Decoupling capacitors needed on P-FET?

Discussion in 'General Electronics Chat' started by jerseyguy1996, Jan 21, 2012.

  1. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
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    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?)
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    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.
     
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  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    • 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.
     
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  4. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
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    9
    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.
     
  5. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
    9
    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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