# How to interpret an ammeter?

I would like to interpret my ammeter. I don't understand what the point with the numbers 2,20,200 means and what is the exact current I am measuring. Thanks

• please add a schematic of the setup that you are using ... there is a button for that when you edit your question – jsotola Dec 26 '17 at 2:19
• I thought I'd share that my multimeter works the same, and I've never come to terms with how to use the current measurement. Neither have my friends. (It shows different currents for different settings, like your images, yet it's the same current flowing through every time). So whenever anyone asks me if I got an ammeter I've had to lie and say "no, the fuse is blown". The solution for me was to just use a 1Ω resistor and measure the voltage across it and get the current that way. - To anyone thinking I'm incapable of using ammeters, no, I've used others that worked fine. – Harry Svensson Dec 26 '17 at 3:43

You've carefully trimmed the photos so we can't see where your probes are connected.

When using the 2m, 20m, and 200m ranges (switch positions) the red meter lead should be in the "Volts/Ohms/mA" socket, and the black lead in the "Com" socket. For the 20A range, the red lead should be in the "20A" socket.

I suspect you are using the "20A" socket for all readings. If so, the readings on the mA ranges will be meaningless.

If so, the correct current is read with the range switch in the "20 A" position, and the current is 0.05 Amp, or 50 mA.

• Yes, you are right. I was doing that – DieDauphin Dec 26 '17 at 16:30

The markings are the full-scale values for the selected ranges. They are 2 milliamps (.002 amps), 20 mA, 200 mA and 20 A. The readings are 0.005 mA, 0.05 mA, 0.5 mA and 0.05 A (50 mA).

• But I am reading the same current. They can't be different – DieDauphin Dec 26 '17 at 2:11
• Then your meter error indicates tolerance error of 0.05/2.00 or 2.5% which make the 1.99x full scale reading with appearance of a 3 1/2 digit display but only has an accuracy of a 2 digit display ( 1%) with effectively 1x,10x,100x,1000x current shunt Resistor gain scales. ( el cheapo DMM) – Tony Stewart Sunnyskyguy EE75 Dec 26 '17 at 2:23
• I suspect that the impedance of the meter is affecting the actual current. With the current only registering in the least significant digit, the reading may not be very accurate either. Can you describe or provide the diagram of the circuit? – Charles Cowie Dec 26 '17 at 2:24
• So, What is the current that I am reading? – DieDauphin Dec 26 '17 at 2:27
• What does it mean when the leads are shorted? Disconnected? Or connect the two leads? It marks 0 in both cases. Don't think there is a blown fuse – DieDauphin Dec 26 '17 at 2:32

That is the display you would get with a constant voltage applied to the meter (probably 0.5mV).

The input current would vary with the range setting.

You may be getting some kind of false reading due to EMI since the current drawn from the power supply is switched at relatively high frequency to feed the stepper motor.

I have a feeling that meter is only accurate to three digits, the forth being the 5 which is probably either 0 or 5. It may have only 8 bit sampling resolution, or effectively 9, which would pretty much put you in that ballpark.

When you up the scale the rounding error then moves up from the mA range up to the tenths of an amp at the 200A scale.

As such, the least significant bit is pretty much useless on it's own. It shows you the least significant bit is indicating that digit is more than zero and possibly as much as half that digit point.

This is really only a guess though, but it sort of fits the symptoms. It would be interesting if you can force it to show other than a 5 in the last digit.

• Multimeters, even 3 1/2 digit models, never use 8-bit ADCs. They use dual-slope ADCs with about 11 bits or more. – WhatRoughBeast Dec 26 '17 at 5:04
• @WhatRoughBeast perhaps. It does fit the evidence though and 8 or effectively 9 is sufficient for a cheap knock-off 0-2xx meter with three and a half digits accuracy. – Trevor_G Dec 26 '17 at 9:38