Abstract:
A protection device, for a telephone (or other load) 27 in series with an external protection transistor 2, includes a sensor 6 for detecting common-mode current into the load. The output of the sensor 6 is connected to affect a current source/sink combination, and imbalance in this source/sink combination produces a voltage shift which is indirectly connected to control the protection transistor.

Description:
This is a continuation of Ser. No. 08/225,495, filed Apr. 11, 1994 now abandoned, which is a continuation of Ser. No. 07/780,615, filed Oct. 23, 1991 now abandoned and claims priority therethrough from Italian application 21859 A/90, filed Oct. 24, 1990. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to a device for limiting the power dissipated within an electrical and/or electronic circuit connected to a load, such as a telephone. 
     In circuits of the aforesaid type, the problem frequently arises of limiting the power dissipated within them in order to limit for example the heating of their components. 
     In the case of telephone circuits, various devices are available for achieving this limitation. 
     One of these comprises usual means for limiting the maximum current which can be supplied to the line plus a thermal protection circuit which limits the maximum temperature attainable within the circuit connected to the load. 
     Although this device functions properly during normal operation of the telephone circuit, it cannot operate effectively when the load is subjected to a longitudinal or common-mode current. 
     In this respect, when the load absorbs a common-mode current from the feed line, devices of the aforesaid type are no longer able to perform their function because the operating threshold of the said limiting means is usually higher than the current passing through the telephone in this situation. 
     However, the thermal protection circuit continues to operate, and when it does it changes time operating characteristics of the protected circuit to the extent that its operation becomes unacceptable. 
     To overcome these drawbacks, it is known to couple the load-connected circuit to an external component within which part of the power absorbed by the circuit is dissipated. 
     Hence in the case of the said known devices, the power dissipated within the circuit is controlled indirectly by controlling the power dissipated within said external component. 
     This method enables good results to be obtained during normal operation of the telephone, but is problematic when the load to which the telephone circuit is connected becomes the centre of a common-mode current. 
     This is because the known said devices again in this case suffer from the aforesaid drawbacks, and in particular do not enable the temperature increase in the said external component (caused by the increase in the power dissipated within this component) to be limited. 
     An object of the present invention is therefore to provide a protection device for an electrical and/or electronic circuit, in particular a telephone circuit of the type connected to an external load, which is able to intervene and effectively protect said circuit (and the load connected to it) under any operating condition. 
     A particular object of the invention is to provide a device of the aforesaid type which is also able to intervene and effectively protect said circuit when the load is subjected to a longitudinal or common-mode current. 
     A further object is to provide a device of the aforesaid type which is of simple construction and low cost. 
     A further object is to provide a device of the aforesaid type which also very reliably protects the external component or components associated with the said circuit. 
     These and further objects which will be apparent to the expert of the art are attained by a device of the aforesaid type, characterised by comprising sensor means able to determine a common-mode current absorbed by the load, and to control generator means arranged to generate a current signal, the signal emitted by these latter varying in accordance with the determination effected by the sensor means and being at least partly absorbed by absorber means controlled by a control unit connected to measurement means measuring the current passing through the line containing said load, said absorber means being connected at one end to a point downstream of the load, said absorber means, on determination of a common-mode current passing through the load, varying the absorption of the signal emitted by the generator means, to produce a variation in the potential difference across its ends and hence a variation in the potential at the load, this variation limiting the current passing through this latter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be more apparent from the accompanying drawings, which is provided by way of non-limiting example and in which: 
     FIG. 1 is a block diagram of one application of the device according to the invention; 
     FIG. 2 is a schematic representation of one embodiment of the device shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a circuit 1 for feeding a load 27, to which an external component 2 is connected in order to at least partly dissipate the power dissipated by said circuit. The circuit 1 and component 2 are connected between two known feed lines 3 and 4. 
     The circuit 1 is connected to a device for limiting the power dissipated within it and within the component 2; this device comprises a control unit 5 which measures the current passing through said circuit, said unit 5 being connected to sensor means 6 which determine the current absorbed by the load 27 and operate on means 7 for generating a current signal In proportional to the acceptable limiting current passing through the circuit 1. This limiting current is the maximum allowable current through said circuit for the power dissipated within this latter (and within the component 2 connected to it) to remain within values which are not dangerous for the circuit components. 
     The means 7 are connected to a ground 8 by a branch 9 comprising a diode 10. This branch extends from a node 11 from which a further branch 12 extends to terminate in means 13 able to at least partly absorb the current I L  generated by the means 7. A further diode 14 is contained in the branch 12. 
     Finally, the absorber means 13 (the function of which is described in detail hereinafter) is connected to the unit 5 and to a node 15 located downstream of the circuit 1, between this latter and the component 2. 
     Specifically (see FIG. 2), the circuit 1 comprises a branch 20 extending from the line 3 and terminating at the non-inverting input of a first voltage follower 21. The branch 20 contains a usual battery 22. 
     A branch 24 comprising a resistor 25 and a diode 26 extend from the follower 21. The branch 24 terminates in an active two-terminal network or load 27 from which a further branch comprising a diode 29 and a resistor 30 extends. 
     A second branch terminates in a second voltage follower 32, the non-inverting input of which is connected to a usual battery 33. 
     The other terminal of battery 33 is connected to the said node 15, to which the component 2, which in the illustrated example is a transistor 34, is also connected. 
     Operationally associated with the two voltage followers 21 and 32 there are two usual current measurement means (of known type) 37 and 38. These latter are connected to the control unit 5. 
     As stated, the unit 5 is connected to the sensor means 6. These determine a common-mode current by comparing a signal V K  from the unit 5 with a threshold signal V S . To achieve this, these means comprise in the example illustrated in FIG. 2 a comparator 39 (advantageously of the type with hysteresis) connected at one input to a branch 40 extending from the unit 5, the other input receiving the threshold signal V S . 
     The comparator 39 is connected via an output branch 41 to the means 7, which in the illustrated example are a current generator 42. This latter is connected at one end to a point 63 of positive voltage, and as stated is of the type which generates a variable-amplitude current signal determined by the output state of the comparator 39. 
     The generator 42 is connected via the branch 12 to the absorber means 13, which in the illustrated example is a current generator 44. 
     This latter is able to absorb in known manner the current generated by the generator 42. The characteristics of the generator 44 are such that it generates during normal operation of the circuit 1 a current which is the image of the current passing through the load 27. 
     As stated, this latter generator is connected to and controlled in its operation by the control unit 5. This is achieved via a branch 47. 
     Specifically, in the example illustrated in FIG. 2, the generator 44 is connected at one end to a node 49 from which a branch 50 extends to terminate at the non-inverting input of a voltage follower 52. 
     This latter is connected to the inverting input of an amplifier 53 via a branch 54 containing a resistor 55. 
     The amplifier 53 has its non-inverting input connected to a voltage divider 56 and its output connected to the base 57 of the transistor 34. 
     Finally, a branch 59 containing a resistor 60 extends from the inverting input of the amplifier 53 to terminate at the node 15. 
     It will be assumed that the circuit 1 is applied to a telephone, and that this circuit together with the device of the invention are to be used. 
     During normal operation of the telephone, a usual transverse current flows from the line 3 to the line 4 via the branches 20 and 24, the load 27, the branch 28 and the transistor 34. 
     The measurement means 37 and 38 measure this current and feed their data to the unit 5. 
     This latter causes the generator 44 to generate a current equal to said transverse current, which it absorbs at the same time. The generator 42 is controlled by the comparator 39, which is itself controlled by the unit 5. This comparator does not act on the generator 42, which remains at a predetermined value, until the unit 5 feeds a signal V K  of amplitude greater than V S , i.e. a common-mode signal. 
     At the node 11 the current I L  divides into two parts, namely a part flowing to the ground 8 and a part flowing to the generator 44. 
     A small portion of current also flows to the amplifier 53 to control the transistor 34. This latter portion is of very small value because of the presence of the voltage follower 52, which has a high input impedance. 
     It will now be assumed that the load 27 is subjected to a common-mode signal, for example because the user, while telephoning, presses the usual earth pushbutton provided on the telephone. 
     In this situation the unit 5 determines via the measurement means 37 and 38 the presence of a common-mode current through the load 27. 
     This unit also feeds a signal V K  to the comparator 39 proportional to the amplitude of the determined common-mode current, to reduce time current I L  generated by the generator 42 and at the same time enable the generator 44 to absorb greater current from the node 12. 
     In this latter case, said generator tends to absorb increasingly more current from this node until substantially all the (reduced) current I L  leaving the generator 42 flows into the generator 44. As a result of this there is a considerable voltage drop across this latter generator. 
     This voltage drop is sensed at the node 49 to produce, via the follower 52 and the branch 59 (connected to the inverting input of the amplifier 53), an increase in a voltage at the node 15, or an increase in a regulator voltage V REG  at the transistor 34. 
     This voltage increase produces a corresponding voltage increase at the output 32A of the follower 32. 
     As a result of this, the current passing through the load 27 falls below the reduced I L . 
     This results in a limitation in the current passing through the load 27 and transistor 34, in which there is therefore a reduction in the dissipated power. The regulating loop, defined substantially by the unit 5, the comparator 39 and the generators 42 and 44, therefore enables said current to be maintained at a value such that the power dissipated within the circuit 1 and external component 2 is less than that which would represent a danger to their stability. 
     A device constructed in accordance with the invention allows direct control of the power dissipated within the circuit 1 when a common-mode current absorbed by the load 27 is present. 
     The device is of reliable operation and very simple construction. 
     It also requires no thermal protection circuit for the circuit 1 and external component 2. 
     One embodiment of the invention has been described, however different embodiments are possible. 
     For example, as a modification the direction of the currents emitted by the generators 42 and 44 can be suitably varied to simplify the circuit part between the node 49 and the transistor, 
     This modification and other embodiments equivalent to that described fall within the scope of the present document.