Re: Putting Red LED's In Osram Dot it light

On Wed, 20 Feb 2008 03:28:14 -0800 (PST), "Dave.H"
<the1930s@xxxxxxxxxxxxxx> wrote:

On Feb 20, 9:29 pm, "Dave.H" <the19...@xxxxxxxxxxxxxx> wrote:
I want to put 16,000 mcd red LEDs in an Osram Dot it light for working
outside when I don't want my night vision affected, the light in
question has 3 white LED's and a resistor (red-yellow-black-gold-
brown) voltage measured from the pins of the LED's is 3.122 VDC. How
would I go about installing these 2.0 volt red LED's? Do I just
change the resistor, if so what value? Unit is powered by 3 AAA
batteries.

The resistor is red-GREEN-black-gold-brown not red-yellow-black-gold-
brown as I mentioned earlier. I think the difference is only one ohm
or so. The green was light in colour making me think it was yellow,
but it does measure 25 ohms.

---
From your description, (and neglecting the switch) it appears that
the flashlight is currently wired like this: (View in Courier)


4.5V 3.1V
/ /
+--[25R]--+--[LED>]--+
| | |
|+ +--[LED>]--+
[BAT] | |
| +--[LED>]--+
| |
+--------------------+
<-- Iled

which means that the current into the LEDs is:


Vbat - Vled 4.5V - 3.1V
Iled = ------------- = ------------- = 0.056A = 56mA
Rs 25R


Unfortunately, since Vf/If is different for each LED (unless they
were very carefully matched) the current won't split equally, with
the result being that one LED may be hogging current beyond its
rating, resulting in a shortened life.


I suggest that you rewire the flashlight like this:

+--[R1]---[LED>]--+
| |
+--[R2]---[LED>]--+
| |
+--[R3]---[LED>]--+
| |
+-----[+BAT]------+


In order to determine the value of each of the resistors use:


Vbat - Vled
R = -------------
Iled


Where R is the value of the resistor, in ohms,

Vbat is the battery voltage, in volts,

Vled is Vf(min) for the LED, from the data ***, and

Iled is the nominal forward current, in amperes, also from
the data ***.


For example, If your LEDs are rated for a Vf(min) of 2V at 20mA,
then you'll have:


Vbat - Vled 4.5V - 2.0V
R = ------------- = ------------- = 125 ohms
Iled 0.02A


The closest standard 5% resistor on the low end is 120 ohms, so the
current it would allow through the LED would be:


Vbat - Vled
Iled = ------------- = 0.0208A ~ 21mA,
R

Which would be fine.

--
JF
.