Powering led's, 9v battery not enough.

Thread Starter

hashmaster

Joined May 30, 2012
42
Resistor for LED==(Supply Voltage - LED Vforward)/20 mA

Example:
(9 V - 2.71 V - 3.73 V)/20 mA= 128 Ohm (he rounded down to 120 Ohm)
Power for resistor == I*I*R == 20 mA*20 mA*120 Ohm=0.048 W (he round it up to 1/8 W which is 0.125 W for extra safety margin)
I see what u did there. Im used to doing source voltage/ desired current= resistanceto use. Calculating from his schematic I took the led forward voltage divided by 20mA to get a rough led resistance, added those resistance values together to get a rough led total resistance, subtracted that from the 470 ohm value and ended with about the same resistors used.
 

#12

Joined Nov 30, 2010
18,224
I wrote down all the LED voltages and played mix&match until I got the least number of strings that added up to less than 7 volts, then I did what shteii said, then I figured the drive resistor for 7 volts, rounded down, and wasted the excess drive current with a 4.7k resistor. Finally I got a copy of a circuit I did 2 days ago, erased, "PIC", 3 LEDs and changed the voltage labels.
 

Thread Starter

hashmaster

Joined May 30, 2012
42
Alright. Cool. Now the fun task of pulling everything off the breadboard and wiring it all up to the car body and a small board.
 
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