[simpits-tech] LED Resistors

[simpits-tech@simpits.org]

Stig Joergensen wrote:
> 
> 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....

Thanks for the help. I'm wasted on a billion kinds of drugs right now
and having a bit of trouble digesting all that, but I'll be better in
the morning ;) I'll breadboard the thing, plug it in, and see what
happens.

I just had a thought. You know how most turn signals have a thermal
flasher that makes an audible clicky, springy noise as it cycles? Well,
this bike doesn't make that noise. I suspect that the flasher is
actually solid state (prob. a 555 or maybe just a simple RC circuit). It
makes sense as there's a red box that controls the automatic cancelling
signals anyway. If the engineers had a few spare brain cells sitting
around they would have done away with the flashers and made it solid
state, or at least that's what I would have done. If that's how they did
it, likely with a transistor doing the switching, this whole discussion
is a bit wasted since the resistance wouldn't matter.

I'll take a look at some things tomorrow and see what I can figure out.
Thanks again for the help, Stig. Ya know, this group is one of the best
resources I have for more reasons than I can think of in my current drug
induced state ;)
-- 
Steve
mysticz28@swbell.net
Jesus saves, Budda enlightens, Cthulhu thinks you'll make a nice
sandwich.