I have 3 12VDC/40A automotive relays (datasheet) that I want to use with my Arduino. Based on the tutorial I am following (link) I need transistor, resistor, and diode. I am not an electrical engineer therefore I am unsure about the parts and calculations I made.
For the start, relay coil resistance is 90+-10% Ohm per datasheet. So I proceed by calculating the current flow.
Ohm Current = 133mA
For the transistor I can get 2N3904 or 2N4401. At this point I have to calculate resistance for the base of transistor. In tutorial its as following
hfe = Ic / Ib
Ib = Ic / hfe
Ib = 0.03 A / 75 Ib = 0.0004 A => 0.4 mA
R1 = U / Ib
R1 = 5V / 0.0004 A
R1 = 12500 Ohm
2N3904 datasheet states the H(fe) is 30-300 when lc = 100mA (mine is 130mA) and Vce = 1V. At this point I have no clue what is going on, thus I need help.
Edit: Here is what I ended up with. RLY1 in the picture is 12VDC/40A (link)