 mA scale on each measured with the 200 ohm
scale of the other meter is:

GE2524 13.2 ohms
DT-830B 10.1 ohms

John,

I've put your numbers for the GE2524 into a spreadsheet, and factored
in the effect of the DT-830B's resistance when used for current
measurements. Even so, your readings are consistent with a
"resistance" interpretation for the Diode Test mode.

Without going into too much detail:
You're actual "short circuit current" values will differ, since your
measurement was actually of the current at 10 (or 13) ohm load, and
not at zero-ohm (short circuit). But the s.c. current values change by
only 0.006 mA.
Likewise, the current-limiting resistance is a little lower, but just
1 or 2 ohms.

Since your meter resistance is only about 0.5% of the current-limiting
resistance (10 ohms vs. 2k ohms), it has a very small effect on the
measurements.
Still, the reading in diode-test mode, should change slightly when
switching the other meter from current-measurement to voltage-
measurement, reading about 1 or 2 mV higher for the voltage
measurement.

Mark

 

[John Popelish]

John,

I've put your numbers for the GE2524 into a spreadsheet, and factored
in the effect of the DT-830B's resistance when used for current
measurements. Even so, your readings are consistent with a
"resistance" interpretation for the Diode Test mode.

Without going into too much detail:
You're actual "short circuit current" values will differ, since your
measurement was actually of the current at 10 (or 13) ohm load, and
not at zero-ohm (short circuit). But the s.c. current values change by
only 0.006 mA.
Likewise, the current-limiting resistance is a little lower, but just
1 or 2 ohms.

Since your meter resistance is only about 0.5% of the current-limiting
resistance (10 ohms vs. 2k ohms), it has a very small effect on the
measurements.
Still, the reading in diode-test mode, should change slightly when
switching the other meter from current-measurement to voltage-
measurement, reading about 1 or 2 mV higher for the voltage
measurement.

Yes, I saw the least significant digit change a few counts
when I inserted the milliamp meter in series with the
junction, and a little larger change when I inserted it into
the short circuit measurement. But a little back of the
envelope math indicated to me that even with this small
perturbation, both my meters are indicating resistance
instead of millivolts. I would have found millivolts more
useful.

But now I see that if any meter does not display units, I
will have to test each to find out what it is displaying on
the diode test range. Knowing that is worth something.

I may also write the resistance of each current meter range
on the dial, also, since this is good information to have
when using those ranges.

 

[redbelly]

Yes, I saw the least significant digit change a few counts
when I inserted the milliamp meter in series with the
junction, and a little larger change when I inserted it into
the short circuit measurement. But a little back of the
envelope math indicated to me that even with this small
perturbation, both my meters are indicating resistance
instead of millivolts. I would have found millivolts more
useful.

But now I see that if any meter does not display units, I
will have to test each to find out what it is displaying on
the diode test range. Knowing that is worth something.

I may also write the resistance of each current meter range
on the dial, also, since this is good information to have
when using those ranges.

Definitely good to know those meter resistances. You can also measure
resistances for "voltage" mode, directly with the other meter since
you have 2 meters.

And if the resistance (in voltage mode) is out of range of the other
meter's resistance measurement, there's a neat trick for getting it
within 10%. With your second meter set to measure resistance, measure
a 1M resistor. Then measure the parallel combination of 1M and the
1st meter in "voltage" mode. Use some algebra to figure out what
resistance the 1st meter is, in order to change the resistance from
the 1st measurement value to the parallel-combination-value.

Mark