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Created June 11, 2019
Last modified June 16, 2019
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Summary

MOSFET driver for LED strip with passive pull-up and active pull-down to switch/control 12V LED strip with 3.3V TTL PWM input


Description

A corrected (see stack exchange question and first revision) and working-if-basic circuit with a simple gate driver. This is an inverting driver - the MOSFET gate is driven low when the control input signal is high and vice versa. It has an active pull-down via the NPN transistor and a passive pull-up via the 10k resistor.

This won't work great for high-frequency PWM, as you can see in the circuit simulator, because the passive pull-up makes the turn-on speed is too slow. If you oscillate too fast, the MOSFET spends too much time in turn-on and wastes too much power.

(I revised this further in this circuit that in simulations copes with the 100kHz PWM frequency recommended for comfortable LED lighting by manufacturers by using active push/pull).

So you're not going to get super fast flicker-free 100kHz+ LED dimming from this.

Basic circuit courtesy of https://arduinodiy.wordpress.com/2012/05/02/using-mosfets-with-ttl-levels/ . That article also does an excellent job of explaining everything. In particular it mentions there are MOSFETs available that work well at 3.3V TTL level gate inputs.

What it doesn't really explain is that you still don't want to drive those logic level gate MOSFEts over a long wire from a microcontroller somewhere else because of the wire inductance etc. But a driver like this should work.

Because this has active pull-down but passive pull-up it won't necessarily perform well for very fast switching. It'll waste a bit more power than an active/active circuit both in the MOSFET (due to slower transitions) and in the pullup resistor. But it's easy.

A MOSFET driver IC like a TC4426A might be a much easier way to do this. Or an LED driver IC that integrates PWM if you can find one that's available in DIP or a similar convenient non-surface-mount package at a reasonable price.

Also handy basic reading: https://www.electronics-tutorials.ws/logic/pull-up-resistor.html


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