Electronics > Beginners
74HC14 for debouncing - question about the type of capacitor
wraper:
--- Quote from: Zero999 on August 15, 2022, 01:29:13 pm ---What's the rationale for the diode?
When the switched is closed the time constant is C*100R and when C is opened, it's C(R1+R2). You could get rid of the diode and use 100R for R2 and 56k for R1 and get virtually the same result.
--- End quote ---
I guess it's fine too. The idea was to not decrease wetting current, however as capacitor is discharged into a switch through low resistance, keeping low pull-up resistance should be pretty much rudimentary.
EDIT: although there is a downside actually, and I'm uncomfortable with high resistance pull-ups. When repairing TVs, monitors and other stuff in the in the past I often replaced TACT switches which became leaky. IIRC usually they had resistance above a few tens of kOHm. In a variant with a diode and low resistance pull-up device will continue to function, with high pull-up resistance it will fail. With microswitches this is not really relevant though unless they were in really harmful environment. I actually posted pictures of such TACT switches under microscope on this forum years ago.
Zero999:
--- Quote from: wraper on August 15, 2022, 01:34:48 pm ---
--- Quote from: Zero999 on August 15, 2022, 01:29:13 pm ---What's the rationale for the diode?
When the switched is closed the time constant is C*100R and when C is opened, it's C(R1+R2). You could get rid of the diode and use 100R for R2 and 56k for R1 and get virtually the same result.
--- End quote ---
I guess it's fine too. The idea was to not decrease wetting current, however as capacitor is discharged into a switch through low resistance, keeping low pull-up resistance should be pretty much rudimentary.
EDIT: although there is a downside actually, and I'm uncomfortable with high resistance pull-ups. When repairing TVs, monitors and other stuff in the in the past I often replaced TACT switches which became leaky. IIRC usually they had resistance above a few tens of kOHm. In a variant with a diode and low resistance pull-up device will continue to function, with high pull-up resistance it will fail. With microswitches this is not really relevant though unless they were in really harmful environment. I actually posted pictures of such TACT switches under microscope on this forum years ago.
--- End quote ---
It sounds like case of crappy switches and contamination. Decent switches with gold plated contacts with an appropriate IP ratings shouldn't have that issue.
If leakage is a problem, use a lower value for R1. I don't see what difference the diode makes.
wraper:
--- Quote from: Zero999 on August 15, 2022, 06:45:47 pm ---It sounds like case of crappy switches and contamination. Decent switches with gold plated contacts with an appropriate IP ratings shouldn't have that issue.
If leakage is a problem, use a lower value for R1. I don't see what difference the diode makes.
--- End quote ---
The difference is you can use smaller capacitor as said in the post with schematic (while keeping the same pull-up resistance). TACT switches generally have silver plated contacts which are flush with plastic. And silver is very prone to electromigration in presence of some moisture, therefore dendrites easily grow over the plastic. You could use switches with gold plated contacts instead but they are way less common and an order of magnitude more expensive.
Zero999:
--- Quote from: wraper on August 15, 2022, 07:07:36 pm ---
--- Quote from: Zero999 on August 15, 2022, 06:45:47 pm ---It sounds like case of crappy switches and contamination. Decent switches with gold plated contacts with an appropriate IP ratings shouldn't have that issue.
If leakage is a problem, use a lower value for R1. I don't see what difference the diode makes.
--- End quote ---
The difference is you can use smaller capacitor as said in the post with schematic (while keeping the same pull-up resistance).
--- End quote ---
Does it matter? A larger capacitor is still cheaper and uses less space than an extra diode.
--- Quote --- TACT switches generally have silver plated contacts which are flush with plastic. And silver is very prone to electromigration in presence of some moisture, therefore dendrites easily grow over the plastic. You could use switches with gold plated contacts instead but they are way less common and an order of magnitude more expensive.
--- End quote ---
The last time I looked, switches with gold plated contacts weren't expensive. The amount of gold is tiny. I suppose it's the usual trade-off between cost and reliability.
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