Re: How to determine what size resistor is needed??


<dsuperbee@xxxxxxx> wrote in message news:a5124f08-4d8f-4c50-a989-f3b18bb3218a@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
While helping my friend with his Black Rose I had an idea for a cool
mod.

It involves running a component LED off of a coil, to say the least.
Here is where the problem lies...I have NO idea at all on how to
figure out what size resistor I will need so the LED does not burn
up! I am looking here:

http://www.superbrightleds.com/cgi-bin/store/commerce.cgi?product=LEDS&keywords=&cart_id=9502165.5482&next=50

At the LED model RL8-R25-360.

What kind of power is needed to run these? I know it is MUCH less
than the 40v or so that a coil needs. So how do I figure out what
size resistor to put in? I have looked online and am trying to figure
out the color band scheme, but how does the ohm relate to wattage
resistance?

If anybody has some good links or info for a newbie on this, I would
really appreciate it!

Brian

You need more info on the LED than is provided in the above link.
There are a number of factors involved in calculating resistor value.
1 -- the operating voltage -- in your case -- 40VDC
(but I'm not sure what you are saying - you mean you are connecting the LED in parallel with the coil so that it lights when the coil is firing?)

2 -- forward voltage drop of the LED (Vf). This isn't given in the specs of the LED you mentioned. Typically 1.4 volts or so but varies widely depending on manufacturer, LED color and LED size.

3 -- forward current of the LED (If). They normally give maximum forward current and often (but not always) give typical forward current. Use the typical value which is always less than maximum. Often the max value is about 20mA and the typical is ~10mA. Again, varies widely depending on characteristics of the LED and that webpage does not give specifics.

Once you have these -- resistor value is easy:

Resistor = (operating voltage - forward voltage) / forward current

So -- if operating voltage is 40, forward voltage is 1.4 and forward current is 10mA:
Resistor = (40 - 1.4) / 0.01 = 3860 (use 3900 ohms as closest typical value).
Determining power of resistor = voltage drop x current = (40-1.4) x 0.01 = 0.386 watts -- use 2 watts for lots of leadway.

-- Ed


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