Suggestion on the impact of removing a capacitor in parallel with a tactile Switch

[Sink0]

Hi,

I have a design that have a tactile switch with the following circuit and an internal pull-up of a microcontroller (about 47k). The capacitor was placed in order to reduce the bounce, but the real debouncing is made inside the firmware.



Would removing the capacitor have any meaningful impact on EMC or EMI of the circuit. It will be tested for IEC 60601-1.

Regards,

Luis

 

[barry]

The capacitor would not hurt emi/emc performance. How much it would help really depends on everything else around it. I would leave the pads for the cap on the board, that way when you go to test it you could easily add the cap if needed. You might also want to put a ferrite bead on the trace running to the switch.

 

[Ishaan Karnik]

Switch bouncing tends emitt EMI but at low voltage like 3.3V that won't be much of an issue provided you are not working in RF band or some high speed switching protocol like PCI.

This may cause issue with sensitve, high gain amplifier. I have not faced this issue but have heard my co-workers complain about the same. They were working with ultra-sonics.

 

[ravindragudi]

Is there a serious reason to omit the capacitor? If not, then have it on PCB. Even though there is a debounce algorithm in the firmware, containing the switch bounce in the electronics circuit by means of hardware deisgn is best practice. Instead of leaving out the capacitor, you can think of changing the value of capacitor from 100nF to a lower value like 10nF or something.

 

[Sink0]

Is there a serious reason to omit the capacitor? If not, then have it on PCB. Even though there is a debounce algorithm in the firmware, containing the switch bounce in the electronics circuit by means of hardware deisgn is best practice. Instead of leaving out the capacitor, you can think of changing the value of capacitor from 100nF to a lower value like 10nF or something.

Definately there is a reason: Board space. We need to place a new resistor in the board and we are out of space. When i was checking what could be done, I realized I placed this capacitor inside the board for good practice purposes only... So i was wondering.

Another possibility would be to replace a series resistor we have between microcontroller pin and a logic N-Mosfet's gate (DMG1012UW-7). It is switched in a very low frequency, but I placed a series resistor to avoid ringing. I could start driving it with the microcontroller pull-up resistor. Would that be a solution?

 

[ravindragudi]

It is riskier to cut down on Cs and Rs to free up board space. Instead of that, another tactile switch with smaller profile can be selected. I am not sure if that is an option for you.

 

[Sink0]

It is riskier to cut down on Cs and Rs to free up board space. Instead of that, another tactile switch with smaller profile can be selected. I am not sure if that is an option for you.

Well,

Not if they were placed there just for good practices and not for real calculated purpose. So i am just revaluating their utility as i see many designs with no capacitor at all at the tactile switch.

Inside the board there is no chance the switch will create problems. The pull-up is about 47k, so very small switching currents. My concerns is regarding the emited radiation to outside the board. It is a werable device so it is quite well isolated circuit. No RF communication.

 

[Ishaan Karnik]

Well,

Not if they were placed there just for good practices and not for real calculated purpose. So i am just revaluating their utility as i see many designs with no capacitor at all at the tactile switch.

Inside the board there is no chance the switch will create problems. The pull-up is about 47k, so very small switching currents. My concerns is regarding the emited radiation to outside the board. It is a werable device so it is quite well isolated circuit. No RF communication.

Whats your application?

 

[Sink0]

Whats your application?

Something similar to these activity tracking watch, but for medical applications.