Re: meter with continuity test and diode test on same setting

John Popelish wrote:
ehsjr wrote:

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?

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


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.

Ed



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