Sunday, July 17, 2016

Measuring tiny currents to assess power consumption

The original EEVblog µCurrent (David Jones).
Way cheaper than real industrial probes,
but still too expensive for my needs ($80+)
Optimizing power needs requires power tools that can read tiny currents (nano or pico amperes)...

Nothing happened like I wanted to the last N days and had to work on unexpected stuff, like a remote water level alarm for our swimming pool after more leakages (the liner is 14 year old). So I made an RF alarm, which is powered by a small ultracapacitor (0.47F) backed by a tiny 3x2cm solar panel I scavenged from a led keychain.

I wanted it to be small, so I can duplicate the project and put sensors all around (I should end up at around 3x2x2 cm all included). But small means low power.






Optimizing power requires tools that can read tiny currents (nano or pico amperes). In this range, not only my existing tools (scope, multimeters, etc) will see nothing but noise, but they also have a significant impact on the circuit itself: the so called "burden voltage" lowers significantly already tiny and critical 1.8V-2V circuitry, to the point it is may not be worth to interpret the measured data...

Note that measuring milli-ohms also raise issues, which are not the subject of this post though (see there instead!)

Interestingly, this problem happens also with terribly expensive high-end tools.

David Jones made his own probe for this reason, that is being sold at EEVblog ... when it is in stock (i.e. rarely!). It can ready tiny currents in the range of the picoamp without harming the existing circuit. It is very well made and gorgeous, but it is still expensive (often 80$+) and I do not trust my own proper use of it... what if I feed 100mA to this? Will it blow?

Someone at dorkbotpdx relied on the same principle, i.e. amplifying the tiny voltage drop along a very low resistance, to plot the quick variations of the tiny currents of a PIC microcontroller while it boots (the interesting read is here). He used a $20 AD8428 and a high quality regulator. Straightforward but still too expensive for me.

Dynamic micro-amps measured at 3MHz.
It gives very interesting results, but the components are
not cheap though ($20 for the amplifier and a $10 regulator)

Eventually, I stumbled upon +Ben Hencke project below, which suits my needs well enough. This is a "cheap" copycat of David Jones's classy probe, that reads "only" nano amps (no pico amps). Most notably, it relies on a cheaper $8 MAX4239ASA amplifier, that I can blow without crying when I do it wrong. And it is a one-sided PCB, so it can be made it at home as soon as I have the amplifier (or in ~20 minutes when I have an equivalent one in stock). Good to have.

Here is a simpler, single sided PCB, "cheap" variant of David Jones' µCurrent.
It reads no pico-amps due to the cheaper amplifier ($3), but it is well enough for me.


Refs:
(... ignoring real "industrial grade" stuff...)
http://www.eevblog.com/projects/ucurrent/ (the original, deluxe)
http://hackaday.com/2013/08/04/measuring-tiny-current-with-high-resolution/ (intermediate)
https://github.com/simap/ucurrent (this one)