Hi everyone, just need some quick advice.

I want to use the BC337 transistor as a switch to control an arduino lilypad twinkle via an output of a lilypad MP3 player.

With 4 LEDs being driven by the twinkle the max Ic is around 100mA.

What I'm not sure about is what gain value I use to calculate the base resistor. The datasheet says min is 100 on the data table but one of the graphs shows a curve when Ic/Ib = 10.

Which one do I use?

 

If you are using the transistor as a switch assume a gain of ten to make sure the transistor is saturated so you get the minimum voltage across the 'switch'. This also keeps the power loss in the transistor to a minimum.

 

Gain will change with collector current and which BC337 you have, At 100 mA gain will be 100 minimum on most types.BC337-16 is different than BC337-25 or -40.

 

The datasheet says min is 100 on the data table but one of the graphs shows a curve when Ic/Ib = 10.

100 is for operation in active mode. When operating in saturation mode, many designers use 10. That's what you're seeing in the graph you mention. Some use 20, but I was taught to use 10.

No one will use the stated beta value of 100 for saturation mode operation. Note that that value is specified for a Vce significantly higher than the saturation voltage. In addition to being dependent on collector current, beta also has a dependency on collector emitter voltage.

 

For minimum saturation voltage, when using a BJT as a switch, a current gain of 10 is typically used.
If you want to minimize the base current, and a possibly slightly higher saturation is acceptable, then you could use a gain of 20 to calculate the required base resistor.
But I would never use a gain value higher than 20 for good design practice.

 

Hi everyone, just need some quick advice.

I want to use the BC337 transistor as a switch to control an arduino lilypad twinkle via an output of a lilypad MP3 player.

With 4 LEDs being driven by the twinkle the max Ic is around 100mA.

What I'm not sure about is what gain value I use to calculate the base resistor. The datasheet says min is 100 on the data table but one of the graphs shows a curve when Ic/Ib = 10.

Which one do I use?

A logic level MOSFET in the same TO92 package wouldn't need any base resistor.

 

thanks everyone. that helped and my circuit worked first time.