What size resistor would I use to convert a 24v LED to 12v?

Answer 1

i assumed that you have a circuit which power supply is 24 volts and have a LED connected by means of a resistance to that power supply, and you want to know what resistance you need to use if you change the power supply to 12 volts, right?

The voltage drop in the LED is approx. 2 volts and the current needed 20 mA so:

R=Voltage / Current = V / I

R=(12-2)Volt /0.02Amp= 10 / 0.02 = 500 ohms

the nearest standard value is 470 ohms which you can use.

Answer 2

it doesn't work that way - what you need to do is to drive the 24V LED with an amplifier powered from a 12V source - you can drive a 12V LED with a resistor and a 24V source but not the other way around - you need to boost the voltage to at least that required by the LED -

or you could get a power regulator and up-convert the 12 V to 24 V - [[EDIT: I just looked through my parts catalog and couldn't find any reasonably priced 12 to 24 up-converters - it's probably best just to get a new power supply if you really want to drive a 24 V LED]]

I would probably go the second route myself

[[EDIT: are you sure it's a 24 V LED - how much current does it draw @ 24V - that'll help me engineer a solution]]

Answer 3

If the LED is made to run on 24V and you only have 12V you don't want a resistor in series to lower the voltage available to the LED - it is already way too low.

Maybe it was a 4V LED. If it is a 4V and draws 5mA you want to lose 12-4 = 8V while drawing 5mA. Using Ohm's law R=E/I
you get R=8/.005 = 1600 Ohms. To complete the specifications the power (W=Ex I) for that resistor = 8 x .005 = .04 Watts so even a 1/8 Watt resistor would be OK.

Answer 4

Recheck the LED, typically LED's are rated around 2VDC and 20 mA and you just calculate the dropping resistor for your circuit voltage.

http://www.theledlight.com/LED101.html


I used 'dropping resistor' as the more or less slang term in electronics. No it doesn't change the circuit voltage, more technically it's a current limiting resistor so the higher voltage/current doesn't blow the low voltage/current semiconductor LED...

See the following: "Why do you need a resistor with an LED? The short answer: to limit the current in the LED to a safe value."

http://led.linear1.org/why-do-i-need-a-resistor-with-an-led/

Answer 5

Resistors DON'T Change Voltage.

You can have apparent voltage drops checking with a meter, but a 12 volt system is always a twelve volt system. Transforming the voltage is the only way to change it.