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Project Notes

#106 PFET Switch with Delay

Using a small-signal p-channel MOSFET as a switch, with turn-on delay.



P-Channel Enhancement MOSFETs (like their n-channel cousins) are easy to use as a digital switch:

  • ON when gate-source voltage is much higher than threshold voltage (into saturation region)
  • OFF when gate-source voltage is below threshold voltage (-0.1 to -3.5V for BS250)

This circuit demonstrates the basic switch, with the addition of a turn-on delay.

With the switch normally open, the gate is pulled low and therefore Vgs is below the threshold voltage. When the switch is closed, Vgs is brought up to 0V and the FET turns on.

The turn-on delay is implemented with a simple RC circuit: the R1=220kΩ C1=10μF pair having a time constant of 2.2 seconds. When the switch is opened, C1 takes some time to discharged, hence then FET is delayed in turning back on.





Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.