Wednesday, July 5, 2017

Measuring micro amps, the easiest way

You can build yours (see my former review), but you can buy them already made: these $10 sub micro ampmeters on ebay work pretty well from 50mA down to 0.001mA. This range matches nicely the currents of many sensors, including when they are in deep sleep mode. This range is very convenient for IoT autonomous projects and sensors, and the 5 digits makes it unnecessary to switch between milliamperes and microamperes like on a multimeter.

You lose one digit when plugged backwards. The feature is not really interesting except that it makes it safer (you will not burn it by accident).

$10 for these micro current meters is a bargain!
They really are straightforward to use, and they do measure very low currents: the display goes as low as 0.001 mA, ie. 1µA, but I am not sure it is reliable at that level -- anyhow, below a few micro amps, the self-discharge rate of a battery usually becomes not negligible, so there is often no point in fighting further to reduce the sleeping currents. A CR2032 coin cell can run for years at this level, so you would better start optimizing also the wake-up consumption (make sure to read this impressive in-depth review on the matter!).

I soldered a USB male plug on the input voltage (5V is OK but it accepts a wider range), and I added 2 hooks for the sensed current (put the ampmeter in serie of course: the provided cables & plug are thick -- it is a good thing given the small currents).
Here is my complete, workable setup.
Using an USB powerbanks gives a steady, independant power source.
I only had no red hook anymore, so I used a yellow one,
but the devices withstands reverse polarity anyway.
The burden voltage is quite low (they are measuring the tiny voltage drop across a small resistor, thanks to an I2C amplifier), so it has a very small impact on the circuit compared to 99% of the usual multimeter even in µA range (mostly worthless imho).

All I think it is a must have for less than $10 if you are trying to reduce the power consumption of an IoT device, and it will certainly postpone the day I build one for me :)

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