If you don't have the led
's yet, try Ebay, you get a much better price (11$ for a hundred, or 6$ or so for 50) and they come with resistors rated for 12VDC... most eletronics shops try to gouge you for 2$/led
The value of resistor depends on the DC
adapter voltage. the coles notes breakdown for the average value can be calculated by.
(InputVoltage-.7)/.03 = Resistor value.
So, assume 12 volts in.
12 -.7 = 11.3 / .03 = 376Ohms , or the nearest common resistor which is greater (probably 380Ohm)
Most 12v adapters put out 13.8v, so putting it through the works it is 470Ohms ~
The resistors colour codes are
0 - black
1 - brown
2 - red
3 - orange
4 - yellow
5 - green
6 - blue
7 - violet
8 - grey
9 - white
The breakdown of the bands are
moving towards the gold/silver band (tolerance, gold = 5%, silver = 10%)
first band = first digit
second band = second digit
third band = multiplier (unless a 4th band is present, in which case, this is the 3rd digit, the next is the multiplier. But typically you won't see this)
So for example, the resistors that came with my LED
's rated for 13.8v are
yellow violet brown gold
4 - 7 - 1 - 5%
using what was said above.
47 * 10^1 = 470Ohms at 5% tolerance (+/- 5%)
The ones in this example are
red red brown gold
or 220 Ohms.
And the adapter he was probably using is a 6v DC
Hope it's not too winded, I just think it's better for people to know how to calculate the right resistor rather then assuming the same resistance is used for every transformer.
For those that care beyond that where I got those numbers..
.7 is the voltage that the LED
.03 is the maximum current that can travel to the LED
(.03a or 30mA)
the formula is basic ohms law. *Voltage divided by current = resistance*, resistance * current = Voltage and Voltage / Resistance = Current
You subtract the .7 volts from the source voltage because this is the voltage drop across the LED
. The remainder must be dropped across the resistor that must have 30ma travelling through it. Knowing this, you can divide the remaining voltage you need to drop by the 30mA you need to flow through to get the resistance. This formula is fundemental to electronics.
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