A detecting circuit comprises a magnetic core having a rectangular magnetic characteristic, a first winding coupled magnetically to the magnetic core, for applying a current to be detected, and a second winding coupled magnetically to the magnetic core, for applying a reference current. An output signal is derived from the second winding.
A temperature control system for electric surface heater of piled layers of a heating element across which a commercial AC source current is fed and a thermosensitive electrode opposing the element through thermosensitive element showing negative impedance characteristics with temperature rise. In the system, an AC signal voltage different from the source current is applied between the heating element and the thermosensitive electrode, and excessive variations in the signal voltage responsive to the negative impedance of the thermosensitive element is detected to disconnect or connect the heating element and the source.
A device and method for diagnosing shorts and fault conditions in electrical circuits such as heating, ventilation, and air conditioning (HVAC) control circuits. The device includes a positive temperature coefficient (PTC) member to protect the electrical circuit from overcurrent conditions during diagnosis. The device also includes a light emitting diode (LED) and resistor electrically connected in parallel across the PTC member. An overcurrent condition causes the resistance of the PTC member to rise dramatically, creating a potential difference across the PTC member and lighting the LED. The invention also includes a method for locating a short in an HVAC control circuit using the short locating device.
