Turning on LED panel with a NPN transistor [duplicate]

Question

I have a NPN transistor (2N3904), I would like to be able to turn on my LED panel - 37v (don't know the amps) with my Arduino Uno, which gives output of 5v, 0.2 amps.

As far as I understood, the Arduino will be connected to the base of the NPN transistor. if I put a diode or a resistor between the Arduino and the NPN transistor's base, following the tutorials it "should" work, but I don't want to burn my Arduino down.

I don't understand, if the transistor is a 'switch', when I give electricity to the base, why is there electricity coming into the base from the collector? shouldn't it go straight to the emitter? how can I know how big of a resistance to put, between the Arduino and the base of the transistor? In addition, should do 'GRD' from Arduino go to the LED's negative terminal? Thanks

Answer 1

IF your LED Panel draws a good bit less than 200mA (say 100mA~150mA), I'd suggest the following:

1 - As per datasheet 2n3904 hFe is minimal 30 (in practice more). In Ib = Ic/hfe, we get:

Ib = 150mA/30 = 5mA

2 - VBE max at closest match (50mA) is 0.95V, say 1V (in practice less).

So 5V - Vbe = Rb * Ib, gives ~4V = Rb * 0.005mA

Rb = 800R

3 - Schematic:

^{simulate this circuit – Schematic created using CircuitLab}

Note that you get uncomfortably close to 2n3904 limits: Vce 40V, Current 200mA, etc..

So I'd actually suggest the following:

^{simulate this circuit}

I just used this particular mosfet because its available at Sparkfun and gives better margins (60V, 30A). You can use anything that suits your needs. (R2 optional)