How to determine R at base?

Determine R

Any formula to determine R at base

Re: Determine R

It seems that the LED is placed in wrong direction. Please tell me what is the marked part in your circuit:

Re: Determine R

like leotim said you have to reverse the connections of the LED for the circuit to work

to calculate R:

from your diagram

beta = 100
IC = Iled = 100mA
beta = Ic / Ib

to find R first we need to find Ib

Ib = Ic / beta
Ib = 100mA / 100 = 1mA
Usually Vbe of NPN = 0.65
so Vr = 5 - 0.65 = 4.35V
R = V / I = 4.35 / 1mA = 4350

A 4.7K will do....

Re: Determine R

leotim said:

It seems that the LED is placed in wrong direction. Please tell me what is the marked part in your circuit:

Click to expand...

Logic And Gate( operate as mixer )

Re: Determine R

The 38 does not allow the LED to have 100mA current. If we suppose the voltage across the LED to be about 2 volts (it is probably more) and the transistor saturation voltage to be 0.2 volts, then remains 5-2-0.2=2.8 V for 38 Ohms resistor. so it can pass 2.8/38=73.7 mA current at most.

You must decrease this resistor (use 27 Ohms).

Re: Determine R

leotim said:

The 38 does not allow the LED to have 100mA current. If we suppose the voltage across the LED to be about 2 volts (it is probably more) and the transistor saturation voltage to be 0.2 volts, then remains 5-2-0.2=2.8 V for 38 Ohms resistor. so it can pass 2.8/38=73.7 mA current at most.

You must decrease this resistor (use 27 Ohms).

Click to expand...

Actually, the circuit is apply for IR transmitting and valid light LED.
I though Vdrop = Vcc - Vled = 5 - 1.2 = 3.8
then R=vdrop/Iled= 3.8 /100m =38 ohms?

Added after 6 minutes:

tajrin said:

like leotim said you have to reverse the connections of the LED for the circuit to work

to calculate R:

from your diagram

beta = 100
IC = Iled = 100mA
beta = Ic / Ib

to find R first we need to find Ib

Ib = Ic / beta
Ib = 100mA / 100 = 1mA
Usually Vbe of NPN = 0.65
so Vr = 5 - 0.65 = 4.35V
R = V / I = 4.35 / 1mA = 4350

A 4.7K will do....

Click to expand...

But i trying use multisim to simulate the circuit.
R below 4k, the Transistor is working and above 4k( doesn't work).

Re: Determine R

By now, the LED current is limited by 38Ohms resistor (what ever R value be). so you should just choose an R value to turn the transistor ON. your AND gate may have an iutput curren limit. if so it is not needed to use any R, place a wire (short circuit) instead of R. If you prefer to use an R (to limit the AND gate output current) you can use simply a value much less that what tajrin have calculated (4350 Ohms), for example use a 1KOhms.