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I always put a decoupling capacitor at the VCC pin of every component on my pcbs and this question derives from my habit to put decoupling capacitors on every power pin of the components I place.

I wonder about using decoupling cap on the Drain pin on N-Channel Mosfet, when using them in saturation mode (fully ON/ fully OFF).

Lets say I have this circuit, to make the question more specific (as this is the circuit I am working now):

schematic

simulate this circuit – Schematic created using CircuitLab

Here is my thought: The Drain pin will originally have the voltage of lets say 12V, but when I give a signal to the Gate of the mosfet (driving it to saturation) the Drain pin will have a near-Zero volts. So the decoupling capacitor seems pointless to me at that point, as I do not need to "de-couple" the Drain of Mosfet from anything. I need it to do the opposite actually, short itself to GND and draw current. Using a capacitor (C1) there will only make the "short" delay some nanoseconds more, since it will have to discharge as well.

My thought is, I do not need to use a capacitor at the Drain pin of a mosfet, when Driving it in saturation mode?. Is there a reason I should use a Capacitor there when I simply need to turn on and off a resistive load?

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    \$\begingroup\$ No. It will only cause trouble. Also, note saturation means different things for BJT and MOSFET. You are using saturation in the BJT sense but talking about MOSFETs (and yes, it's a big huge terminology mess so you never really know what someone really means to say unless they provide context) You should just learn what a decoupling cap is supposed to do and how it is supposed to do it then this question will never come up again. \$\endgroup\$
    – DKNguyen
    Oct 6 '21 at 14:10
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    \$\begingroup\$ You'll only be (potentially violently) dumping the energy of the capacitor to Ground via the FET (and then recharging the cap via the load when you turn the FET off). You are more likely to cause the problems that bypass caps are meant to solve. \$\endgroup\$
    – Wesley Lee
    Oct 6 '21 at 14:11
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    \$\begingroup\$ That, C1, is not a decoupling capacitor. To be a "decoupling cap", it has to be on the "rail". For your scheme; one end on 12V side, and the other end on the ground side, of whatever you are concerned, the device/circuitry you wish to "decouple" from the "rail voltage/current disturbances". \$\endgroup\$
    – jay
    Oct 6 '21 at 15:27
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The capacitor is not necessary and can only degrade performance (speed) of the circuit. In an extreme case (if you toggled the FET at a high frequency), the energy stored in the capacitor could overheat the FET when it is discharged into it.

If there is a possibility of higher voltage, high energy spikes on the drain line, then perhaps the capacitor could be considered as filtering those spikes (when the FET is off), but since your FET is already a 30V device, this is unlikely to be a problem with a 12 V supply.

Note that if your '12 V' is really a 12 V car battery, that '12V' can reach 40 V in certain extreme conditions (load dump), and you really should use a 40 V or higher rated FET.

Be aware also that when an incandescent lamp turns on, there can be a very high (10x the normal value) inrush current when the filament is cold. Be sure your FET can handle that surge.

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