Re: Larson Scanner with a 9v Battery

Windell Oskay

(A) The IC is capable of sourcing enough current (40 mA) to destroy the resistor (typically rated for 30 mA maximum). Also, LEDs blow instantly, whereas the IC has some protection circuitry, and usually withstands being out of spec for a little while. So… no surprise.  However, it’s not unheard of for an LED to be out for more mundane reasons, such as an incomplete solder joint that opened up.

(B) That would be correct only if the current draw were constant.
(C) Diodes are very different from resistors. They drop an essentially _constant_ amount of voltage, over a wide range of currents, whereas resistors have vanishingly small voltage drop at small current.   If using diodes to drop the extra voltage, you won’t need a resistor in there.