Testing the low power characteristics of the device

A project log for LoRa darkStar

Low Quiescent Current, Solar Powered +30dB LoRa

capt-flatus-oflahertyCapt. Flatus O'Flaherty ☠ 03/14/2023 at 17:460 Comments

At last, the long awaited low power performance of the device has been tested!

I selected the TinyCurrent test module from I-Fuse for sensing the very small currents involved in this device. I liked the idea that it could be bolted right onto an oscilloscope and measure nA directly as mV. There are 3 dfferent settings on the TinyCurrent: nA, uA and mA (per mV). One word of warning though: DO NOT CONNECT ANY OTHER PROBE TO THE DEVICE CIRCUIT !!!!!! Trying to connect multiple probes causes some serious short circuiting and nasty smells.

The TinyCurrent is easy to use and I started off on the nA setting with the AEMLION uninitialised (It needs a short flash of sunlight on the solar panels to boot up). In this state, the CMOS AND gate has no voltage on either input and the total current flow for the whole device was a meager 60 nA, which equates to a discharge time of about 1900 years on a 1000 mAh lithium battery. In real life the AEMLION energy harvester cuts off the voltage to the AND gate if the battery, for some reason, discharges below 3.6V, so it's actually quite useful.

In the next test, I flashed a light onto the solar panels to initialise the AEMLION and the current shot up to a massive 240 nA, which equates to about 476 years life on the 1000 mAh battey.

Fantastic low power performance by any stretch. But what about the self discharge of the battery?

Obviously, the self discharge characteristics of the battery far out weigh any of the advantages of the the elaborate low power design and using a super capacitor is about 10 times worse. So what's the point of all this effort?

One thing to remember is that we're using an energy harvester which can pull out electrons from very low light levels and be enough to keep a 40 mAh rechargeable coin cell topped up. In this scenario, the current draw of the device is much more relevant than on, say, a 1000 mAh battery with plenty of juice from the solar panels. When the device is set up with a coin cell with a very limited light source, the low power performance of the device as a whole is more critical.