Re[2]: ATM LED Brightness, was Sources for tester plans

[Dwight Elvey <elvey@hal.com>]

Bradley Davy <bdavy@fred.net> wrote:
> Dwight Elvey wrote:
> > recall ( I could be off ) the green LEDs run at about
> > 2.1 to 2.2 volts forward. This might be where you got that
> > 2.2 number.
>  
> No, the package is clearly marked 2.2 mA rating. 

 This may be the case for smaller LEDs but the jumbos are
all rated at least 20mA.

> 
> >  Peter's estimate of the current was incorrect because of this.
> > Allowing 2.1 volts for the lamp, the current would have been,
> > 13.8mA. This is well within what any jumbo lamp I've seen
> > can handle.
> 
> At the fully conductive mode, the resistance of the diode effectively
> becomes 0

 The dynamic resistance is about 0 Ohms. This is not the same as
saying that the voltage drop isn't there. Measure an LED your
self to prove it. You subtract the drop across the diode. It
is Kirchhoff's voltage law requires it. No diode I've ever
seen has 0 volts across it while current is flowing through it
unless it was shorted. Besides, this violates the conservation of
energy law. In order for the lamp to emit light, there must be
power dissipated in it. This means both voltage across and current
through it. Otherwise, no energy, no light.

> > 
> >  Alkaline 9V transistor batteries are about 500mAH. Using the 500
> > Ohm value should give about 36 hours of use.
> 
> 500mAH/2mA=250 Hours

 The amount of current that goes through the lamp is controlled
by the current limiting resistor and the voltage drop across it.
My calculation was for the 13.8mA using the 500 Ohm and a non-internal
resistor type green LED. Yours isn't a jumbo green LED.

> I use a heat sink clip.  Much easier to attach than Needle Nose Pliers.

Most people don't even have heat sink clips any more. These
were a hold over from the germanium days.
Dwight
PS I used to design these circuits for a living. Trust me.
If you don't think I'm wrong, a voltmeter will prove it.