Upper limit of timing resistor value in a tropical setting

[Pimpom]

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?

[Liz Tuddenham]

Post by Pimpom
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.

[Pimpom]

Post by Liz Tuddenham

Post by Pimpom
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.

I made a gadget with a similar purpose back in 2008 and it's still in
use in a video studio, although this one is of a completely different
design.

I used an opamp as a Schmitt trigger as the timer in that earlier one
with the timing set by a 100k resistor and an electrolytic cap. The
current one is to be used with CMOS gates.

[Jan Panteltje]

On a sunny day (Fri, 20 Sep 2024 14:18:26 +0530) it happened Pimpom

Post by Pimpom
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?

100 k would be a high limit
10k 22k better
But s Microchip PIC with internal clock is slightly temperature sensitive
but can create any timer, uses almost no power, and can do many more things.
Requires some programming though.
No timing caps and resistors needed...

[Pimpom]

Post by Jan Panteltje
On a sunny day (Fri, 20 Sep 2024 14:18:26 +0530) it happened Pimpom

Post by Pimpom
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?

100 k would be a high limit
10k 22k better
But s Microchip PIC with internal clock is slightly temperature sensitive
but can create any timer, uses almost no power, and can do many more things.
Requires some programming though.
No timing caps and resistors needed...

Yes, I suppose I could use one of the simpler ATTiny MCUs or an Arduino
Nano. But the whole thing uses a common 12V supply and using an MCU will
require a buck regulator and scaling of the input and output.

[john larkin]

Post by Pimpom
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.

[b***@www.zefox.net]

Post by john larkin
Why not use a uP? An RP2040 is 70 cents in any quantity, and has an
oscillator on-chip. ADCs and temperature sensing too.

What is the time-keeping mechanism?

bob prohaska

[Lasse Langwadt]

Post by john larkin

Post by Pimpom
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

Loading Image...

[Jan Panteltje]

On a sunny day (Sat, 21 Sep 2024 02:07:30 +0200) it happened Lasse Langwadt

Post by Lasse Langwadt

Post by john larkin

Post by Pimpom
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

Nice overview thank you
even a PIC12C509 clone!

[Joe Gwinn]

Post by Pimpom
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?

Maybe 1 Kohm. Others have said 10K and 100K.

But why not dip the board in hot wax, to seal the surface? One can
repair the board without removing the wax, and just re-wax when done.

Back in the dead-bug era (late 1960s), we would put the vectorboard
with wired and ICs in a shallow tray of molten wax, and let it cool
and solidify, protecting the circuitry both physically and
electrically from damp and salty dust. If repairs or upgrades are
needed, melt wax, pull board out, make changes, return to wax tray.

Joe Gwinn