Electrical schoolwork...

[Louis-Alain RICHARD]

Wonderful ! (LOL)
Thanks guys!

I knew a question like this one would raise interest...

So, after all this, here is what I can add:

Like Ameer said, the 150 000 ohm figure is probably not useful since it
depends on the DMM voltage. I did not knew this, now I (and all of us) do.
This figure comes from a batch of green 3V LED I have here. 

After all these posts, I went to the basement and made some measures.
However, I don't have any useful source for 12V in the house so let's play
with 3V.

2 new AA batteries (2.8V total) 
various ohm resistances
3 green LEDs.

1 LED connected to 2xAA: very bright green, 2.7 Volt across the leads, not
able to measure any current with my DMM (when series-wired).

3 LEDs in parallel connected to 2xAA: 2.48 Volt across, still bright green,
still no current figures.

1xLED and 1x 500 ohm in series: dim LED, 1 volt across the resistance, 1.8
volt across the LED.

Does this confirm Ti and Ameer numbers? I guess so.

But then I have a problem (like Huw said): my initial thought was that a
small LED and a small resistance would be easy to insert IN the switches at
the burned-bulb place (I already opened all the switches last year to check
if it was feasible to replace only the bulbs: negative...). But now if I
have to package a big 1W resistor IN the switch, it might me very
challenging. 

Hum...

The other solution would be to insert only the LEDs and find a way to place
a single resistor elsewhere and carry an additional 3V wire to the different
LEDs... A good way to ruin perfectly good switches and wiring...

Or maybe if I can find some 12V LEDs, all this discussion would be a huge
WOB...

Yeah, thinking of it, this would be the ideal solution.

Is such a LED exists? A long life, wide temp range 12V red LED?

Answer tomorrow, after my visit to my NFNKLEPS (not friendly, not
knowledgeable local electronic parts store)!

Louis-Alain
1983 urQuattro
85-D-900463




-----Message d'origine-----
 De la part de Ameer Antar

Although the current wasn't given, this really the only way to calculate
anything for an LED. The resistance of an LED depends on the voltage used by
the meter, and that's usually not something written anywhere. So the
resistance reading of an LED is not very useful, and I've never seen written
on a spec sheet. That's b/c an LED is not a simple resistive load like a
light bulb. It's basically a diode which changes states rapidly as voltage
changes. That's how transistors and such can switch on or off in the digital
world.  Oh, and the 15mA is just a usual figure for red LED's... it maybe
more or less, but it's unlikely to be far off from that unless it's one of
those ultra-bright's or blue-colored LED. Hope this clears things up a bit.

-Ameer

---Original Message---
Date: Tue, 10 Feb 2004 01:57:48 -0500
From: Huw Powell <audi at humanspeakers.com>

I prefer this answer to Ti's current based calculation.

I would recommend testing your choice "out in the open" before 
rebuilding a fistful of switches, etc., to make sure you got it right.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/