It seems the more I read about transistors the more confused I get. I think the most frustrating thing is how the datasheets can be a little overwhelming.
I figure I’ll explain what I’m trying to do and what I think I need to make it work, and if you guys can assist me on going the right path, I’d really appreciate it.
I have a couple RGB led strips that I want to connect to 3 pins of the arduino. Below is the current draw for just one led bar:
- Red: 100mA @ 12v
- Green: 80mA @ 12v
- Blue: 87mA @ 12v
So now I need 3 transistors. I start by finding Vload and Iload(max). The Vload on the led bars is 12v and for the Iload(max) I doubled red’s current draw for the two bars and added 100mA for safety. I got Iload(max) = 300mA.
I chose the PN2222 NPN transistor(datasheet) because the Ic(max) of 1A and the Vceo of 40V seems perfect and so far I think I’m on the right track.
The next step is to calculate the base current needed for saturation. On figure two of the datasheet I find ß=10 and assume this means that for an Ic of 300mA or lower I need a base current of Ic/Ib = 10 or Ib = 30mA.
At this point I think I’m doing alright. Then comes the choosing of the base resistor and I become unsure of myself.
One source says to look at Vbe(sat) which is in figure three of the datasheet. I see 3 lines, and I choose the line marked 25°. I try to guess where 300mA would be on the chart and it looks like it’s above 0.9V but below 1.0V, so I (probably incorrectly) assume 0.9V and do the equasion:
Arduino is outputting 5V, so the resistor has a voltage drop of (5V - 0.9) or 4.1V across it. Ohm’s law gives me V/I = 4.1/(30mA) = 136 Ohms. I should choose a 150 Ohm base resistor.
The other source I found says to use this equasion: V/Ib with a 0.7v drop across the base junction. So, V/Ib = (5V-0.75V)/30mA = 141 Ohm. I should choose a 150 Ohm resistor.
Did I do this right? Should I have done something different?