Transistor

You will need to provide a lot more information to get a sensible answer.

What are you trying to do ?

To know the actual value of resistor for construction of a transistor as to prevent it from burning out by a high current or voltage

That would depend on the transistor, the load, the application and  a whole load more things.

When I said explain what you are trying to do I meant like:

turning an LED on and off

controling a room heater

Transistors come in thousands of different types , capable of handling currents from a few nA to a few thousand amps - to calculate the compoents in a circuit then first you need to knwo what it supposed to do.

MK

Turning on and off an LED 

The transistor am using is NPN 

I need the formula of getting it right, how to know for instance that either 1 kilo ohm resistor is to be used

1 Decide the current you want in the LED (check the data sheet) call it Iled
2 Choose the power supply voltage V1
3 Choose the control voltage to switch the transistor on, V2
4 If the transistor is suitable and switched on the voltage on its collector will be low so the voltage across R1 and D1 will be V1
5 Look up the forward voltage of the LED (Vled) in the data sheet
6 The current through R1 will be (V1 - Vled)/R1, so Iled = (V1 - Vled)/R1
7 R1 = (V1 - Vled)/Iled
8 if V1 = 5V, Iled = 10mA, Vled = 1.2V then R1 = (5 - 1.1)/0.01 = 390R
10 For the transistor to be really well turned on the base current should be at least collector current/25
11 In our example base current = 10mA/25 = 0.4mA
12 So R2 = (V2 -0.6)/0.4mA
13 if V2 = 5V then R2 = 11k

When you build this, please send a picture so we can see it working, it's no fun answering questions if you don't get feedback !

MK

1 Decide the current you want in the LED (check the data sheet) call it Iled
2 Choose the power supply voltage V1
3 Choose the control voltage to switch the transistor on, V2
4 If the transistor is suitable and switched on the voltage on its collector will be low so the voltage across R1 and D1 will be V1
5 Look up the forward voltage of the LED (Vled) in the data sheet
6 The current through R1 will be (V1 - Vled)/R1, so Iled = (V1 - Vled)/R1
7 R1 = (V1 - Vled)/Iled
8 if V1 = 5V, Iled = 10mA, Vled = 1.2V then R1 = (5 - 1.1)/0.01 = 390R
10 For the transistor to be really well turned on the base current should be at least collector current/25
11 In our example base current = 10mA/25 = 0.4mA
12 So R2 = (V2 -0.6)/0.4mA
13 if V2 = 5V then R2 = 11k

When you build this, please send a picture so we can see it working, it's no fun answering questions if you don't get feedback !

MK

Thank u.

I will send a picture of the work