Does a mosfet require PWM?

I want to use a mosfet to dim an LED light strip at 0.5 amps. Do I need to use an arduino and PWM or can I just connect a POT across the mosfet gate? I'm using a 12v light strip as a brake light and wish to have it illuminated when just the tail lights are on like a Dual element tail light bulb

Thank You

If you just use a pot the FET will operate in the linear region. That means it will get much hotter than if you use PWM. In effect it will burn off the unwanted power.

Well I have an irf520n mosfet running a 0.5 amp load. I have heat sinks and an arduino but it seems like an overly simple task for an arduino. What would you recommend?

Also I have the strip cut in half and one half wired to each tail light but I know wiring in the mosfet will connect the two halves in parallel so I need to use a couple diodes. Would a 1n4007 diode running at 0.25 amps per half suffice? I'm not very good at understanding the data sheets

What Grumpy_Mike wrote is more theory than practical.
I think it is okay to say that a mosfet does require PWM. That is when the mosfet is at its best.

Can you make a drawing or schematic with the diodes ? I don’t understand why you would need them.

Peter_n: What Grumpy_Mike wrote is more theory than practical.

Why do you think that then?

Since wikitjuggla is a beginner (sorry wikitjuggla), and it is about a led strip. Using a pot meter would be very hard and the mosfet could get too hot without heatsink. So yes, in this situation is pwm needed, and a pot meter would even be wrong :cold_sweat:

A mosfet can be used in an analog circuit, like in a radio transmitter amplifier. :open_mouth:

With the dual element light bulbs, the brake light filaments of both bulbs are wired independent of each other in order for the blinkers to work but the second tail light elements are wired parallel so if I connect the mosfet, it will wire the two LED strips in parallel to each other and the blinker circuit in parallel with the tail light circuit also, causing my entire blinker system to be wired in parallel and not work so i need the diodes in order to keep the two circuits independent of each other and to keep both blinkers independent of each other

Peter_n: Since wikitjuggla is a beginner (sorry wikitjuggla), and it is about a led strip. Using a pot meter would be very hard and the mosfet could get too hot without heatsink. So yes, in this situation is pwm needed, and a pot meter would even be wrong :cold_sweat:

A mosfet can be used in an analog circuit, like in a radio transmitter amplifier. :open_mouth:

I can't remember for sure but a while back i'm pretty sure I ran a light strip at 2.0 amps off of the same mosfet with only a POT for about 10 minutes. It got hot enough to where you couldn't keep your finger on it for more than 5 or so seconds but that was without a heat sink. Now I only require 0.5 amps and have a heatsink

Hi, I know its an "instructables" page but this would be easier than just using an arduino to dim your LEDs to a fixed level.

http://www.instructables.com/id/Simple-PWM-Control-555-for-Motors-lights-heat/

Unless you want to use the arduino in the car for other reasons, it seems like using a piledriver to knock in a pin.

Tom...... :)

wikitjuggla:
Well I have an irf520n mosfet running a 0.5 amp load.

I’m not very good at understanding the data sheets

IRF520
IRF520

VGS, output & transfer characteristics not a good match with 5V, separate drive ckt req’d.

In practice, what you want to do is to divide the light strip up into a number of sections, according to where the division points are (generally every three LEDs with a resistor).

For the "tail light", you use one quarter of the sections, fed with a diode from the tail light circuit. A 1N400x should be fine. You also supply that quarter section with a diode from the stop light circuit, and you power the other three quarters of the sections from the stop light circuit as well. Thus only a quarter operates from the tail light, but all from the stop light (or blinker).

Your only concern is how you arrange the sections. If they are all laid side by side, then every fourth one is the tail light. If you want a long strip, then not all would light up as the tail light - but I do recall seeing some cars or vans which do just that - part of the strip shows as the "tail" (generally the outer part to mark the side of the vehicle) but all of it as the "stop".

{The other alternative is of course, to power the strip from the tail circuit through a diode and resistor, and the stop circuit through a diode only.}