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value of resistors to drive transistor as a switch
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dkeerthi170022



Joined: 04 Mar 2008
Posts: 8
Post05 Apr 2008 23:11   

value of resistors to drive transistor as a switch
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.

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mister_rf



Joined: 27 Feb 2008
Posts: 243
Helped: 59
Location: Romania
Post06 Apr 2008 1:00   

Re: value of resistors to drive transistor as a switch
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
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dkeerthi170022



Joined: 04 Mar 2008
Posts: 8
Post07 Apr 2008 22:04   

Re: value of resistors to drive transistor as a switch
i could not find the load current to saturate the transistor .could u pls help me out in this aspect.
thank u
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Audioguru



Joined: 19 Jan 2008
Posts: 1145
Helped: 95
Location: Toronto area of Canada
Post07 Apr 2008 23:19   

value of resistors to drive transistor as a switch
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?
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