On Sunday, 8 October 2017 19:53:50 UTC+1, wrote:

please can anyone show me how to calculate the base resistor for an NPN transistor used as a switch. the supply voltage is 12 volts, the load is a 12 volts relay with a resistance of 300 Ohms.
thank you.

12v 300ohms is 40mA.
Assume beta of at least 30, so we need 40/30mA into the base = 1.3mA
V across base R is 12v - 0.7v = 11.3v
R=V/I so R = 11.3/1.3m = 8.5k. Preferred values below that to give a bit more i are 8k2 or 6k8 or 4k7.


NT