Decoupling capacitors needed on P-FET?

Thread Starter

jerseyguy1996

Joined Feb 2, 2008
206
I understand the need to supply decoupling capacitors on every IC but I am curious if one should be placed from source pin to ground on a p-fet. It seems that when the p-fet is switched on it is going to jerk the powersupply if there is any significant load attached to it. What is common? I have attached my schematic and I am concerned about the need for caps on U4 and U5 (or should those be labeled with a Q perhaps?)
 

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Wendy

Joined Mar 24, 2008
22,146
It depends. It can not hurt, it just may not help. Normally I would say not, because if you spike (as in a quick voltage drop) those parts there is no logic that would switch. With parts that have logic and counters you need to keep their power supplies as stable as possible.

I looked at the schematic. You are missing a connection to the cathodes of the LEDs.
 

Adjuster

Joined Dec 26, 2010
2,148
I understand the need to supply decoupling capacitors on every IC but I am curious if one should be placed from source pin to ground on a p-fet. It seems that when the p-fet is switched on it is going to jerk the powersupply if there is any significant load attached to it. What is common? I have attached my schematic and I am concerned about the need for caps on U4 and U5 (or should those be labeled with a Q perhaps?)
  • Decoupling capacitors from the source connections of PMOS FETs to common would be good practice. Power supplies should generally have such capacitors.
  • The "common" or ground connection in your circuit is shown in many places by a triangular symbol. I can't see a connection to the outside world for this - normally there would be one at least for the power input, and probably other places.

  • In your diagram D1 to D9 anodes seem not to be connected to anything at all. As shown, they will not function.
 

Thread Starter

jerseyguy1996

Joined Feb 2, 2008
206
It depends. It can not hurt, it just may not help. Normally I would say not, because if you spike (as in a quick voltage drop) those parts there is no logic that would switch. With parts that have logic and counters you need to keep their power supplies as stable as possible.

I looked at the schematic. You are missing a connection to the cathodes of the LEDs.
The cathodes are connected to U2 which is an LED driver. The anodes will eventually be wired up to the 3 pin connector on the right. It is just a little 3X3 Led cube so there are two more layers of LED's that are not showing because their pads won't be part of the PCB. I'm trying this out before I make a larger 5X5 cube.
 

Thread Starter

jerseyguy1996

Joined Feb 2, 2008
206
  • Decoupling capacitors from the source connections of PMOS FETs to common would be good practice. Power supplies should generally have such capacitors.
  • The "common" or ground connection in your circuit is shown in many places by a triangular symbol. I can't see a connection to the outside world for this - normally there would be one at least for the power input, and probably other places.

  • In your diagram D1 to D9 anodes seem not to be connected to anything at all. As shown, they will not function.
When I make the PCB I am going to have a ground plane that will cover everything that isn't a track and the ground of my power supply will connect to that. This in an LED cube so the anodes will be connected by a wire to the 3 pin connector on the right hand side of the schematic.
 
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