24 volts to 9 volts

Raw leds! About two blue ones! Can't remember the specs on it off hand!commit your way to jehovah and he will act in your behalf. psalms 37:5

What i don't understand is how i'm coming up with the value of the resistor! Ok assuming the leds are 5watts each what would be the value of the resistor! Thanks for being patient i know i can get irritating! show me how you calculate it and i shouldn't have to be a bother to you again!commit your way to jehovah and he will act in your behalf. psalms 37:5

Do you have a link for the led's you purchased?  All that is needed from the spec sheet is their Vf...forward voltage and If..forward current.  Blue led's typically have a Vf of around 4 volts.  Let us know what you have and we'll go from there.

m

Thanks KP for the night shift. LEDs are not 5W unless they are for lighting (torches, headlights etc). If they are typical "bullet head" LEDs they are probably 20mA. Hence 0.02A x 2V or 4V = 40mW or 80mW. A 5W LED would require a 10W dropping resistor (24V to 9V). Find their current rating. The the resistance required is easy... V=IR hence R = V/I (Volts, Amps, Ohms; or Volts, mA & k-Ohms) Where V is the voltage drop required & I is the current through it. EG, 20mA blue 4V LED on 24V; R = (24-4)/.02 = 20/.02 = 1,000 Ohm = 1kOhm (1kΩ). If 2 blues in series, drop 24-4-4V = 16V, hence 16/.02 = 800 => 820Ω. (In practice, probably the 1kΩ would also be ok.) If 9V@20mA, R = 9/.02 = 450Ω => 470Ω. The power dissipated by each resistor is P=VI (Watts = the Voltage across it x the current Amp through it). EG - for 1kΩ at 20V/20mA, P = 20 x 0.02 = 0.4W hence a 0.5W (1/2 Watt) resistor (or bigger) is required. The others will consume less (same current, lower voltage). FYI shortcut: From V=IR & P=VI we get P=VV/R or IIR. EG - for 1kΩ at 20V/20mA; P = 20x20/1000 = .4W or P = .02 x .02 x 1000 = 0.4W.

Ahhhh! Thank you old spark! That's all i wanted! Thanks fellas for your time and info, it's appreciated!commit your way to jehovah and he will act in your behalf. psalms 37:5

Sorry - WRONG ANSWER! You are supposed to knock my long explanations. (LOL! I write for novices etc, not for those in the know.) And though you may have underspecified your OP, I (we?) should have checked before continuing... Yes - we know better, but when 9V is mentioned, from experience we assume some 9V device that requires regulated voltage... The key was "LEDS" (only). As to the V=IR bit, a search would have been quicker (excellent tuts & references on this site as well as elsewhere; and I find Wiki invaluable). But the "5W" LED assumption was one well suited to this forum. And I could have added that many ignore the LED voltage when calculating for 12V or more - ie, 2V has little impact; 4V may impact - but round resistors UP to then (next or next-next) preferred value, and LEDs are not that touchy anyway... But KIS... (?) Thanks for the thanks. And to the other repliers (good stuff; as usual).