On Sat, Nov 2, 2019 at 12:46 PM, Rich NE1EE wrote:
I didn't know why you liked the BAV99 until I just looked them up. Nice!
It's a $0.02 device!
Puzzled because the BAV99 data sheet says VF=0.7V max @ 1mA, and we are well
below that...so I'd expect much less than 0,7V offset. Yet you report 4.2V for
a 1V offset. On the whole, I really like the approach.
I am using both diodes in the BAV99 package in series hence Vf ~1V with VBATEN set and ~0.5V with VBATEN unset.
We're interested in Vf @ 1.5uA and 40uA. See attachment from a Nat Semi data sheet 0.25V and 0.425V respectively. Conveniently very close to my measurements ;) YMMV depending on the manufacturer.
A lot of manufacturers don't specify Vf properly at <1mA. So I measured my sample device using a 0-20V bench supply, a 200kOhm 1% resistor and a calibrated meter.
If I try the resistors, I'll just put in vbat_offset=1000mV, and live with the
increasing (small) error as voltage drops.
It would be interesting to see how well it works. It has the advantage that it is more or less self calibrating with 1% resistors. No doubt the firmware could be modified to suit.
