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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Upper limit of timing resistor value in a tropical setting
Date: Fri, 20 Sep 2024 10:21:52 +0100
Organization: Poppy Records
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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?

I've experienced damp getting into the laminated structure of a printed
circuit board and causing tracking in all directions.   Under those
conditions it would be better to ignore all the usual advice and use 10k
or less and an electrolytic capacitor. 

Remember the capacitors and ICs have exposed terminals which can
track-across, so the resistor might not be the only limiting factor.  In
really damp circumstances, or with high voltages, I would consider using
PTFE standoffs with wire-ended components - they should be good up to
about a Megohm.  


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk