# R calculation in attached circuit - help needed

Discussion in 'The Projects Forum' started by mishra87, Sep 10, 2016.

1. ### mishra87 Thread Starter Member

Jan 17, 2016
305
3
Hi All,

please find the attached schematic and let me know how to calculate the R vale for below parameter.

Les say Q119 =Q120 =BC547
Vf = 1.9- 2.4V
IF =15-20 mA

How do calculate R value !!!

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2. ### mishra87 Thread Starter Member

Jan 17, 2016
305
3
Comment awaited still in this particular post...!!!

3. ### AlbertHall AAC Fanatic!

Jun 4, 2014
7,399
1,790
First, unless you select matched transistors the currents in the two strings will be different. It would be much easier to dispense with the transistors and use two resistors.

Maximum voltage across each string of LEDs is 5 * 2.4V = 12V! So you don't have enough suply voltage to feed 5 LEDs in series and leave enough headroom for the voltage drop across the resistor. Note that this problem is even worse if you use the transistors as you lose 0.7V from the supply because of the Vbe of the transistor.

Minimum voltage across each string of LEDs is 5 * 1.9V = 9.5V
To restrict the current to a maximum of 20mA, we calcuulate at the maximum voltage across the resistor 12V - 9.5V = 2.5V
The value of the resistor is then 2.5V / 20mA = 125Ω
Now we can calculate the voltage available for the LEDs when the current is 15mA,
12V - 125Ω * 15mA = 10.125, so the voltage for each LED will be ≈2V

This value of resistor will safely limit the current to 20mA but note that if the LEDs need 2.4V they may not be very bright.

Jun 4, 2014
7,399
1,790