value of resistors to drive transistor as a switch

[dkeerthi170022]

if the voltage is 24v instead of 23v.wat should be value of the two resistors in the circuit so that the transistor acts as a switch(driven in to saturation mode).pls help me out.also give me value of current to drive tip120 transistor in ckt into saturation mode and formulas required to calculate current going in circuit.

 

[mister_rf]

Read the TIP120 datasheet.
You’ll need to have Ib=Ic/250 for low Uce.
So depending on the load current you’ll find the resistors values.
Ib=(Vcc-Vbe)/(Ra+Rb)
(Ra+Rb)=250(Vcc-Vbe)/Ic
for example
Ic=1A, Vcc=23V, Vbe=1.5V
Ra=Rb
Ra=250(23-1.5)/2*1=2.687K or the standard 2.7K

 

[dkeerthi170022]

i could not find the load current to saturate the transistor .could u pls help me out in this aspect.
thank u

 

[Audioguru]

The example given by Mr RF is for a load current of 1A or less.

The load current is caused by your load. What is the resistance of your load?