Voltage regulator for cars
In older cars, the battery charging voltage is controlled mechanically. The regulator consists of a relay that switches the stator windings of the alternator on and off. This arrangement is prone to breakdown, inaccurate regulation and sensitivity to load variations.
An electronic alternative as shown has the advantages of not containing any moving parts and of providing much more accurate regulation. Moreover, the voltage is measured at the battery terminals, so that losses in the wiring are not included in the measurement.
The Regulator. IC), is essentially a device that continuously compares the battery voltage with a reference potential. This comparator drives a power transistor that switches the excitation of the alternator.
Terminal Z1 is connected to the + terminal of the battery; Z2 to the ignition switch: and Z3 to the stator winding of the alternator. The battery voltage is reduced to about 5 V by potential divider R1-R2-Pi and applied to lie non-inverting input of Schmitt trigger IC1. The inverting input of this op a reference potential of 5 V provided regulator IC2. Power transistor Ti switched the output of IC1 and transistors T2 andT3. Diode D2 functions as an indicator, while D1 is a free-wheeling diode. Capacitor C6 attenuates the pulse generated when T1 is switched on so that far fewer harmonics are generated and ‘interference on the medium-wave radio is suppressed.
The regulator is calibrated by connecting a 12 V lamp or 15 S2, 10 W resistor between Z3 and earth and a variable power supply and multimeter, set to 15 V, between Z1 and earth. Set the power supply output to 14.3 V and adjust P1 until the lamp just goes out. When the power supply output is reduced slowly, the lamp should come on again at 13.9 V.
The regulator is best built in a small aluminum case that also serves as heat sink for T1. The case can be made water-tight with a suitable (hardening) silicone paste.