MAILBAG

摘要

In your December column, you analyzed a small battery charger in a weed whip. While your circuit analysis was correct (as always) based on a TO92 NPN transistor for the pass element, I believe the original part was an SCR. The give-away was the remaining letters "MCR." Motorola used to make a line of SCRs in TO-92 packages. The MCR100 series were .8A, and the MCR22 series were 1.5A. These are sensitive gate types requiring only 200 microamps of gate drive.