Wide input voltage range relay drive circuit for universal defrost timer

A circuit for switching an AC input voltage to a load, comprising:

• a relay having a coil and at least one of a normally open or a normally closed contact;
• a transformer-rectifier configured to generate a bulk voltage from the AC input voltage to drive the coil;
• an electronically controlled switching device operatively coupled in circuit with the coil to actuate the relay with the bulk voltage;
• a power dissipating electronic switching device coupled across the coil;
• and
• a controller operatively coupled to the switching device and the power dissipating electronic switching device, the controller further operatively coupled to the transformer-rectifier to monitor the bulk voltage generated thereby;
• and
• wherein the controller enables the switching device to energize the coil for a first period long enough to ensure actuation of the relay but short enough to prevent damage to the coil, the first period being dependent on a value of the bulk voltage.

A method of switching a widely varying voltage with a relay, comprising the steps of:

• rectifying the widely varying voltage to form a bulk voltage;
• sensing the bulk voltage;
• determining a period sufficient to ensure actuation of the relay but short enough to prevent damage to the coil based on a magnitude of the bulk voltage;
• applying the bulk voltage to a relay coil for the period;
• and thereafter
• when the magnitude of the bulk voltage is greater than the rated coil voltage, modulating the bulk voltage to the relay coil to maintain energization of the coil and to prevent damage thereto.