Help needed: MOSFET gets hot when PWMing car highbeams as DRL lights

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Thread Starter


Joined Sep 1, 2014
Hi folks.

I need some help with my little project here.

The idea of this project is to have my car's high beams work as daytime running lights (DLR), with reduced light output. The DRL would turn on with the ignition on, and turn off, either when the parking lights are turned on or the ignition turned off. I found a PWM circuit online that I used for reference: , but instead of putting the load on the high side, I put it on the low side.

The MOSFET I am using is BUZ11A

Now, I temporarily assembled the circuit on my car to see whether it would work, and it did, everything worked as I planned, but the MOSFET I used got so hot that it started to burn off the tape which was wrapped around its legs, so I pulled the plug. I did use a heatsink.

Can anyone help me out with this, what am I doing wrong here, what am I missing? I want it to work as it did, but I can't have this this burning up things under the hood :). I'm no expert when it comes to MOSFETs, I do have an understanding of basic electronics, as I took electronics classes in college.

Please don't be hating on my text, English is not my first language :)

Here's the whole circuit.
(click on the schematic to zoom in)

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