Simulations of simple shunt voltage regulator circuits to regulate the rectified output of a permanent magnet alternator such as may be found on many motorcycles. Several variations of the basic circuit are possible. *Please note, however, that not all variants are illustrated in this project*: 1. Using a bipolar power transistor as the shunt regulator element. Suitable for low power alternators up to about 100W / 8.3A output. 1. Using a MOSFET power transistor as the shunt regulator element. Suitable for medium power alternators up to about 300W / 25A. 1. An extendable version based on (2), using a paralleled array of MOSFET power transistors as the shunt regulator elements. Suitable for higher power alternators up to about 3kW / 250A. 1. Simply by reducing the zener diode to a 6.2V part, design version (1) is particularly suited to regulating lower voltage outputs such as the 6V DC outputs of some smaller motor cycles and scooters. 1. By reducing the zener diode to a 4.7V part, connecting the regulator across the +/- outputs of a suitably rated (and heat sunk) bridge rectifier and then connecting the alternator output across the AC terminals of the bridge rectifier, design version (1) is also well suited to regulating lower AC voltages such as the 6V AC outputs of some trail bikes. 1. Similarly, by changing the zener diode to a 6.2V part, and using a lower threshold voltage MOSFET, design version (2) is also well suited to regulating lower voltage outputs such as the 6V outputs of some trail bikes. 1. Similarly, by changing the zener diode to a 4.7V part, using a lower threshold voltage MOSFET; connecting the regulator across the +/- outputs of a suitably rated (and heat sunk) bridge rectifier and then connecting the alternator output across the AC terminals of the bridge rectifier, design version (2) is also well suited to regulating lower AC voltages such as the 6V AC outputs of some trail bikes. Some of the designs simulated here have been built into a number of bikes and run successfully for many years however, because the power devices in these regulators operate in shunt circuits, i.e. directly across the rectified alternator output at all times and therefore dissipate very high powers - up to the maximum power output of the alternator: they even work if the battery itself is disconnected - it is **essential** that these power devices be mounted on adequate heatsinking. Whilst such heatsinking may have the benefit of airflow over it when the bike is moving, bear in mind that there may be times when the engine is spinning at high rpm - so the alternator output will be around the maximum - but the bike itself may be virtually stationary. This is particularly true on trail bikes, which may be climbing steep inclines in low gear or stuck in mud. Always mount such devices as far as possible out of the way of water and mud splashing onto them and ensure that the enclosures are fully watertight. Remember that these devices will be subject to severe vibration, some mechanical shock and a pretty wild temperature range so they should be built using good quality components and assembly techniques such as using strain relief loops and cable restraints in the wiring. Note that in these simulatons: 1. the alternator is very crudely modelled as a set of low resistance 3-phase voltage sources. This is adequate for the purposes of these simulations but does not fully represent the operation of a real permanent magnet alternator; 2. the battery is also very crudely modelled simply as a very large capacitor with a small series resistance. (Schematic 1a shows a way to start the simulation off with a fully charged battery.)