Overcurrent protection circuit

Claim 2

In a signaling system in which a line driver supplies a signaling current to a communications line, means to protect said line driver from damage due to a short circuit on the line, comprising a monostable circuit having an input circuit coupled to said line driver to be triggered into a relatively long timed state when the current through the driver exceeds a safe threshold value, and having an output circuit coupled to said line driver to inhibit conduction in the line driver during the timed state of said monostable circuit, whereafter excessive conduction can resume in said line driver for only a very short time before the monostable circuit is again triggered and inhibits the line driver for a relatively long time.

Claim 3

Means as defined in claim 1 wherein said line driver includes a resistor effectively in series with the current path to said communications line.

Claim 4

Means as defined in claim 2 wherein said monostable circuit includes a first normally non-conducting transistor having input electrodes coupled across said resistor to render the transistor conductive when the current through said line driver exceeds a safe threshold value.

Claim 5

Means as defined in claim 3 wherein said monostable circuit includes a second normally non-conducting transistor coupled to the first transistor to be rendered conductive by conduction in the first transistor.

Claim 6

Means as defined in claim 4 wherein said second transistor includes an output circuit coupled to said line driver to inhibit conduction in the line driver when the second transistor is conductive.

Claim 7

In a system which includes a transmission line comprising two conductors, a driver transistor connected to one of the lines for supplying a signaling current thereto and a driver transistor connected to the other of the lines for supplying a signaling current thereto, and in which, when there is a short on the line, excessive current may flow through the one of these transistors which is on, a circuit for protecting this transistor comprising, in combination: a monostable circuit of the type which switches from its stable state to a second state in response to an enabling signal and which returns to first state after a given time interval, the given time interval being much longer than the time, after the occurence of an enabling signal, for the monostable circuit to switch to its second state; means responsive to a flow of current of greater than a given amplitude through the one of said transistors which is on for supplying an enabling signal to said monostable circuit; and means responsive to said monostable circuit, when the latter is in its second state, for turning off the one of said transistors which is on.

Claim 8

In a system which includes a transmission line comprising two conductors, a driver transistor connected to one of the lines for supplying a signaling current thereto and a driver transistor connected to the other of the lines for supplying a signaling current thereto, and in which, when there is a short on the line, excessive current may flow through the one of these transistors which is on, a circuit for protecting this transistor comprising, in combination: a monostable circuit of the type which switches from its stable state to a second state in response to an enabling signal and which returns to first state after a given time interval, the given time interval being much longer than the time, after the occurence of an enabling signal, for the monostable circuit to switch to its second state; means responsive to a flow of current of greater than a given amplitude through the one of said transistors which is on for supplying an enabling signal to said monostable circuit; and means responsive to said monostable circuit, when the latter is in its second state, for turning off the one of said transistors which is on.