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I am beginner enthusiast. I was trying to make lights under my bed by following this tutorial:

http://aruljohn.com/blog/raspberrypi-christmas-lights-rgb-led/

It worked for me with a 1 meter color RGB led strip. However, when I put a white LED 5 meter strip, it can not go bright enough and it seems to be at 30% brightness.

The only MOSFET I have at home is IRF520. Is this the reason for my dim light? If yes, what kind of MOSFET would fit? Is there a way I can learn to calculate it?

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  • \$\begingroup\$ what are you using as power supply? \$\endgroup\$
    – Wesley Lee
    Jul 23, 2016 at 0:44
  • \$\begingroup\$ White LED's are actually a blue-violet LED firing into a special yellow diffuser to create narrow band white light. These LED's need at least 3.0 to 3.5 volts to turn ON. RGB LED's will come on at a lower voltage. \$\endgroup\$
    – user105652
    Jul 23, 2016 at 2:16

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Two causes.

Power supply too weak. At 5 meters 5050 rgb or 5050 3 diode white you are looking at 1.2 Amp per meter for all the channels. So 6 Amps minimum power supply. For single diode white like 3268 you are only looking at 2+ Amps.

More important, the IRF520 is not suitable for 3.3V logic level applications. It is barely on at that gate voltage. Its resistance will be too high, resulting in the brightness issue you see. At 400 mA is okay, but not at 2 Amps per channel.

Look for a logic level mosfet, or a mosfet driver circuit. A simple npn transistor setup would do it.

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    \$\begingroup\$ I have a single white diode with 3.3A, so power is good. It was the Mofset that was problematic. Unfortuantely, i got a few other ones that had a Vth with 1V or 2V and they still didn't work. I had to use a npm transistor setup, which did work! thanks! \$\endgroup\$
    – mgPePe
    Jul 23, 2016 at 16:02

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