SparkFun Forums

[adam.g]

Hi folks,

While it's a clearly established fact I'm obsessed with low-power operation (I think I need help), one of the biggest constructive-criticisms I have of the MicroMod Carrier Boards is that they all have a 10 kΩ/20 kΩ voltage divider that cannot be disabled without cutting a PCB trace or desoldering a 0603 SMD resistor.

Even for your average user, a constant 0.2 mA draw at 6 V can be non-trivial.

For future versions of MicroMod carrier boards, I'd highly recommend putting a jumper (or FET) between VCC that would allow users to easily disable the voltage divider.

While I would suggest using higher value resistors with a decoupling capacitor to decrease the quiescent draw, I suspect the whole point of the low resistance is to be compatible with a wide range of processors, some of which handle high impedance worse than others (e.g. the Artemis really struggles with this).

Cheers,
Adam

[TS-Chris]

Thank you for the feedback!

[luca0074]

@adam What value of resistors do you recommend and where to place the capacitor?

[adam.g]

Hi @luca0074,

Typically, Arduinos that emphasize low power operation will use a resistor divider in the megaohms. Something like 2MΩ/1MΩ, which would result in a quiescent draw of 2 uA at 6 V. The resistor values used are dependent on the maximum battery voltage, which needs to be scaled down to be able to be measured within the ADC's 0-3.3V range. The capacitor helps out the ADC when it goes to perform the analog measurement, and in this case, would be across the 1MΩ resistor.

However, for the MicroMod boards, Nate made a good suggestion in the following GitHub issue that a fuel gauge IC is the better way to go: https://github.com/sparkfun/MicroMod_Ar ... -742869375

In the case of the MicroMod Artemis Processor, which has its own battery voltage divider, you end up needing to do calculations for a loaded voltage divider. In theory possible, but it requires correction factors, and this functionality was never really completed for the Artemis Processor.

In the MicroMod Asset Tracker schematic, you can see it made the switch from the voltage divider over to the fuel gauge:
https://www.sparkfun.com/products/17272

Cheers,
Adam

[luca0074]

Thanks @adam.g !

Im planing to use a SAMD51, so no Artemis math required

What should be the capacitance of the capacitor?

Bildschirmfoto 2021-04-21 um 22.56.26.png

Edit: R10 has 2M

Cheers, Luca