# 3mm led RAGE!!

#### Spidey82

##### Enlightened
i riped open my rage and put in a 3mm white led,it has a nice beam, liked it alot.

i used a dummy battery in it(foil wrap), and 3 AAA (abt 1.25V~1.2V).
how long do u think a typical set of fresh alkaline would last??

but when i test another led accross three fresh AA, (abt 4.6V NL) it light up bright initially and within seconds, it go toasted and the brightness is less that when i putting 3v accross it.

do u think i need a resistor?
how whould u suggest i caculated the resistor value?
whould the resistor dim the led a lot when the batt gets weak??

20hr has pass and it is still running like it was 20hr ago
and the battery inside is not new!!
is rated at 1.2v each.
if that batt can last so long,
wonder how long i can run it wif new batt.
of cos i need a resistor, and i need that value

Spidey82:

Originally posted by DaveH, 8-22-00:
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>For parallel hookup the ohm calculation works out this way,
(voltage source - LED voltage )/ LED current. White LED's are usually rated at 20 ma, but some products their driven at 50 ma, so lets set the LED current at 30 ma.

So substituting in, (6 - 1.7)/.03 = 143 ohms more or less. You can probably get 150 ohm resisters at radio shack in a pack of 6 for \$1.
<HR></BLOCKQUOTE>

I located this using the board's search engine ... it's a very useful tool. Amazing how much data is posted here.

Mark

((Voltage supplied)-(Voltage required)) / Current = Ohms resistance

(Vs-Vr) / I = Ohms

so to apply to the above problem

if Vs = 4.5
and Vr = 3.6 (single LED or Paralled LEDs)
and I = .020 Amps

then you would need a (4.5 - 3.6)/.020 resistor = 45 ohms if you play "by the rules" and keep it in the factory specs for Nichias (if that's what you're using)

The LED will dim when the battery gets weak no matter what you do. The resistor just "uses up" the extra voltage that would destroy the LED. Essentially the resistor (in the example above) becomes a little tiny .9V heater that you run whenever the LED is on, wasting the .9V as heat but protecting the LED. If you want, you could build a switch into the circuit that jumps the current around the resistor when the LED starts to get dim.

(WOW! I almost sound like I know what I'm talking about)

Thanks Doug! I've quoted your reply to the Batteries included forum so others can easily find it (like me) ... that question gets asked so often!

Mark

thanx guys.
think i will run the led at 4v
how much current should i let it pass??
a switch will be good

For resistance, just use the formula above.

(4.5-4.0)/.020 = 25 ohm resistor per LED assuming .020 Amps (= 20 milliamps)

If you want to push it a little more,

(4.5 - 4.0)/.030 = 17 ohm resistor per LED assuming .030 Amps (= 30 milliamps)

The calculations above are based on an LED that required 20 milliamps (e.g. Nichia white). How much current (milliamps) you let through should be based on the manufacturer's specification for the LED.

DP