Primary current in the coil flows when the pnp transistor T is switched-on, which is controlled by T3. The switching of T3 is controlled by the current supplied through R5 and the state-of-charge of the capacitor C. During the charging of the capacitor with current from R5 no current passes to the base of T3, and so T3 is switched-off. Once the capacitor is fully charged, current passes to the base of T3 and switches it on to start the dwell period (i.e. to initiate current flow in the primary winding of the coil). The time taken to charge the capacitor governs the dwell period. The RC Time Constant, in this case, is determined by the amount the capacitor is discharged prior to receiving its charge from R5.
At low engine speeds, the transistor T2 is switched-on for a comparatively long time. This allows the capacitor plate, adjacent to T2, to pass the charge to earth that it received from Ra when T2 was switched-off. At this slow speed, there is sufficient time for the capacitor to fully discharge to a point where the plate potential becomes similar to earth. This causes the capacitor to attract a large charge from R5 when the transistor T2 switches off. Since the time taken to provide this charge is long, the switch-on point of T3 is delayed and a short dwell period results.
At high speed, the T2 is switched-on for a short period thereby allowing only partial discharge of the capacitor. Consequently, the time taken to charge the capacitor is shorter and the dwell commences at an earlier point providing a longer period. Interruption for the primary coil occurs when T2 is switched on. This is dictated by the trigger signal due to which the end of the dwell period always occurs at the same time. At the instant T2 switches-on, the capacitor starts to discharge, which causes T3 to switch-off to trigger the spark.