Both the main and commutation SCRs have their gates driven by PUT (Programmable Unijunction Transistor) circuits ... function needed when the SCR is not conducting). Q2 receives its charge signal from ...

The reason for this circuit placement is the ease of driving the SCR gates via the positive ... charger tapers off when the battery voltage is above about 14V. For this to function, D6 is a 5.1V shunt ...