i suddenly remember i did some basic explenation a while back, did a quick search, maybe this will help:
 viper_mp wrote: |
Sometimes thats all you need to do. It really depends on if you are running it for long or short time periods. I set up mine for short, so its just two 12v LEDS hooked to a single 9v. Gives it a little more run time.
| |
im afraid i dont agree.. the problem is not the voltage, but the Amps.
a normal led on full intensity pulls something like 20 Amperes, so 2 leds: 40 Amps. now check your battery to see howmuch amps its delivering (depends on battery) obviously, it will give more than 40 amps, the voltage is not that important. You need to take away the rest of the amps so the leds will only get 20 amp, otherwise they will burn out way too soon... (the wattage will be too high in leds..)
you can compare it with a river:
the width of the river: VOLTAGE (U)
the quantity of water streaming through the river: AMPERE (I)
controlling the quantity of water (like a dam): RESISTANCE (R)
energy consumed by components: WATTS.
With a lot of water in a small river, the water will stream fast and hard (low voltage lots of amps, meaning preasure, stuff gets really hot..)
With a very wide river, but only a glass of water, no preasure at all.
What you want to do, is create a river wide enough to let enough water through to power your components, and than create a dam, so you control how much water is entering. Obviously the width of the river controls the amount of water that CAN be put through, so the resistance depends on this.The minimum width of your river depends on the led, normally they require 2-3 Volts.
how? again: U=IxR
it will give you 100.000 hours of life for a led if you screw down the amps, and since resistors dont cost anything really, imho its really worth wile protecting the leds.
so in your case you have 2 leds:
2x2Volts = 4 volts (so a 5 volt battery will do) 5 volt - 4 volts = 1 volt too many..
so u=ixr => 1=20xR ==> R=1/20 ==> 50 Ohm. (next high value in E12 series will do nicely)
in your case you have 9 volts:
2 leds = 4 volts ==> 9 volt - 4 volts = 5 volts too many:
u=ixr ==> 5=20xR ==> R = 5/20 = 250 Ohm
next resistor from E12 series in your case: 270 Ohms.
Hope its a bit more clear now, and you see why you really DO need to hook up resistors to a led, nomatter the voltage.
ps: if you dont trust my or your own calculations, or if you are just too lazy for it, google for: resistor calculator, lots of them! just give the power supply, nr of leds et voila, you have your resistor... :)
gr. S.