Engineer's Asylum

How to choose the right NPN transistor for Switching?


#1

I want to switch a resistive load which need 5V and consumes about 280mA current using an Arduino for my electronics projects. I am a little weak in basic electronics. I know the circuit diagram for an NPN transistor with the load. But I don’t know how to choose the right transistor for my project. All I know is that collectror current and hfe are taken into account while choosing one.


#2

In most case, these two factors are only needed while choosing a transistor for switching.

  • Firstly, you have to calculate Ic = Vs/R ( Where R is the load Resistance ). In your case, you already know the current consumed by the load which 280mA. So you have to choose a transistor which is capable of delivering more than 280mA ( Ic > 280mA ).

  • Secondly, you have to consider the gain of the transistor or the hfe. hfe can be calculated by, hfe = Ic/Ib. Since you are using Arduino, the maximum current from a single pin is about 5mA. So hfe = 280/5 = 56. Choose hfe value ( > 56) in between 100 and 280 ( 5 times 56 ).

  • Thirdly, calculate the base resistor Rb = ( 5 - 0.7 )/ Ib

  • Also, have a clamp diode across the load if you are using an inductive load like a solenoid valve. It protects the transistor when the switch is turned off.


#3

Thanks for the valuable insights. Can you suggest any transistor for my above application? Its will be very helpful for me.