meter with continuity test and diode test on same setting

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

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Regards,

John Popelish

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

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

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Regards,

John Popelish

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

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.

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

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.

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Regards,

John Popelish

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