Hey guys, don't know if you missed my post here:

http://www.acura-tl.com/forum/showth...1&pagenumber=2

But I wanted to run this by you and make sure I'm right.

The LEDs are 3.5v and 10ma-25ma (low-max I guess) so:

(7.5v - 3.5v) / (20ma / 1000) = 200ohm

Right? Should I be using the 20ma number? I can buy 220 ohm SMT resistors so:

7.5 - (220 * (10ma / 1000)) = 5.3v
7.5 - (220 * (20ma / 1000)) = 3.1v
7.5 - (220 * (25ma / 1000)) = 2v

So I'd be running the LEDs at roughly 2v-5.3v! I'm confused... what milliampere rating should I be using for my equation and what resistor should I buy?

I'm going to buy the resistors in 2 hours and should have the radio finished tonight, so pleace respond quickly! Thanks in advance!!

 

7.5 supply volts/220 ohms= 34 milliamps. This is too much current and will burn out the LED.

Is this your setup: An LED in series with a resistor connected to a DC power supply? If so, when the LED is turned on it will provide no resistance to the circuit, the only resistance in the circuit will be the resistor.

 

So what resistor do I need??

 

Lets try some algebra.

We will say 20ma running through the circuit.

Ohms law. V=I*R

7.5V=20ma*R
7.5V/20ma=R=375 ohms. This is assuming that your circuit consists of 7.5 volt DC running to a series resistor/LED circuit. This is also assuming that I haven't forgotten everything in my degree.

If you ran the Voltage directly to the LED, once the LED hit breakover voltage it would short out. So, the resistor will dictate the current for the circuit.

I haven't been to radio shack in awhile, but if you can find 400 ohm resistor, that would be good. The amount of current in an LED doesn't matter, it only matters that the LED turns on. Its brightness will be the same regardless of its current load.

 

I'm not sure what SMT resistors are available at the place I'm going so...

7.5 / 320 = 23
7.5 / 340 = 22
7.5 / 360 = 20
7.5 / 380 = 19
7.5 / 400 = 18
7.5 / 420 = 18
7.5 / 440 = 17
7.5 / 460 = 16
7.5 / 480 = 15

...looks like between 360 and 440 would ideal. Thanks SoundSpeed!!!

 

No problem. Let me know how it turns out so that I can comfort myself in knowing that my education wasn't a total waste.

 

Nope, LED brightness is definitely governed by its current draw. The 10-25mA figure is the maximum current the LED can handle.

Nate, where are you getting the 7.5V figure from?

Don't worry if you don't get the precise current draw. LEDs are finicky anyway... don't sweat it. 400 ohm sounds about right.

 

Hmmm, that is not how I remember all the LEDs I have ever played with, but I am out of practice.

At least I got everything else right, no?
And the 7.5V is probably the voltage he got by taking a meter to his terminals at each of the gauges, switches, etc...

 

Damn I typed a post and then accidentally nuked it. Just click this link.

http://www.acura-tl.com/forum/showth...threadid=32274