Looking at the specification of the IRF540 mosfet a rise time of about 35ns should be possible. Finally the voltage at the gate rises to 5V which will slightly lower the on-resistance of the mosfet and the final Drain-Source voltage should be about 100mV.Īs mentioned all this takes about 600ns which is not very fast. After this drop the Drain-Source voltage for this particular mosfet will be a bit above 150mV and the remaining 11.85V will be across the LED strip. This drop happens across the relatively flat part of the peak in the middle called the Miller plateau region. At this point the current through the mosfet is already at the peak but the voltage across the mosfet from drain to source still has to drop from the 12V it starts at. The first sharp rise is from 0V across the threshold voltage of the mosfet to the Miller plateau voltage and takes about 100ns. Whats interesting to see is that it takes about 600ns in total to go from 0V to 5V at the mosfet gate. More specific lets look at the rise and fall of the PWM signal at the gate of the mosfet.Ī detailed look at the voltage at the rising edge at the gate of the mosfet This is actually good enough for simple dimming, but lets look at it more in detail.

Not shown in the video is that this also nicely dims the LED strip. This gives a PWM signal as shown in the (crude) video below. The following Arduino sketch does a simple PWM sweep in 10 steps per second: On the scope you can see the yellow PWM signal driving the mosfet and the blue one that is not connected to any load. Digital 9 is used to drive the mosfet and digital 10 is only used for displaying the unloaded PWM signal on the scope. They both use the Timer2 facility of the atmega328p microcontroller which means I can have them output exactly the same PWM signal.

Digital pin 9 and 10 (also known as OC1A and OC1B) are used. In this experiment I'm using 5 meter of cool white LED strip running at about 800 mA giving 10 Watt.Īn Arduino Duemilanove (my trusty lucky white one) is used to provide pulse with modulation ( PWM ). Dimming a 12V LED strip with an N-channel power mosfet is pretty straightforward.

Arduino pwm fet