Variable form factor periodic heating load controller - uses differential analogue demand voltage to control VCO with two periodic outputs and counter producing control state with override

摘要

The control signal is cyclic with a period of about a minute and overcomes the linearity and switching noise problems associated with conventional range and threshold control circuits. The two input voltages are fed into a differential amplifier with complementary outputs which control a VCO with periods T1 and T2 which together have the period of the supply frequency. A chosen number of periods T1 are counted and a control output in one state produced, then the same number of periods of T2 are counted whilst producing the control output in the second state. An override circuit switches the load off or on when the differential outputs exceed one or other preset limit. The control output is synchronous with the VCO and is delayed by one or more periods to avoid signal races. It is passed to the input of a D type flip flop by the oscillator which is asynchronously controlled by the differential amplifier outputs. The VCO uses a small thyristor to discharge a capacitor charged by constant current.