P 100 ohm pot
C1, C2, C3 100nF ceramic
D2 BZX6V2 zener, 6.2V
D4 no markings, but a 10A, 50V schottky should do the job here. Strangely enough, the diodes in my Siemens databooks that use this package go up to about 3A, which I consider a little low for this application (BY245, BY246).
From the wiring diagram in my manual:
Correctly adjusted, the voltage on the pot wiper is slightly less than half D+ (appx. 0.47*D+) and Q1 will conduct if (D+)-(Vp)>6.2+0.7+0.7, or 0.53*(D+) > 7.6V, (D+) > 14.3V. If D+ is lower than 13.7V, Q1 will not conduct, Q2 will get driven via R5, and Q3 will conduct. Df will carry a voltage. When D+ rises, Q1 will start conducting, Q2 will get pinched gradually, and so will Q3. Voltage on Df will drop.
This was originally investigated by Matt Bennett, but the diagram, functional description, and many corrections were done by Rik Steenwinkel, firstname.lastname@example.org or email@example.com.
Revised on 10/3/95 by Matt Bennett