I want to control a 12V relay with coil current of 37.5mA using a 2n2222 transistor and arduino pro mini 3.3V. How can I compute the base resistor considering the worst case scenario?
2 to 4 mA of base current should be enough for 37mA collector current.
Ohms law to get base resistor value.
Always measure Vce to confirm saturation.
0.3V is saturation voltage when IC = 150 mAdc, IB = 15 mAdc as per data sheet
it wont be that precise! just assume 40mA.
you can pretty much scale the current needed;
IB (Ic = 40mA) =( IB(150mA) = 15mA) * 40 / 150
IB (Ic = 40mA) = 4mA
so you have 3.3V - 0.7V = 2.5V 4mA = 625 ohms.
using common values choose 470, 560 or 620.
There are saturation graphs you can look at in the data sheet, interpolate for 37mA collector current.
Rule of thumb 37mA ÷ 10 = 3.7mA
(3.3v - .7v) ÷ 4mA = 650 Ω as John has shown too.
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