I'm trying to figure out if i could use 16 white 5cm 12v 5050 led strip with the TLC5940
The Array/paralell setup on the strip is something like this:
+12V + -|>|- -|>|- -|>|- -/\/\/\- + R = 150 ohms + -|>|- -|>|- -|>|- -/\/\/\- + R = 150 ohms + -|>|- -|>|- -|>|- -/\/\/\- + R = 150 ohms
Using the http://led.linear1.org/led.wiz to calculate the values i came up with this:
- each 150 ohm resistor dissipates 43.35 mW the wizard thinks 1/4W
- all resistors dissipate 130.05 mW together
- the diodes dissipate 489.6 mW total
- power dissipated by the array is 619.65 mW
- the array draws current of 51 mA from the source.
This is probably correct as with a regulated 12v led driver i measured 15cm and it was around 120-130mA
The TLC is able to sink 120mA (with a proper heatsink).
So i guess that it could handle 51mA..... but it's at 12v
At 120mA a 15 Ohm resistor will swallow 1.8V.
So...if the LED takes 2.4V, the resistor takes 1.8V, that leaves 0.8V for the TLC5940.
0.8V*120mA*16=1.536W total dissipation - that's within spec.
My LED's use 3.2V at 120mA so 5V-3.2V = 1.8V.... 1.8V * 120mA == 0.216W per channel. 2.5W/ 0.216W = 11.57 ..... So I can run 11 of these LED's.
So: 9.6v (3x3.2v) @ 51mA so 12v-9.6v = 2.4v * 51mA = 122.4mW * 16channels = 1,958W.
"2.0 Watt on the TLC5940NT" is this calculation correct?
Do i need a heatsink for 2 watt on the TLC5940?
The TLC5940NT is nice because you just need one resistor to impose the resistance on all leds. but as on those 5cm strips the resistors are already soldered in place what should i put on the iref pin (20)? no resistor ? to ground? a small resistor?
IF the above does not work
Most examples using transistors and mosfets to increase the wattage use PNP / P-type transistors. While the official datasheet suggests NPN transistors or N-Type Mosfets.
Using NPN transsistors it's probably inverted at the end.... but more efficent ... right? But also with PNP transistors it's inverted right? Why are there more examples with PNP transistors?
If the above calculation is correct i probably need to use transistors.
The mosfets i own are all n-type but i have not enough power to fully saturate each mosfet considering 5v . so i think i go for transiostors. i have some p2n2222a NPN. those can sink 500mA each.. that should allow me to not overheat my circuit. this also would allow me to use 15cm per channel.
In that case how should i setup the TLC5940NT? Looking at the TI pdf it looks suspicious that the emitter goes directly to the ground. it says that using an NPN allows to do all the math based on the transistor, so it would actually be able to sink 500mA(with proper heatsink on NPN)? Vcc comes from 5v or 12v? and what resistor should i put at the base of the npn?