Although you can buy voltage regulators fairly cheaply, particularly second hand, I decided to build my own. There is a single chip solution available, the L9911 from ST Microelectronics. This is constructed with an easily mountable package like a large transistor and the pins are on a 0.1" pitch so it is easy to mount on standard stripboard (Reinforce the tracks with solder as currents up to 3 amps will be flowing). The tab on the package is earthed so this can be fastened directly to the aircraft chassis via the aluminium project box I have chosen. Some thermal conductive paste was used to ensure that any heat generated would be dissipated to the firewall. The regulator needs a light or LED connected between the "ignition" pin and the regulator supply which it senses and this turns it on. I used a panel mount LED with a 1K ohm resistor as a current limiter.Unlike most aircraft regulators it needs a phase input from the alternator which you take from between any one of the pairs of diodes. Some alternators have a connection for this. Mine didn't but it was easy to add an extra wire to the alternator. The regulator uses this to sense that the alternator is turning and switch the field in sync with the rotation. This helps to reduce any chance of radio interference. The regulator also only applies a small current to the field if the alternator is not turning unlike most aircraft regulators which apply full current any time the voltage is below the regulator set-point. The LED illuminates if the alternator is not turning and goes out when the regulator senses the phase input.
Other than that you only need to connect the +ve supply and the field output as per any other regulator. The large capacitor is used to smooth the regulator +ve supply which it uses to compare with the set-point and control the field current. Total cost of the completed regulator, including the box, was less than $20.
Today, I also installed the oil temperature sender and replaced the internal battery in the KMD150 GPS. This is a bit of a fiddle as it requires the unit to be stripped down. However, the new battery should last about 8 years. There are good instructions on the web how to do this if you google "KMD150 battery replacement".
So that is another three items ticked off the check list:-)
