#Bjt as a switch how to#

Calculate the value of base resistor (RB) : RB = (I/P – Vbe)/(Ibx3).Note down the base current (Ib), and the control input voltage (I/P) : (1mA and 5V).From the transistor datasheet, find the values Vbe (sat) and hFE : (0.77V and 100).Note that the base-emitter junction of a BJT behave likes a forward-biased diode with typical 0.65V drop across it. But to ensure that the transistor operates in the saturation region, usually the base current (Ib) is multiplied by a factor of three.

Here, formula for base resistance R B = (I/P – Vbe)/Ib. Since the general-purpose BC547 transistor has a minimum gain just above 100, base current required to saturate the transistor will be I B = Ic/hFE = 100mA/100 = 1mA. Assume that the 70mA relay may require 100mA to operate, hence Ic=100mA.

In principle, BJT is a current amplifier hence Ic = Ib x hFE. The parameter “hFE” denotes the DC current gain, and this is the vital parameter to consider while driving/switching loads. For this, refer the datasheet of BC547 transistor first and look at the values Vbe, Vce, and hFE. Since the transistor (TR) is a driver/switch here, suitable value of base resistance (RB) is required for conduction in the saturation region. The base resistor (RB) is crucial here to control the amount of current flowing into the base – the base current (Ib), which in turn controls the amount of current flowing through the collector – the collector current (Ic).

The BC547 have a maximum collector current (Ic) rating of 100mA, seems fine for the 70mA relay. A sensible choice here is the use of a BC547 NPN transistor as a low-side driver/switch as shown below. Say you wanted to drive a 5V/70mA rated electromagnetic relay by one I/O port of your microcontroller, and it can cater a maximum current of 20mA at 5V. Admit it, analog electronics is not a piece of butter cake for the most except a few willing scholars!

#Bjt as a switch how to#

Recently I got an unusual query from a newbie microcontroller hobbyist on how to drive an electromagnetic relay using a bipolar junction transistor (BJT).