Overheating protection circuit for electric blanket - has logic circuit cutting off power in response to small currents in detector wire

摘要

The output of a flip-flop, consisting of two NAND gates (P1,P2) cross-coupled by two resistors (R3,R4), is fed to a buffer consisting of two parallel NAND gates (P3,P4). The buffer output (13) is fed to a current supply cut-off circuit, e.g. a crowbar thyristor which blows a replaceable fuse. When the blanket is plugged into the mains supply, a reference voltage (Uref1) is applied to the flip-flop, setting it in the normal, i.e. safe, state. The hot-spot detector wire is connected via a diode (D1) and resistor (R2) to the flip-flop input. If the detector wire current exceeds 1 mA, the flip-flop changes state and operates the cut-off circuit. The flip-flop can be reset by operating a reset switch (S) which shorts out a dropper resistor (R1) between the setting reference (Uref1) and a second reference (Uref2).