Here is a drawing of Two Circuits that you can Tackle your problem:To Calculate your resistor value you need just use Ohm's law and a bit of algebra:
We know The supply voltage you have is
9 VoltsWe know The LEDs require
3.3 Volts.But we Need to know the Voltage drop across the LED if it doesn't use a resistor.
So to find this you just subtract:
Supply - vLED = Surplus Voltage9v - 3.3v = 5.7vThe Voltage Dropped across the LED
Never Changes because LEDs are Non-Linear Components. Resistors are Linear Devices and drop a proportional percentage of voltage upon the supply.
OK now we have to calculate the
percentage ratio of
3.3volts to
5.7volts.(smaller number on top)
3.3v / 5.7v = 0.579 (57.9%)
The resistor must drop 57.9% of the voltage.So just multiply the percentage to the supply...
0.579 * 9.0v = 5.21v============================================
Time to use Ohm's Law:
V = I * RPlug the numbers in the equation:
R = V / I
Resistance = Voltage To be Dropped / Current Through the Resistor
Resistance = 5.21 / .02AResistance =
260.5 OhmsOne Last Step. I promise.
:
Finding Power Consumption:
Ohm's Law
Power =
I *
VEach LED Consumes
20mA of current.You want to use
34 LEDs in paralell.
0.02 =
20mASo.
34 *
0.02A =
0.68ACurrent total =
680mAVoltage Supply =
9vPlug it in the equation:
P =
I *
VP =
.68 *
9Power =
6.12 WattsThat's One Huge Resistor!!!!!Normally Resistors come in 1Watt 1/2Watt and 1/4Watt Packages.
You must put 1 Resistor for each LED individually since the Power is so great; if you don't then you run the risk of smoking the Resistor possibly starting a fire.
I would Round this value down a bit no more than 50 Ohms.
I recommend 1/4 Watt 220 Ohm Resistors for your circuit. 1 Resistor for each LED.
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Remember, current isn't dropped across a resistor its dropped through a resistor. When resistors are put in series together they use the same current but drop different voltages, when resistors are paralleled they use different current and drop the same voltage.(assuming they are all the same value)
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Extra Info:
Resistors are Linear Components since their voltage drops' change as the voltage changes; resistors drop a percentage of the total supply.
The LED will drop 3.3volts in a 9volt circuit and it will drop 3.3volts in a 12volt circuit; a resistor will drop a proportional percentage of the voltage in that circuit.