how to calculate the base resistor

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.

Reply to
fynnashba
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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.

Reply to
natp

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.
Reply to
mike

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

Reply to
tabbypurr

Thanks all your suggestion are well taken

Reply to
fynnashba

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

Reply to
Clifford Heath

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