Inductance measurement to detect fused relay contacts

A relay driver, comprising:

• a high side terminal adapted to be coupled to a first terminal of an activation coil of a relay;
• a low side terminal adapted to be coupled to a second terminal of the activation coil;
• one or more drive transistors configured to apply a drive signal at one or more of a high side terminal and a low side terminal;
• a clamping device, coupled to the low side terminal and to one or more of a power supply voltage and a circuit ground, and configured to clamp the low side terminal to a selected clamping voltage;
• an inductance measurement circuit coupled to the clamping device and comprising:
• a first comparator configured to compare a coil current conducted from the low side terminal with a first current level while the one or more drive transistors are applying the drive signal, and to issue a start signal responsive to the coil current increasing to the first current level, the first current level selected to be less than a pull-in current of the relay;
• a second comparator configured to compare the coil current conducted from the low side terminal with a second current level while the one or more drive transistors are not applying the drive signal, and to issue a stop signal responsive to the coil current decreasing to the second current level from the first current level;
• and
• a timer for measuring a time elapsed between the start signal and the stop signal, and having an output;
• and
• control circuitry configured to control the one or more drive transistors to stop applying the drive responsive to the start signal.

A method of detecting a contact failure in a relay actuated by an activation coil, the method comprising:

• providing a coil current through the activation coil, the current increasing to a first current level that is less than a pull-in current of the relay;
• stopping the coil current through the activation coil after reaching the first current level;
• discharging energy stored by the activation coil;
• determining a time to discharge the energy stored by the activation coil;
• based on a time to discharge the energy stored by the activation coil, determining an inductance of the activation coil;
• and
• indicating the relay has the contact failure when the inductance of the activation coil is different from an inductance of a relay that does not have a contact failure.

A relay driver, comprising:

• a high side terminal adapted to be coupled to a first terminal of an activation coil of a relay;
• a low side terminal adapted to be coupled to a second terminal of the activation coil;
• one or more drive transistors configured to apply a drive signal at one or more of a high side terminal and a low side terminal;
• a clamping device, coupled to the low side terminal and to one or more of a power supply voltage and a circuit ground, and configured to clamp the low side terminal to a selected clamping voltage;
• and
• an inductance measurement circuit coupled to the clamping device:
• wherein the inductance measurement circuit indicates the activation coil of the relay has a contact failure when a measured inductance of the activation coil is different from an inductance of a relay that does not have a contact failure.