Date:         28 Jul 1996 13:34:24 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         "Harvey Chao" <Harvey_Chao@smtp.svl.trw.com>
Subject:      Re- basic electronics quest

Been watching this one from the sidelines, lurking you could say ('cepting a
private cheap shot at Cetin the other day :-)   ) -  and as Popeye would say "
I can't stands it no more"  

In digest 1052, Dan said  " using a cheapo 12v power supply, that puts out
about 15v unloaded the  LED drew about 80ma with a 280 ohm resistor.  1/2
watt.  the resistor "

Folks, Ohms law is:  Volts = amps x ohms.  280 ohms times 0.80 amps  =  22.4
volts .  That is the voltage drop across the resistor.  If the power supply
max unloaded is 15 volts, there has to be a measurement error somewhere here. 
How accurate is the resistor compared to whatever it may be marked ????

the critical things here are:  
1)  Does the LED light to an acceptable brightness?
2)  Is the LED gettting excess current that could cause it to fail
prematurely, and  - do we care if it fails prematurely, and if so, HOW
prematurely?
3)  Is the 1/2 watt resistor running too hot - to the point where it could
fail and be a fire hazard???

Harvey