This energy saving, a 12-V controller is nearly universally applicable. In addition to controlling battery powered lighting in a car, boat or caravan it is also quite able to control the speed of a model train. In essence, this circuit converts the 12-V DC voltage into a rectangular pulse train with a duty cycle that is adjustable from 0 to 100%. The circuit can be divided into four sub-circuits: the saw-tooth generator built around IC1, reference network P1/R3/C3, comparator IC2, and driver stage T1/T2. The comparator compares the generated sawtooth voltage (1) with the reference voltage (2).
This reference voltage is adjustable between the lower and upper limits of the sawtooth voltage with P1. When the saw-tooth voltage is greater than the reference the output of the comparator will be ‘high’. Since the saw-tooth voltage, with its fixed frequency, is continuously crossing the reference voltage, a rectangular waveform (3) appears at the output of the comparator, the duty cycle if which can be determined with P1. The driver stage, with its large current amplification, ensures that the voltage up to a load current of 3.15 A will remain sufficiently square. By varying the reference with P1 we can change the width of the pulses in the pulse train. This affects the average voltage to the load and therefore also the power drawn by it.
The current through T2 is always largest when the voltage drop across it is smallest (saturation) and smallest when the voltage drop is the greatest. T2, therefore, needs to dissipate only very little power and needs to be cooled only when used with highly inductive loads. Diode D2 protects against reverse connections and acts as a freewheeling diode for inductive loads. Returning to the sawtooth generator, IC1 is a 555 timer configured as an AMV, which is tuned to 65 Hz with R2/C2. With such an AMV the square wave at pin 3 is typically used, but this time we are more interested in the charge/discharge voltage across C2. This is, strictly speaking, not a pure sawtooth but is nonetheless very suitable as a sawtooth for this controller. If it turns out that the controlled lamp flashes visibly, it is possible to raise the frequency by lowering the value of C2. Because of the behavior of the load and the dissipation of T2, it is not recommended to increase the frequency beyond 200 Hz, even though the circuit will work without problems at frequencies greater than 10 kHz. C2 is charged and discharged by the 555 between the bottom limit of 1/3 and the upper limit of 2/3 of the power supply voltage. These limits are defined by three internal resistors of 5 kΩ each. These also give the IC its name. In order to adjust the frequency, the upper limit is made available to the outside world via the control input (pin 5). This voltage is stabilized by C3 and directly made available to P1. The lower limit is defined by making the resistance of P1 and R3 equal, so the voltage division is the same as the internal resistors in the 555, with which they are effectively in parallel.