led stripmosfetraspberry pirgb
I am able to control an RGB LED strip using Raspberry Pi, using 3 x TIP120.
However, I would like to add multiple strips, maybe about 7 to 10 RGB LED strips. I will use 12V 30A power supply.
I am wondering why the light is not so bright when I connect 2-3 series of RGB LED strip.
Do I need to use 3 x TIP120 for each RGB LED strip?
Any suggestions is appreciated.
Thank you!
EDIT:
Image from http://popoklopsi.github.io/RaspberryPi-LedStrip/#!/
I used that tutorial related to this question.
You're nearly there, a couple of things though:
The base resistor calculation ins't correct - remember you only need 1.6mA according to your calculations (the collector current is separate).
Looking at the datasheet, the minimum gain is 1000, and maximum base-emitter voltage is 2.5V, which means we need to adjust the calculations, 1.6mA will do for the base current (always good to have extra for a switch as the gain drops at saturation) but we need to use 2.5V rather than 1.3V for worst case (it's better to use worst case/maximum values to design with, though looking at the graph it seems the extra Vbe is unlikely at this current, so somewhere between the two figures below should be okay):
So:
(3.3V - 2.5V) / 1.4mA = 570Ω
or
(3.3V - 1.5V) / 1.4mA = ~1.2kΩ
This should work okay, but is not the most efficient way to do things - the transistor dissipation will be at least 0.4A * Vce(sat) which is about 0.4A * 0.75V = 0.3W, plus your R-pi needs at least a couple of mA or so to drive it.
A modern logic level MOSFET can be much smaller, be driven with (almost) no current) and have almost no dissipation. Here is an example part, the FDC637BNZ , chosen at random from thousands at Farnell:
Dumb RGB LED strips like you have are simple. Each segment consists of 3 channels. Each channel has 3 diodes and 1 resistor in series. Each segment is in parallel with each other. Cutting and extending the sections with wire does not change that. You will not experience an increase in current because of it (in fact, extending with wire will actually cause a decrease in current, due to the non-zero resistance of the wire. For your short runs, this is negligable).
That said, adding 100 segments, each with 3 ~20mA channels (so 20mA x 3 x 100 = 6 Amps!) To the dome circuit of your car will A, cause the fuse to blow, probably, and B, cause a fire as wiring for the dome light is not sized for multiple amps. A relay in line with the dome switched circuit, to a new fused circuit (or the cig circuit, that's normally fused for 10 Amps) is the safe approach.
Update: a 1.5 Amp controller will not work well with a full 300 led/5 meter length of rgb led strips... At most you can power 25 segments! Thats 500mA per channel/color
Best Answer
A TIP120 is already a darlington with a reasonably high beta... if you're having trouble saturating it with the GPIO output of an Arduino, adding another transistor is just going to raise the Vbe drop another 700 mV and make the problem worse, not better.
You don't say how large a resistor (if any) you are using between the Arduino and the base of the TIP120, but using a lower value there would probably help.
The ATMEL datasheet specifies the minimum tolerable voltage for a "High Level" output as 2.3V ... typical value is probably going to be never more than one diode-drop below Vcc - or 2.6 at the very best.
While the datasheet for the TIP120 says the maximum Vbe is 2.5 volts, a much more typical value is going to be on the order of 1.7V ... To drive the transistor into saturation, you're going to need at most 5mA , well within the current limit for a GPIO pin.
A solution that adds parts count, would be to invert your logic, drive the base of the TIP120 with 12V through a suitably sized resistor ( around 2K would be adequate ) and use a small signal PNP driven by the arduino to "turn off" the base current. Schematic forthcoming.