I have been doing much research to learn about transistors and to find the right resistor/transistor coupling to power a laser driver that I am building. I want to complete the circuit through the 5V/40mA output on an Arduino, as the base, to turn the laser on and off. The circuit will have between 0.2A and 1.25A current flowing through it to power the laser and that will either flow or be cut off by the transistor (the laser driver does not measure voltage, only current.) I want to use the NPN transistor as a switch so it will saturate completely.
Any help with the resistor and transistor I am looking for? I am having so much trouble reading through these data sheets. :/
Here is an image of the laser driver diagram for those who requested!
Best Answer
You are likely to be better off using a MOSFET than a BJT.
We typically assume a BJT will have on the order of 0.2 V between collector and emitter in saturation. With 1 A collector current, that means 200 mW of power dissipation in the transistor.
MOSFET's drain-source channels on the other hand act a lot more like low-value resistors when fully switched.
Going to On Semi, I found several dozen available FETs with channel resistance (rDS(on)) below 50 milliohms for gate voltage of 4.5 V and current handling capability above 1.5 A. With this level of on resistance, the power dissipation is reduced to ~50 mW.
Lower power dissipation means a smaller package, probably lower cost, more power available for other functions, and less worry about thermal problems with your laser.