초록 ▼

A transistor circuit (18) is series connected with a solid state switching device (14) and a load (4) controlled by that device (14) to protect that device (14) from overcurrents resulting from short circuits such as across the load (4). The transistor circuit (18) detects the overcurrent and switch

A transistor circuit (18) is series connected with a solid state switching device (14) and a load (4) controlled by that device (14) to protect that device (14) from overcurrents resulting from short circuits such as across the load (4). The transistor circuit (18) detects the overcurrent and switches to a high impedance state in response to that detection. The circuit (18) is self-resettable to restore to the low impedance state each half-cycle of applied voltage, thereby to automatically restore the switching device (14) to normal operation immediately upon removal of the short, and is set to initially respond to a level of current sufficiently high to preclude switching in response to inrush currents. Initial switching of the circuit (18) to its high impedance state latches a reduced level detection circuit (R1-R2-Q4) in the transistor circuitry (18) whereby the latter switches to its high impedance state upon detecting lower overcurrent values for each successive half-cycle until the short is removed. The predictability of the response range for the circuit, especially in the reduced level voltage detection mode, is improved by placing a resistor (R5) in series with the control switching transistor (Q3) for the circuit.

대표청구항 ▼

An overcurrent protection circuit for limiting current flow to a solid state switching device connected in series with said circuit, a load controlled by said switching device, and a full wave rectified DC source, said circuit comprising, in combination: means rendering said circuit conductive in a

An overcurrent protection circuit for limiting current flow to a solid state switching device connected in series with said circuit, a load controlled by said switching device, and a full wave rectified DC source, said circuit comprising, in combination: means rendering said circuit conductive in a low impedance state upon application of DC power to said circuit; first detection means for sensing current in said circuit and causing said circuit to switch to a high impedance state when said current exceeds a first predetermined value and for connecting a second detection means in said circuit, said second detection means being responsive to a second predetermined current value less than said first predetermined current value for causing said circuit to switch to said high impedance state; means switching said circuit to said low impedance state at the end of each half-cycle of DC power applied to said circuit; and capacitor means for maintaining said second detection means connected in said circuit for subsequent successive half-cycles until said current does not exceed said second predetermined value.