The invention concerns a method for indicating an overload in a switching element by measuring the overload of a switching element that is provided with a control switch and supplies a signal path.

In known solutions, an overload current in direct current switches is not examined until after the switch, i.e. the voltage of the supplied conductor is examined. High voltages are then unavoidable, and there¬ fore separate interface circuits have to be used to enable processing of information about an overload current in the switch e.g. by means of microprocessor logic.

It is known to indicate an overload current e.g. by current mirrors or voltage division couplings in connection with comparators, implemented with resist¬ ances. Comparators have been used to examine the current supplied from a current mirror or voltage dividers; a comparator has indicated a possible overload current in the switch. Direct current switches are often required to have a low ohmage, whereby the voltage drop in the coupling is also too small for current mirror measure- ment. In other words, current mirror measurement cannot always be used.

The aim of the present invention is to provide a new kind of method, in which the problems of the known solutions are eliminated. The aim is achieved with the method of the invention, which is characterised in that to detect an overload, the current passing through the control switch of the switching element is examined, and that on detecting a change in the current passing through the control switch of the switching element that supplies

the signal path, a control signal controlling the over¬ load current is generated from the main current path of the control switch to an indicator switch, which generates an indicator signal indicating an overload current.

The idea underlying the method of the invention is that an overload is measured on the low voltage side by measuring the circuit controlling the switching elements instead of measuring the supplied conductor located after the switch. The idea is that a change in the current of the circuit controlling a switching element or switching elements is utilized in detecting the overload of the switching elements.

Several advantages are achieved with the method of the invention since it is possible to use low voltage components in the arrangement according to the inven¬ tion. In the new solution, no separate interface circuits are needed for a microprocessor connection, whereby an overload current can be indicated directly on the logic level. The new solution also operates independently of variation in the criterion voltage, i.e. the voltage that is switched on by a direct current switch and loads the switch. Unlike in a voltage divi¬ sion coupling, no resistances, which in a breakdown would reduce resistance measured against ground poten¬ tial, are needed for measuring the overload, and so the amount of leakage current is reduced. The number of components required by the solution of the invention is small. The invention also concerns an arrangement for indicating an overload in a switching element, the arrangement comprising a solid-state switch that supplies the signal path and comprises a control elec¬ trode and a main current path electrode; a means limit- ing the current of the control electrode, the means

being arranged between the control electrode and the signal path; and a control switch connected to the con¬ trol electrode of the switching element, the control switch comprising a control electrode and main current path electrodes.

To eliminate the disadvantages of the known methods, the arrangement is characterised by further comprising a resistance arranged in the main current path of the control switch and an indicator switch, which comprises a control electrode and main current path electrodes, the control electrode being connected to a resistance arranged in the main current path of the control switch.

In the following the invention will be described in greater detail with reference to the attached drawing, in which

Fig. 1 shows a circuit diagram of an overload current indicator according to the invention.

In accordance with Fig. 1, the arrangement for indicating an overload in a switching element comprises a solid-state switch Q4, Q5 supplying a signal path A ( -VCRN potential) . The -VCRN potential represents the criterion voltage switched on by the switching elements Q4, Q5. The criterion voltage may vary e.g. from -20 volts to -70 volts. The solid-state switches are e.g. npn transistors comprising a base B, an emitter E and a collector C. In respect of operation, no more than one transistor, e.g. Q4, is needed. The solid-state switch Q4 comprises a control electrode 1 and main current path electrodes 2 and 3, i.e. a base B, a collector C and an emitter E. Correspondingly, the second switching element or transistor Q5 comprises a control electrode 4 and main current path electrodes 5 and 6, i.e. a base B, a collector C and an emitter E. The arrangement further comprises a means, such as a zener diode Z3, limiting

the current of the control electrodes 1 and 4, the means being arranged between the control electrode 1 and signal path A or, respectively, the control electrode 4 and the signal path A. In parallel with the zener diode Z3 is connected a resistance R55. Between one main current path electrode 3, i.e. emitter, of the switching element Q4 and the signal path A is connected a resist¬ ance R33; and correspondingly, between one main current path electrode 6, i.e. emitter, of the switching element Q5 and the signal path A is connected a resistance R34. The collectors or electrodes 2 and 5 of the switching elements Q4, Q5 have ground potential.

The arrangement further comprises a control switch Q3 connected to the control electrode 1 or 4, i.e. base B, of the switching element Q4 or Q5, respect¬ ively, the control switch comprising a control electrode 7 and main current path electrodes 8 and 9. Advantage¬ ously, the control switch Q3 is also an npn transistor with a base B, a collector C and an emitter E. The control electrode 7 of the control switch Q3 is connected through a resistance R53 to a control point 10, which is in practice ground potential. Between the emitter E or electrode 9 and the base B or electrode 7 of the control transistor Q3 is connected a high ohmage resistance R30. Between the control electrodes 1 and 4 or bases B of the switching elements and the collector C or electrode 8 of the control switch controlling them is arranged a resistance R54, which provides the voltage difference needed between the switching elements and the control switch.

The arrangement further comprises resistances R52 and R29 arranged in the main current path of the control switch Q3. The main current path or the col¬ lector-emitter circuit of the control switch Q3 is connected to V5P potential (5 volts, positive) at a

point 11. The arrangement further comprises an indicator switch Q12, which comprises a control electrode 12 and main current path electrodes 13 and 14. The control electrode 12 of the indicator switch is connected to the resistance R52 arranged in the main current path or emitter circuit of the control switch Q3. The indicator switch Q12 is advantageously a pnp transistor, one main current path electrode 14 or emitter E of the transistor being connected to V5P potential at a point 15. The base B or control electrode 12 of the transistor or indicator switch Q12 is connected to the resistance R52 of the emitter circuit of the control transistor Q3. One main current path electrode 13 or collector of the indicator switch Q12 is connected through a resistance R51 to ground potential. One end of the resistance R51, and thereby the collector C of the indicator transistor Q12, is connected through a line 16 to a microprocessor 17, in which a possible overload current in indicated. In other words, the overload current is indicated directly on the logic level or in the microcircuit 17 with a voltage of 5 volts.

The switching elements or transistors are controlled by control current I generated from the control switch Q3. Q3 in turn can be activated by a control signal from the point 10 by connecting the control to zero or ground potential. In a normal situ¬ ation, i.e. with no overload current, current is generated from the point 11 or the positive V5P poten¬ tial through Q3; said constant current is supplied to the bases 1 and 4 of the transistors Q4 and Q5, respect¬ ively. The collector current of Q3 is thus used for controlling the switching elements Q4 and Q5.

When there is an overload, the zener diode Z3 limits the current at the transistors Q4 and Q5 in such a way that when the voltage is sufficiently high in R33

and R34, the zener diode Z3 starts to suck and limit the control current flowing to the base. The threshold volt¬ age of the zener diode Z3 is 3.3 volts. When base current is generated from the switching elements or switching transistors Q4 and Q5 through the zener diode Z3 to the signal path A, the current passing through the main electrodes 8 and 9 of the control transistor Q3, i.e. the current in the collector-emitter circuit, increases since the potential of the switching tran- sistors Q4 and Q5 at the point between their bases B in the electrodes 1 and 4 turns more negative because of the zener diode Z3. When the voltage at the bases 1 and 4 turns more negative, the current passing through the control transistor Q3 increases and a point 18 between the resistances R52 and R29 turns more negative. The voltages then become such that the base B or electrode 12 of the indicator transistor or indicator switch Q12 receives control current, i.e. opens, whereby it connects R51 to positive potential 15, and the micro- processor 17 receives information that the output is '1', i.e. there is an overload.

In a preferred embodiment, the resistances R52, R29 and R51 are such that when the control point 10 is inactive, no current passes through the control tran- sistor Q3, whereby the voltage at the point between R52 and R29 cannot fall sufficiently low to activate the base 12 of the indicator transistor Q12 and thereby the microprocessor 17 through the line 16.

An overload is thus detected from the resist- ance R52 in the emitter circuit or main current path of the control transistor Q3 controlling the base current of the switch Q4, Q5. An overload is detected as an increase in the current of the controlling circuit.

The arrangement is employed in the method of indicating an overload current in the switching element

Q4, Q5. In the method is measured an overload of the switching element Q4, Q5 provided with a control switch Q3 and supplying the signal path A. To detect an over¬ load, the current passing through the control switch Q3 of the switching element Q4, Q5 is examined, and when a change is detected in the current I flowing through the control switch Q3 of the switching element Q4, Q5 that supplies the signal path A, a control signal K for controlling the overload current is generated from the main current path or emitter circuit of the control switch Q3 to the base 12 of the indicator switch Q12. An indicator signal L indicating an overload is gener¬ ated from the indicator switch through the line 16 to the microprocessor 17. In a preferred embodiment of the invention, a change in the current passing through the control switch Q3 of the switching element Q4, Q5 is brought about by passing current by a current limiter, such as a zener diode Z3, to the other side of the switching element Q4, Q5. This is an effortless way of generating a signal for detecting an overload. Also, in a preferred embodiment an indicator signal L is provided on the basis of the voltage U(R51) provided over the resistance R51 arranged in the main current path of the indicator switch Q12, the voltage being switched on to the indicator switch Q12 by the control signal K for controlling the overload current generated from the main current path of the control switch Q3. In a preferred embodiment of the method, a control signal K is provided at the base 12 of the indicator switch or indicator transistor Q12 on the basis of a change in voltage U(R52) provided over the resistance R52 arranged in the main current path of the control switch Q3 of the switching elements Q4, Q5.

In a preferred embodiment of the arrangement of the invention, a high ohmage resistance R51 is

arranged in the main current path of the indicator switch Q12, the voltage U(R51) provided over said resistance as measured at the point 15 can be switched on by the indicator switch Q12 when it has received a control signal K from the control switch Q3. On account of high ohmage, voltage drop in the resistance R51 is sufficient to generate a sufficient signal to the line 16. The resistances R52 and R29 arranged in the main current path of the control switch Q3 have a low ohmage, in order that the emitter circuit of the control tran¬ sistor Q3 might operate in a correct manner.

The solution of the invention can be used e.g in constant current signal devices, whereby the signal path A represents a line to which signal impulses are to be generated. In Fig. 1, constant current switches Q4, Q5 connect ground potential to the signal path A.

Although the invention is described above with reference to the embodiments illustrated in the attached drawings, it is to be understood that the invention is not limited thereto but that it can be modified in many ways within the scope of the inventive idea disclosed in the attached claims.