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

There will be a few variables. I am assuming an NPN setup where there is a resistor in series with LED between V+ and collector. Emitter to ground. And the base used to switch (edit: michaelkellett's schematic above).

Step 1: output side
you need to know

• the led current, from the LED datasheet.
• the voltage over the LED for that current
• VCE for your transistor, at saturation, for the LED current
• you need to know V+

Then you can calculate the resistor:

• V+ may be 5V. Your source.
• The voltage over the LED and resistor will be (V+) - (VCEsat). Maybe 90 mV for a BC547 in saturation
• The desired current for a good LED light. Maybe 12 mA for a red led?
• The voltage over the led, VLED, is the one from the datasheet. Maybe 2V for a red led?

Now you know how to dimension the resistor, because:

• you want 12 mA = 0 .012 A
• you want the resistor to drop: (V+) - (VCEsat) - VLED) =  5V - 0.09V - 2V = 2.91V
• you use Ohms law: R = U / I = 2.91 V / 0.012 A = 242 Ohm.

Don't look for a resistor with the exact value. This isn't high precision electronics. Look for the cheapest one that's close to this.
If you're at the upper range of the LED current, then use a resistor greater than the calculated value. To avoid exceeding the LEDs limits.

Then, you can calculate how hard you need to drive the basis to get the transistor saturated at 12 mA ...

edit : that is step 10 in michaelkellett's reply above

There will be a few variables. I am assuming an NPN setup where there is a resistor in series with LED between V+ and collector. Emitter to ground. And the base used to switch (edit: michaelkellett's schematic above).

Step 1: output side
you need to know

• the led current, from the LED datasheet.
• the voltage over the LED for that current
• VCE for your transistor, at saturation, for the LED current
• you need to know V+

Then you can calculate the resistor:

• V+ may be 5V. Your source.
• The voltage over the LED and resistor will be (V+) - (VCEsat). Maybe 90 mV for a BC547 in saturation
• The desired current for a good LED light. Maybe 12 mA for a red led?
• The voltage over the led, VLED, is the one from the datasheet. Maybe 2V for a red led?

Now you know how to dimension the resistor, because:

• you want 12 mA = 0 .012 A
• you want the resistor to drop: (V+) - (VCEsat) - VLED) =  5V - 0.09V - 2V = 2.91V
• you use Ohms law: R = U / I = 2.91 V / 0.012 A = 242 Ohm.

Don't look for a resistor with the exact value. This isn't high precision electronics. Look for the cheapest one that's close to this.
If you're at the upper range of the LED current, then use a resistor greater than the calculated value. To avoid exceeding the LEDs limits.

Then, you can calculate how hard you need to drive the basis to get the transistor saturated at 12 mA ...

edit : that is step 10 in michaelkellett's reply above