hi i am planning to install two small white light LED on my car as parking lights.

so basically i am using the reflector along woth LEDs from a torch that uses 3 AA cells(4.5v), the power of LED in manual is 3.2 V and the 1/2or .5W, as we know

V=I*R which Implies I=V/R and R=V/I

and P=V*I, by following these equations i get 154ohm resistance to be used but i used two individual resistors of 60 ohms with each LED, so my question is will it work (well ofcourse its working as of now but for longer durations like 2-3 hours continous working and also with charging system engaged, i guess the power source will change from 12v to 13.5 V.

any help will be appreciated.

  • \$\begingroup\$ Does the datasheet not specify forward current? \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 18 '13 at 13:52
  • \$\begingroup\$ unfortunately no info on forward current is given! and sorry i got the wrong calculation page , it turns out the 154 ohm for resistance and the resistors used are 60 ohm each! \$\endgroup\$ – Paul Mann Aug 18 '13 at 14:00
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    \$\begingroup\$ Please provide a link to the datasheet. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 18 '13 at 14:00
  • \$\begingroup\$ Hi Ignacio, here is all the info: LED type 3.2v P=1/2W 40lumens ultra bright LED, so usinf these i get v=12v-3.2v=8.8v, and I=P/V putting values I =.5/8.8=.057, And R=V/I putting values R= 8.8/.057= 154.38 ohms. this is all the info hope it will help. \$\endgroup\$ – Paul Mann Aug 18 '13 at 14:15

If you have no data sheet information about the expected current of the LED (because you extracted it from an existing appliance, or something like that), then you should measure the voltage and the current while still in its original position and use those values for further calculations.

I do not recommend that you base your calculations on the nominal voltage and nominal wattage figures found in non-technical documents.


You're right in that voltage will vary from 13.5-14V when running/just after turning off the engine to 12V or less after some time. LEDs have a "negative differential resistance" which means that small increases in voltage can cause large increases in current and power. An LED which runs fine at 12V may already burn out at "only" 13V.

So to be on the safe side, do your calculations with 13.7 or even 14V. You will have reduced brightness when the engine is not running but the LEDs should survive longer.

(Standard yelling here: LEDs must be driven by a constant current source and not a constant voltage, much less an arbitrarily varying voltage, so use a proper LED driver.)


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