Reading My Multimeter

[Dustin Smith]

I've got an old digital Fluke Meter that has the ability to read two different current readings.

When placing the red lead in one position I can read 0-300mA.

When placing the red lead in a second position I can read 0-10amps.

I was testing an LED light to see if it really does what it says. It claims 9W of power usage. So I hooked it up and tested with the 0-10 amps and read 0.16A.
I decided that was too small for the big scale to be very accurate, so I went to the 0-300mA and it read 68.8mA. That's a huge difference in the readings.

I thought my meter was a good one, it wasn't cheap when I bought it 15 years ago. Around 100USD I believe. Is there something I'm doing wrong, or is my meter just not able to get even close to accurate on the 0-10A scale when reading small amperage?

I believe that 0.16A = 160mA and that 68mA = .068A.

Anyways, if you go by the mA scale, it's using just under 8W of power on my 115V a/c power. If you go by the A scale, it's using like 18W. So I assume that the mA scale is more accurate, I guess I just want someone to confirm. Or tell me what's going on. Thanks for reading to the end.

 

[Harald Kapp]

Some issues here:
1) you are right, of course, 0.16A=160mA. The difference is not easy to explain. The ammeter typically puts a resistance in the current path. This resistance depends on the current range (higher for low currents). I wonder if the LED circut is that sensible to the additional rsistance. I don't think it should.
2) how is the LED light connected to mains? Is it a standard transformer or (more likely) an electronic transformer? Your meter might not be able to handle the high frequencies of an electronic transformer correctly. Can you verify the basic ammeter function using a stable DC-source? Put the ammeter and a resistor in series to the DC source, Set a current of 100mA...300mA, the exact value doesn't matter. Measure usiong both ranges. Any discrepancies? If yes, your meter is not good anymore. If no, the frequency may be an issue.
3) Measuring power from separeate measurements of voltage and current is a tricky thing in an AC circuit. What you get by P=I*V is the apparent power, which is often higher than the real power due to a phase shift between current and voltage. Also, if the power supply for the LED does not have a power factor correction (and I doubt that it has one because the power is so low), you don't even deal with sinusoidal currents and all simple math is moot.

I suggest you first verify the instrument using DC to eliminate any doubt concerning the Fluke.
Next look up the manual whether it says anything about limitation that may differ between the two measurement ranges (e.g. frequency).

Harald

 

[Dustin Smith]

Dear Harald,
Thanks for the reply. Here are my tests results.

White LED (rated at 3.6V 20mA) test, no resistor in circuit ran at 3.5 - 3.6V
19-20mA (0-300mA scale)
0.00-0.01A (0-10A scale)

Small D/C motor
140 - 150mA (fan would not run)
0.20A (fan would run)

I believe that the problem is with the scaling. I mean, what would be the point of putting in a 0-300mA scale if the 0-10A scale would be accurate enough to measure that low? I assume that the resistance values that you've talked about are part of the reason why I get false readings on the larger scale. That's why the DC motor would run on the higher scale, less resistance. (The batteries were dead, I just hooked 3 in series to get the voltage needed, ha ha.)

I believe our power is either 50 or 60 hertz here. I really don't know about the transformers or how to find out.

I can't find my exact model manual (it's too old I guess) but the new one says this.
[4] In mA and A ranges, frequency measurement is specified to 30 kHz.

Well, I know more now than I did before I posted. Thanks again for the reply.

 

[Harald Kapp]

no resistor in circuit

The issue here may well be that you operate the LED without series resistor. An LED has, as any diode, a strongly non-linear characteristic. A small change in voltage can result in a huge change in current. Since the resistance of the ammeter is the only current limiting resistor in your circuit, the circuit becomes very sensitive to this difference. Add a 100Ohm series resistor and repeat the measurement. You should note more consistent measurements.

As for the motor: the effect may be the same, also not as strongly expressed as can be seen from the much nearer values for the current in the two ranges. Still the higher resistance in the 300mA range may be enough to keep the fan from spinning. Drained batteries aren't helping in evaluating the meter. Use new ones (or freshly charged ones) or a mains powered Dc source.

what would be the point of putting in a 0-300mA scale if the 0-10A scale would be accurate enough to measure that low?

You'd be right if the 10A range could measure as low as the 300mA range. But it can't. If your´meter has n digits (let's assume n=3), and the range is 0...300mA, you can measure with an accuracy not better than 300mA/10^(n-1), for n=3 that's 300mA/100=1mA.
If the same meter is used in the 10A range, the resolution is 10A/(10^(n-1) which is 10A/100=100mA. That's why meters have ranges at all.

I believe our power is either 50 or 60 hertz here

That could be anywhere because those frequencies are used worldwide. Anyway, this is not what I meant by " electronic transformer". LED power sources are often constructed from high-frequency switching circuits. The high frequency (a few kHz) allows the use of much smaller transformers.

Harald

 

[Dustin Smith]

Ah, I see. Well I'm not a wiz at electronics, but I'm learning a lot from you. I'll redo the test with the resistor just for curiosities sake. But I think that I may have located the manual for my fluke, which states that the 0-10A scale is accurate down to 300mA, which explains the discrepancies.

 

[Harald Kapp]

which states that the 0-10A scale is accurate down to 300mA

I think that's it.