Fork me on GitHub

Project Notes

#106 SwitchPFET

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

Notes

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.

Construction

Breadboard

The Schematic

The Build

Credits and References

About LEAP#106 MOSFET
Project Source on GitHub Return to the LEAP Catalog

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

LEAP is my personal collection of electronics projects, 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 (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.