Electronic control for 3-wire DC coils

An electronic controller for controlling operation of a contactor that comprises an armature which when in a pulled-in condition completes an electric circuit, and further comprises an electromagnetic pull-in coil which when conducting current above a predetermined magnitude pulls in the armature, and an electromagnet holding coil adapted for conducting current at least of sufficient magnitude for holding the armature in pulled-in condition, the controller including:

• a first switching device having an input and an output and operative to connect and disconnect the input to and from a power source,
• a solid state switching device connected in a series circuit with said pull-in coil and said output of the first switching device, the solid state switching device having a control element responsive to an applied pulse signal by causing the solid state switching device to conduct for a duration corresponding to the width of the pulse signal,
• a timing circuit having an input coupled to the output of the first switching device and an output coupled to said control element of the solid state switching device, said timing circuit responding to power from the output of the first switching device by applying to said control element at least one pulse signal having a duration corresponding to the time required for said pull-in coil to pull-in said armature so as to convert said solid state switching device to a conductive state to energize said coil for a duration corresponding substantially to the pulse duration;
• said timing circuit including:
• first and second logic gates each having at least two inputs and an output,
• a first circuit comprised of a first resistor having an input end constituting the power input to the controller and a first capacitor connected in series, a point between the capacitor and first resistor being connected to one input of the first logic gate,
• a second circuit comprised of a second capacitor and a second resistor connected in series with the second capacitor and a point between the second capacitor and resistor connected to the two inputs of the second logic gate,
• the output of the second logic gate connected to the other input of the first logic gate,
• a third resistor connected to the output of said second logic gate and a fourth resistor connected to the output of the first logic gate,
• the first capacitor starting to charge when voltage is applied to the first resistor from said output of said switching device to thereby cause the output of the first logic gate to switch to a high logic voltage state for initiating conduction of the solid state switching device developing a positive voltage drop across said fourth resistor,
• said second capacitor starting to charge in response to the output of said first logic gate switching to a high logic voltage state to thereby cause said inputs of the second logic gate to be at a high logic level and the output of the gate to therefore be held at a low logic level until said second capacitor is charged to a predetermined voltage level, upon which event the output of the second logic gate switches to a high logic voltage state which is coupled to the other input of said first logic gate, to thereby cause said first logic gate to switch its output to a low logic voltage level for controlling said solid state switching device to switch to a nonconductive state.