Re: Upper limit of timing resistor value in a tropical setting

On 9/20/24 17:17, john larkin wrote:

On Fri, 20 Sep 2024 14:18:26 +0530, Pimpom <Pimpom@invalid.invalid>
wrote:
 

Take a typical domestic environment in a tropical region, rather humid
at times. Assembled board is not conformal coated and some dust is
expected to seep into the box over the years and settle on the PCB. The
unit is low voltage and low power with just a few watts dissipated when
on, and left unpowered for days on end.
>
An R-C combination is used to set timing of around 1 sec. which is not
critical (long-term variation of up to 10% is tolerable). If it gets
very bad, the board can be cleaned but this should not be required as
regular maintenance.
>
Under these circumstances, what's the highest timing resistor value
you'd feel comfortable with?

 
Without condensation, literal drops of water on the board, 1 Meg might
be fine. Some attention to the board layout would help... extra
clearances on the critical node, maybe two resistors in series, small
R and giant C, maybe use an RC oscillator and divide down.
 
Why not use a uP? An RP2040 is 70 cents in any quantity, and has an
oscillator on-chip. ADCs and temperature sensing too.
 

>
an AVRtiny would do the job for ~30cents, and you can get mcus much
cheaper than that
>
https://cpldcpu.wordpress.com/wp-content/uploads/2019/08/3-cent-mcu-1.png
>