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Project Notes

#233 PWM/GeneralPurposeHighSideController

Build and test a general-purpose high-side PWM controller



This circuit cobines a variable-duty cycle PWM generator with a high-side p-channel MOSFET driver circuit, making it suitable for controlling loads up to 15V/23A.

PWM Generator

The PWM signal is generated with a modified 555 Timer astable oscillator.

Steering diodes are used to isolate the 100kΩ pot so that it primarily adjusts duty cycle with little change to the frequency.

A switch allows selection of 1nF & 10nF capacitors, selecting a base PWM frequency of:

  • 10nF - 1.25kHz
  • 1nF - 12.6kHz

High-side Driver

The PWM output switches an n-channel MOSFET which in turn controls the high-side p-channel Power MOSFET.

The shunt resistor R4 limits the bypass current when the n-channel MOSFET is on. At 1kΩ, it wastes up to 15mA (at 15V) but in return allows fast switching of the Power FET. A higher value of R4 (10kΩ, 100kΩ) saves power, but switching time is affected.

A pair of capacitors (220µF, 100nF) provide some smoothing of the power supply to the Power FET and decouple the PWM and FET loads.

Performance - 10kΩ Resistive Load at 1.25kHz PWM

At 50% duty cycle:


At minimum duty cycle (~1.25%):


At maximum duty cycle, the output approaches 99.9% fully on.

Performance - 10kΩ Resistive Load at 12.6kHz PWM

At 50% duty cycle:


At minimum duty cycle (~1.25%):


At maximum duty cycle, the output approaches 100% fully on.



The Schematic


Protoboard/boxed construction:





Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

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.