LED's on a car

[jpatricks1]

Hello everyone. I wanted to replace some of the lights in my car's
cabin with LED's. I want to know what value resistor I would need if
for example I would use 2, 4 or 6 LED's.

I was playing with some LED's today (12volts+ different resistors) and
for some reason when I use 4 LED's one of them wouldn't be as bright
as the other ones even though they were exactly the same. Can anyone
tell me why?

Thanks a lot!

 

[jpatricks1]

The specs of the LED's I got are here: http://www.superbrightleds.com/specs/WW7035_specs.htm

I put them in a series connection and 1 of the 4 didn't shine as
brightly as the others. Anyway I will use a parallel connection when I
put it in the car.

Would you try to explain it more please? I'm just an amateur hobbyist
and I'm still googling "rated forward current"

Thank you very much for the help John!

 

[John Fields]

On Tue, 19 Aug 2008 08:55:39 -0700 (PDT), jpatricks1
<jpatricks1@gmail.com> wrote:

The specs of the LED's I got are here: http://www.superbrightleds.com/specs/WW7035_specs.htm

I put them in a series connection and 1 of the 4 didn't shine as
brightly as the others.

---
Since the luminous output is only given as "typical", then that LED is
just putting out less light than the others. Probably substandard,
but with no minimum stated there's nothing you can do about it.
---

Anyway I will use a parallel connection when I put it in the car.

---
Don't do that, do this: (View in a non-proportional font)

+12>---+------+------+------+
| | | |
[620] [620] [620] [620]
| | | |
[LED] [LED] [LED] [LED]
|K |K |K |K
GND>---+------+------+------+
---

Would you try to explain it more please? I'm just an amateur hobbyist
and I'm still googling "rated forward current"

---
Google "LED tutorial" for some good hits.

On the data sheet, "IF" is the current which is allowed through the
LED in the forward direction and makes the LED light up. It also
heats up the LED, and if you allow much more than 30 mA through the
diode it'll shorten its life. Often, drastically.
---

Thank you very much for the help John!

---
My pleasure!

JF

 

[Jamie]

jpatricks1 wrote:

Hello everyone. I wanted to replace some of the lights in my car's
cabin with LED's. I want to know what value resistor I would need if
for example I would use 2, 4 or 6 LED's.

I was playing with some LED's today (12volts+ different resistors) and
for some reason when I use 4 LED's one of them wouldn't be as bright
as the other ones even though they were exactly the same. Can anyone
tell me why?

Thanks a lot!
are you placing them in series?

If you do not want to place them in parallel with
a single drive resistor if that is what you're doing?

http://webpages.charter.net/jamie_5"

 

[John Fields]

On Tue, 19 Aug 2008 06:00:00 -0700 (PDT), jpatricks1
<jpatricks1@gmail.com> wrote:

Hello everyone. I wanted to replace some of the lights in my car's
cabin with LED's. I want to know what value resistor I would need if
for example I would use 2, 4 or 6 LED's.

---
You take the supply voltage, subtract from it the LED's specified
forward voltage and divide that difference by the LED's rated forward
current.

The quotient will be the resistance required.

Then, in order to determine how much power the resistor will
dissipate, multiply the voltage dropped across the resistor (the
difference between the supply voltage and the LED's forward voltage)
by the LED's forward current.

The product will be the power the resistor will dissipate, and you
should use a resistor which can handle at least about twice that power
in order to keep it relatively cool.
---

I was playing with some LED's today (12volts+ different resistors) and
for some reason when I use 4 LED's one of them wouldn't be as bright
as the other ones even though they were exactly the same. Can anyone
tell me why?

---
How did you have the 4 LEDs connected?

JF