The 12V auto voltage regulator.
In a previous article I promised the schematic for a voltage regulator. Not surprisingly the web is full of articles covering schematics, how it works, etc. Mine was actually taken from a public library book in the early 80s – it was modified to include an extra diode D11 which generates fast and clean switching. I never found it necessary to place any filtering capacitors across the supply rail – it was reliable. My prototype come finished article failed after about a year – it was not all that robust. Errors and Omissions excluded.
The circuit workings are very basic. On switch-on the (+) power rail gets its current through the charge lamp which supplies power to T1 and T2. As the voltage sits below 13V and T1 Vbe switch on , ZD1 does not conduct but T2 does, through forward bias resistor R3. Because T2 is ‘on’, current is drawn through the alternator field winding ‘FIELD’ via slip-rings on the rotor causing the charge lamp to come on. Once the engine turns the alternator rotor field, the excitation current through the rotor field causes an output at the stator three phase windings, rectified by D1 to D6 (charging circuit) and D7 to D9, which is the regulator sense voltage. When the voltage is above a threshold e.g. 13.6 ~ 14.4V T1 switches on pulling down the base voltage of T2 reducing current rapidly through the rotor field reducing excitation and therefore alternator output voltage and load or charge current. The points in green are the connection points- Ground, Field and Charge Lamp.
Most alternators have four connections to it. B – to battery (high current 40 to 60A), IG – to ignition, S – sense voltage to regulator from battery supply and L – Lamp. Alternators also have a point F connection for field tests.
Some extra notes: Alternator stator windings can be either Star or Wye (star is the common term used on motors and alternators) or Delta – self explanatory where the coils are daisy chained in the form of a delta shape. The image above shows a Star connection. The differences are Star for high voltage out at low RPM and Delta for high current out at low RPM.
Voltage regulators can be grounded field (regulator through field to ground) or grounded regulator ((regulator through field to +B)
Voltage Regulator reliability
As can be seen, the voltage regulator is a very simple device. The electronics is old school and in most cases the voltage regulator is extremely reliable – most emphasis is placed on robustness under very harsh conditions.
Making your own
The schematic is above – determine first whether the field goes to (+) or (-). My motorcycle’s went to (+). You can either build this on a PC board or use strip board. There are many enthusiasts out there that hate strip board but remember that the end result is going to be a lot of glue or resin. Encapsulate the thing by all means – just make sure all the connections are easily reached.
If you pick up any problems let me know. If you have access to a variable power supply then it is very easy to test.