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# Newbie Help

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#### eggoz

##### New Member
I need help buying a simple circuit. What I need to do is simple, I need to power 16 white LED's and 8 blue LED's. I tried building a circuit from somebody else and well, it didn't work. **broken link removed** This what I was aiming for, except splitting the amount of lights in half. I could of just messed up the circuit board when I was building it. (It was my first one). Could somebody help me build this circuit. Never done this before. Thanks.

The circuit is extremly simple. But the fault lies in the circuit diagram. The guy has drawn wrong circuit diagram. Reverse polarities of all the LEDs and you'll make it work.
Also there is no need of dual 12V supply as it is shown. Use only one. Now that you have already built the circuit, short both the 12V wires and connect it to Ground Line and Connect your ground line to +12V line. This will have the same effect as if you have reversed all the LEDs.

I'll have to redo the circuit board. Also, just out of curisority, if I connected 2 100 ohm to an LED, would the voltage drop to around 5 volts, if I am using a 12 volt source? I am just making sure he is using the correct resistors. I have no more spare LED's left, blew them all.

To calculate the series resistor value use the blwoe given forumla.

R = (Vs - Vf)/If

R = Resistor Value
Vs = Suppy voltage
Vf = Forward Voltage Drop of LED
If = Forward Current of LED

If I assume that the Blue LED has Vf = 4V and If = 20mA then
R = (12 - 4)/20E-3
R = 400 Ohms
R = 390 Ohms (Standard)

Thanks. That helps a lot. What does the 20E-3 mean?

20E-3 = 20 x 10^-3 = 20mA

duh. thanks.

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