Hi all, I'm looking for some help analyzing this circuit since I'm having a bit of trouble wrapping my head around it.
The device is a voltage regulator for a snowmobile. The engine has a generator winding underneath the flywheel which generates an AC voltage with frequency proportional to the speed at which the engine is turning. I'm not sure how many magnetic poles the flywheel has, but the engine speed should be in the range of 1000 - 8000 rpm.
The regulator serves two purposes:
1 - Regulate the AC voltage to approximately 14VAC to prevent the voltage from destroying the incandescent light bulbs when the engine speed is increased.
2 - To rectify the AC voltage and charge a 12V lead acid battery. The only purpose of the battery is to run the electric starter motor used to start the engine. All light bulbs are powered directly from the AC voltage.
The regulator had stopped charging the battery and would intermittently stop regulating the AC voltage, causing all of the light bulb filaments to blow (very frustrating). I disassembled the regulator (it was potted in a rock-hard potting compound which only softened up with a hot air station) and reverse engineered the schematic, which is attached to this post. The diodes labeled "Power Diode" are large diodes with no markings that are heatsinked to the metal enclosure of the regulator. All 3 PNP transistors are MPSA56.
The nodes labeled "R", "G", and "Y" are the three wire connections to the regulator (red, green, and yellow). The red wire connects to the positive terminal of the 12V battery. The ground reference of the schematic is connected to the negative terminal of the 12V battery. The yellow and green wires are connected to the AC generator in the engine.
Clearly, when "Y" is positive and "G" is negative, current will flow through D9 and D6, charging the battery on that half cycle. However, to charge the battery on the other half cycle, current would have to flow through D8 and SCR1.
I'm having trouble wrapping my head around the gate control circuits for the two triacs and would appreciate if someone could give me an explanation of how this circuit works.
Thanks!