# how to calculate the base resistor

• posted

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.

• posted

When the NPN transistor is saturated (fully on), the emitter-collector voltage will be about 0.2-0.3 volts. Using the other values you have, you can calculate Ic; the current flowing through the collector (and the relay).

To saturate the transistor, the base current must be greater than Ic / beta where "beta" is the minimum current gain of the transistor.

The last piece of information you need is that the base-emitter voltage is one "diode-drop"; about 0.7 volts.

• posted

It's customary to overdrive the base. I normally use a factor of

1. It's also customary to put a reverse diode across the coil so that the turn-off transient doesn't overvoltage the collector.
• posted

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

• posted

Thanks all your suggestion are well taken

• posted

Beta is reduced near saturation. 10x is an attempt to compensate.

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