This question has probably been answered before, but I am having a hard time finding explanations about the capacitors purpose. I have been looking at schematics of led matrix circuits and you can often see the anode row is controlled by a mosfet. The mosfet usually has a 100 uF capacitor connected between the source and gnd. Why is that? is it just to smooth the power supply current drop or is there another purpose. How important is the capacitance value, what would happen, if there was no capacitor.

If you know where I could look this up, just point me in the right direction. Thanks.

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  • \$\begingroup\$ source as in the MOSFET source pin? or the power supply pin? This website has a schematic editor. You can edit your post to add it in. \$\endgroup\$
    – DKNguyen
    Mar 19, 2020 at 13:30
  • \$\begingroup\$ yes mosfet source pin (: \$\endgroup\$
    – jst kiko
    Mar 19, 2020 at 13:39
  • 3
    \$\begingroup\$ Schematic too small. Is that an PMOS or NMOS? It looks like PMOS so source pin and power supply pin are the same. Not true for NMOS. PMOS is less common than NMOS so people may assume you mean NMOS if you just say MOSFET. Cap is from +V to ground so it is a decoupling cap. Value is not critical. \$\endgroup\$
    – DKNguyen
    Mar 19, 2020 at 13:41
  • \$\begingroup\$ It is not my schematic so i cant really zoom it up, but i am pretty sure it is PMOS \$\endgroup\$
    – jst kiko
    Mar 19, 2020 at 13:45
  • \$\begingroup\$ Thank you, i have looked up decoupling capacitors and found what i was looking for. \$\endgroup\$
    – jst kiko
    Mar 19, 2020 at 13:50

1 Answer 1


It is a bulk/bypass capacitor, local storage for providing energy to the circuit that is switched on with the FET, if it needs a large surge when being switched on.


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