# teach me how to calculate resistor value on transistor b,c,e

#### paldep

Joined Mar 3, 2016
41
Hi guys. I build a led blinking circuit. It works very well. I want to use a 5v powersource. Can you teach me how to reduce the resistor value about transistor? This is the circuit.

#### blocco a spirale

Joined Jun 18, 2008
1,546
You only need to reduce the value of the resistor in series with the LED. The value depends on the forward voltage and current of the LED but typically you will need a value of around 1/3 or 1/2 of whatever you have at the moment, it's not critical.

#### paldep

Joined Mar 3, 2016
41
I tried using a 10 ohm and even without resistor the light is very poor. I tried to bypass R1 with a jumper and light is very shiny. But i need the right value. So a Mathematical formula will be appreciated.

#### tracecom

Joined Apr 16, 2010
3,944
What color is the LED and what is the value of the resistor (R3?) connected to the LED?

#### paldep

Joined Mar 3, 2016
41
The led is red and the resistor is 100 ohm. With a blue led the result is the same.

#### tracecom

Joined Apr 16, 2010
3,944
The resistor that is needed in series with an LED depends on the supply voltage, the voltage drop across the LED, and the current desired through the LED. A common red LED has a voltage drop of about 2V and has a maximum current of 20 mA; note that these numbers vary with the choice of LEDs. When you know the particulars of your LED, you can modify the numbers to agree with your LED.

So the value of R3 is calculated as follows. Subtract the voltage drop across the LED from the supply voltage, i.e., 5 - 2 = 3. Then, use ohms law (E=IR) to calculate the resistance required, i.e., 3 = .02R. Thus R = 150 ohms.

Now, if your LED has the same specifications as just described, you are overdriving it by using a 100 ohm resistor for R3 and may have damaged it.

#### paldep

Joined Mar 3, 2016
41
It's not damaged. I think the problem is in R. Maybe its Too big For my audio output. The audio comes from a 3w Class d anplifier pam8403"and goes in the trimmer pot 100 ohm

#### tracecom

Joined Apr 16, 2010
3,944
You said in the first post that you wanted to use a 5V power source (instead of 12V.) I have described how to calculate the value of the resistor (R3?) in series with the LED.

Now, you ask about another resistor value, but you don't say which one. If you want help, you need to be specific with your questions and provide details all together instead of adding information a little at a time.

#### bertus

Joined Apr 5, 2008
22,215
Hello,

Do you really use the BDX53?
This is a 60 Watt power darlington.
For this circuit a 2N2222 or BC547 already would work.
These are simple low power transistors.

Bertus

#### paldep

Joined Mar 3, 2016
41
It's all in my first post! "Can you teach me how to reduce the resistor value about transistor?" Sorry anyways
Hi guys. I build a led blinking circuit. It works very well. I want to use a 5v powersource. Can you teach me how to reduce the resistor value about transistor? This is the circuit.View attachment 102339

#### paldep

Joined Mar 3, 2016
41
I'am using a tip31c

#### bertus

Joined Apr 5, 2008
22,215
Hello,

The TIP31 is a single power transistor.
The schematic shows a darlington transistor.
The led will be turned on earlier as in the original situation.
The potentiometer will adjust the sensitivity.
A red led will have a voltage drop of about 2 Volts.
The transistor will have a saturation voltage of about 0.4 Volts (depending on the base current).
The resistor will get 5 - (2 + 0.4) = 2.6 volts across it.
When you want a current of 20 mA through the led, the resistor should be 2.6 Volts / 20 mA = 130 Ohms.

Bertus

#### tracecom

Joined Apr 16, 2010
3,944
It's all in my first post! "Can you teach me how to reduce the resistor value about transistor?" Sorry anyways
And you still haven't said which resistor. Good luck.