Maker Pro

mA meter 1 day build

A 1 day build of a digital millimeter for my bench.

Project Log

I've been doing a lot of projects lately (mostly involving LEDs) that draw less current than any of my multimeters can measure with any accuracy. I didn't want to spend an arm & a leg on more meter than I really need for anything else so I did some shopping and bought this little meter on eBay

The specs say
Measuring Current Range: 0-50.000mA
Minimum resolution 1UA
Power consumption: <= 15MA,
Operating voltage:5v(If don't need High precision also can use 3.8-30v power )
Refresh rate: about 3 times / sec
Test accuracy: error: ± (0.05% + 2)

It arrived this week and the other project I was working on is waiting for parts so I gathered the parts and got to work. The first step was to tape up the top of a project box, mark out where I wanted everything and drill some holes

A razor saw is good for cutting holes in plastic but I really need to get a smaller one for projects like this

The box isn't deep enough for the big connector on the meter so I removed the connectors and soldered the wires to the board. Other than that the wiring was about as simple as it could be so it didn't take long to connect them to the switch and the lugs for the binding posts. Note that the battery is held between the meter and the switch (I also added a piece of foam tape inside the lid so it doesn't rattle).

The final step was trying it out. I connected it in series with a 4.7kΩ resistor, an LED and a 9V battery. The voltage across the resistor was 7.6V so the calculated current was 7.6V / 4700Ω = 0.1617mA, which is less than 0.3% from the measured 0.162.

BTW: If you are wondering why I used 2 black binding posts with a red bushing under the positive one, when I was in college (about 50 years ago) someone gave me a big bag of them that were salvaged from something but about 95% of them were black so when I use red ones I put black bushings under them so I can use the red bushings under black ones.

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Sidecar Bob
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