[simpits-tech] LED Resistors

[Stig Joergensen]

Lets try that one...

First lets calculate the power usage (p=u*i) : 23w=12*i == 23/12=i ==
i=1.9
then let calc the bulp resistance (u=r*i) : 12=r*1.9 == 12/1.9=r ==
r=6.3
so now we have all the figures (i have rounded them for easier
references) :

u=12 volt, p=24 watt, i=2 amps and r=6 ohm...

how are we going to match that using leds - let look at the leds
figures...
u=2.5 and i=0.02a == 0.05w and to make it work at 12v we need a current
limiting resistor (u=r*i)....

12-2.5=r*0.02 == 9.5/0.02=r == r=475

so now you can compare the led and the bulp, and see that you are going
to need about 100 leds to use the same current, so this is out of the
question - however..... lets try a different approach....

if we take 4 resistors of each 1.5 ohm and place them in series, this
will total 6 ohm and will draw 2amp of current.... that voltage drop
will be approx 3 volt accross each resistor... (remeber the the resistor
must handle 2a, which means they will produce 6watt of heat each
(p=u*i))
      ____       ____       ____       ____
12v--|____|--a--|____|--b--|____|--c--|____|--0v

that mean we have 6 volt in the mittle (point b) and here we can set a
few leds.... but lets calculate the current limiting resistor (u=r*i)...

6-2.5=r*0.02 == 3.5/0.02=r == r=175

and if we place 10 leds (with resistors) in parallel (at point b) this
will use additional 0.5w (and 0.2 amps more).... lets calculate some
figures with this new idea.....

first - we know that the 4 resistors will use 2a and the leds will use
0.2a this gives us 2.2a in total @ 12 volt this gives us 26.4
watt(p=u*i) - now lets calculate the total resistance(u=r*i)  12=r*2.2
== 12/2.2=r == 5.45 ohm (this is not the real value, as we are acutally
drawing more current though the first 2 resistors (a and b), but to
calculate this is very hard work - but if you want to, let me know)

i think it will be close enough, and i think it will work, but havn't
tried it my self....  anyway this is the teory - let me know if it
actually works....

/Stig


> -----Original Message-----
> From: mysticz28@swbell.net [mailto:mysticz28@swbell.net]
> Sent: 5. april 2003 10:36
> To: simpits-tech@simpits.org
> Subject: Re: [simpits-tech] LED Resistors
> 
> 
> Roy,
> I'm a bit lost now. Powering an LED is easy, and I think I 
> have an idea
> of how to power an array of 9 LEDs, but now I have to also make the
> array have the same resistance as a 12v/23w bulb. Any idea how to do
> that? I'm working on a set of tail lights for my bike that 
> will replace
> the cats eye things with LEDs in the mesh vent things on 
> either side of
> the brake light (see 
> http://home.swbell.net/lt1_z28/pics/s2_tail_1.jpg).
> I'm about to whack the huge, useless fender off of there and when I do
> that I'll lose the spot where the signals bolt. Besides, the LEDs will
> be much sexier in those vents ;)
> -- 
> Steve
> mysticz28@swbell.net
> Jesus saves, Budda enlightens, Cthulhu thinks you'll make a nice
> sandwich.
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