CMOS 555 driving BC337 - base resistor

[Cirkit]

Hello,

I am trying to determine the correct base resistor value to drive a BC337 transistor into saturation. The transistor is connected to the output of a CMOS 555 timer via the resistor. The transistor is driving a 3V solenoid which consumes around 200mA when active.

The entire circuit is powered from 3V.

With VBE = 1.2V and hFE = 60, I have chosen a 500 Ohm resistor. Is this correct?

 

[Harald Kapp]

With VBE = 1.2V and hFE = 60, I have chosen a 500 Ohm resistor. Is this correct?

It depends on the operating voltage (or the high output voltage, respectively) of the 555. hFE = 60 means Ib ~ 3.5mA. The required value for Rb is determined by:
Rb = (Vout_high - Vbe)/Ib

 

[Cirkit]

The 555 is operating in monostable mode and the high output voltage is around 3V.

 

[Harald Kapp]

Then 500 Ω is a good choice.