zetec86
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Joined: Tue Feb 26, 2013 6:55 pm

Transistor Advice for RGB LED Strip Lights

Tue Feb 26, 2013 7:05 pm

Hi, I am using my PI to control a 12V analog RGB LED strip lights with PWM (pi-blaster). I am currently using STP16NF06 MOSFET Transistors but they are not providing enough power output to get the lights as bright as they are when connected to the original controller. Does any body know what Transistors can be used to get the brightness on par with the original controller?

Thanks!

ArbitraryY
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Joined: Wed Mar 06, 2013 9:09 pm

Re: Transistor Advice for RGB LED Strip Lights

Wed Mar 06, 2013 9:24 pm

I'm interested in this one too. I am using these transistors (N-Channel MOSFET 60V 30A ): https://www.sparkfun.com/products/10213. I have seen other tutorials that get this to work, but they all use an LED driver board. Works fine in a circuit with an Arduino Uno, but the same circuit when attached to my RPi results in lower brightness when set to full power for any given LED. I am also using pi-blaster and a 12V LED strip. I don't fully understand all of the electronics here, but the uController is apparently the issue; something to do with RPi and 5V max vs Arduino Uno 12V max maybe?

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Burngate
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Re: Transistor Advice for RGB LED Strip Lights

Thu Mar 07, 2013 10:20 am

None of those FETs is ideal when driven directly from the Pi's 3v3 GPIOs.
The first - STP16NF06 - has a gate-turn-on of between 2v and 4v, and so may not turn on at all, and requires up to 10v on its gate to really get going.
The second - FQP30N06L - is better, in that the gate threshold is between 1v and 2v5 so will start to turn on with 3v3, but still needs 5v to attain its low on resistance

I've not seen any FETs around that really fit the bill (someone else may know of one) but you could try adding a bipolar transistor between the GPIO and the FET gate as a switch to allow 5v to drive the gate harder.

EDIT: from the Arduino site, I found a level-shifter that might fit the bill: it uses a BSS138 which has a suitably low threshold
Image

ArbitraryY
Posts: 5
Joined: Wed Mar 06, 2013 9:09 pm

Re: Transistor Advice for RGB LED Strip Lights

Thu Mar 07, 2013 10:40 pm

Thanks for the info Burngate! For my project I am powering the LEDs using an external 12V 2A power supply. @zetec86 I am using the following schematic which may help you with your project as well: http://mitchtech.net/raspberry-pi-pwm-rgb-led-strip . My post may have been a bit preemptive since I compared an LED strip plugged into an Arduino Uno at full power with that of the LED strip plugged intot he RPi and they are definitely on par in brightness.

Appreciate your time and insight @Burngate


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Gert van Loo
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Re: Transistor Advice for RGB LED Strip Lights

Fri Mar 08, 2013 11:37 am

Just a general tip: power transistors are not very good at switching at high frequencies.
I suggest you limit your PWM frequency to 100-200Hz.

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