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Topic: Problem with MOSFET selection for RGB LED strip driver? (Read 1 time) previous topic - next topic

mrvector

Hi there!

I am trying to make a circuit that will allow me to light up some RGB LED strips with PWM (obviously), but I'm having some confusing problems. It should be a really simple thing, but I just can't get it to work properly. After troubleshooting various things, I think I've isolated the problem down to the MOSFETs I am trying to use.

So, for testing purposes, I've hooked up my strips exactly as shown here except I am using different MOSFETs, I'm only testing one colour right now, and I'm not even using PWM at the moment. I've put the Blink test onto my Arduino and connected the gate of the FET to pin 13. +12V on the strip to 12V, red channel on the strip to drain on the FET, and source on the FET to GND.

My strip lights up and it does get brighter on the blink, but it never completely goes off. I've even tried eliminating the Arduino and tried using just a 3.3V or 5V line. With this, when I repeatedly pull the voltage line in and out of the breadboard at the gate pin of the MOSFET, sometimes the LEDs go out when I pull the voltage out, sometimes they don't.  :smiley-slim:

The only explanation I can think of is that there is a problem with the MOSFETs I am using... whether that problem be with the MOSFETs I am using or something else. The first ones I tried (before I did any of this troubleshooting) were IRFZ44Ns and those didn't really work very well. After reading around a lot, I figured that I needed some that were "logic level." I was directed to STP16NF006s. These worked better but I was still having really inconsistent issues. I've learned a lot since picking up this hobby (I'm much better at writing code), but clearly, I am inexperienced and datasheets confuse the hell out of me sometimes. Obviously, I am doing something wrong and I can't quite figure out what it is.

Thanks!  :)



ron_sutherland

The hookup guide you linked is using BJT's, not MOSFETS. MOSFETS are easy to damage with ESD while BJT's are less sensitive.  I would recommend using a BJT like in the guide. With the FET, there are several things to consider beyond ESD, e.g. when power first shows up on the FET's drain it will capacitively couple to the gate, through the gate to drain capacitance (Cgd). The actual voltage on the gate is determined by the ratio of the gate to source capacitance (Cgs) and Cgd. The gate drive pin will be high impedance (set as INPUT) for a while until the bootloader runs the application (the sketch). This is a very bad situation for a MOSFET, as the gate can easily end up with enough voltage to turn on in the liner region and dissipate lots of power that heats up and damages it, so my guess is they have been smoked.
my projects: https://github.com/epccs

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