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?
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?
On a sunny day (Fri, 20 Sep 2024 14:18:26 +0530) it happened Pimpom <Pimpom@invalid.invalid> wrote in <vFaHO.92333$lZG8.68980@fx09.ams1>:
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 <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.
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?
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?
Why not use a uP? An RP2040 is 70 cents in any quantity, and has an oscillator on-chip. ADCs and temperature sensing too.
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.
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
On 20-09-2024 03:45 pm, Jan Panteltje wrote:
On a sunny day (Fri, 20 Sep 2024 14:18:26 +0530) it happened PimpomYes, I suppose I could use one of the simpler ATTiny MCUs or an Arduino
<Pimpom@invalid.invalid> wrote in <vFaHO.92333$lZG8.68980@fx09.ams1>:
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...
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.
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