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