Transistor as a switch

Thread Starter

jirakst

Joined Jun 14, 2014
3
Hello,

I've got a lot of homework and can't solve following example:

Transistor KFY34 has to switch a load with rated voltage 24V and resistance Rs=60Ω.
a)Solve size o the Ib after switching load.
b)Solve size of current and voltages in the circuit Ib = f(t), Ic= f(t), Ube = f(t) and Uce = f(t) after switching on and switching off.
c)Decide if additional cooling is needed.
KFY34: NPN, h21E = 35 až 125, UCB0 = 70V, UCER = 50V, ICmax = 500 mA, UCES ≈ 1,5V, PC = 800mW

Thanks for any guidance in advance.
 

crutschow

Joined Mar 14, 2008
34,281
We don't do homework for you but we will help. You need to show you work and what you don't understand. Post the circuit schematic to start.
 

Thread Starter

jirakst

Joined Jun 14, 2014
3
Oh yes, the emitter resistor is used to set the operating point of the transistor when connected as a amplifier. But do I actually need the base resistor or it has to be used with other then low-power sources (e.g. solar cell)?
How will I determinate the base current?
 

crutschow

Joined Mar 14, 2008
34,281
Oh yes, the emitter resistor is used to set the operating point of the transistor when connected as a amplifier. But do I actually need the base resistor or it has to be used with other then low-power sources (e.g. solar cell)?
How will I determinate the base current?
For a BJT switch the base current should be at least 1/10th of the maximum collector ON current to insure the transistor is fully turned on (saturated).
 
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