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led and resistor current question

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themikestar

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i am trying to run an led from a car, i obviously need a resistor to do this, at the moment i am using a 470ohm 1/4 watts resitor (this is running it about half it's brightness, this is bright enough), but i want to use a smd resistor. The problem is the LED needs atleast a 1/4 watt 470ohm resistor, the biggest smd resitor i can find is .125watt in australia. If i want to run the led at full brightness i need a 1/2 watt 220ohm resitor.

led specs- 3.8v 50ma
power source - car , around 13.5v



the question is, can i put two smd resitors in parallel so they will share the current ?,

i'm guessing if i wanted to run the led at full brightness i might have to put 4 in parallel, is this correct ?.

what would the values of the resistors have to be ?
 

Exo

Active Member
if you put two resistors in parallel then the total resistance will be

Rtotal = (R1 * R2) / (R1 + R2)

I would use 2 820ohm resistors in parallel, this equals one 410 ohm resistor, wich comes close enough to your 470 ohm

voltage across each resistor is VSource - VLed = 13.5 - 3.8 = 9.7Volt
Current trough each resistor is Vr / R = 0,011A = 11mA
Power on each resistor is Vr * Ir = 9.7 * 0,011 = 0,106 Watt
 

Roff

Well-Known Member
Exo said:
if you put two resistors in parallel then the total resistance will be

Rtotal = (R1 * R2) / (R1 + R2)

I would use 2 820ohm resistors in series, this equals one 410 ohm resistor, wich comes close enough to your 470 ohm

voltage across each resistor is VSource - VLed = 13.5 - 3.8 = 9.7Volt
Current trough each resistor is Vr / R = 0,011A = 11mA
Power on each resistor is Vr * Ir = 9.7 * 0,011 = 0,106 Watt
Exo, you obviously meant to say that the resistors have to be in parallel.

Digikey (and I would assume most other big distributors) have SMT resistors up to 1 watt. It is generally good practice to run resistors at about half their rated wattage (or less). Using this guideline, and assuming your battery is 13.8v, your LED drops 2v, and you want to use 220 ohms in series, you will need about eight to ten 1/8w resistors. You could use one 1w resistor, and they are only 6.4mm by 3.2mm.
 

Exo

Active Member
Ron H said:
Exo, you obviously meant to say that the resistors have to be in parallel.
Yes, of course, ... well, it's christmas, ppl get drunk you know :p
 
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