The present invention relates to a method for protecting an electric circuit against interference voltage with a voltage cutter, and corresponding appar¬ atus comprising a fuse component and a voltage cutter. Electric circuits, such as integrated circuits, are often sensitive to variations of the operating volt¬ age and other interference voltages. A sudden overvolt- age can easily cause disturbances in the operation of the circuit, damage the circuit or even completely des¬ troy the circuit. Therefore electric circuits are often provided with overvoltage protection to prevent AC in¬ terference voltages and surge voltages and to secure the operation of the circuit.

In the prior art solution, overvoltage protec¬ tion is achieved by using a fuse component, such as a PTC resistor, and a voltage cutter. The voltage cutter must withstand high, long-lasting interference voltages. A voltage cutter has earlier been constructed for ex¬ ample by means of series connection of heavy-duty Zener diodes. The prior art solution is illustrated by an ex¬ emplary circuit diagram, which is shown in Figure 1. The figure shows a protected circuit 11, a fuse component PTC1 and a voltage cutter 10, which has a forward-dir¬ ected diode D01 connected in series, and four Zener diodes Z02, Z03, Z04, Z05 in reverse direction. With a solution like this there is the problem that when the interference voltage has persisted longer than a prede- termined period, the Zener diodes are destroyed. The number of Zener diodes used in the connection to prevent this has grown unreasonably high.

The object of the present invention is to pro¬ vide a method and apparatus, which can protect an elec- trie circuit against interference voltages simply and

reliably and which can also protect itself from being destroyed.

This is achieved with the method according to the invention, which is characterized in that the dura- tion and level of the interference voltage is monitored, and if the interference voltage persists longer than a predetermined period, the cutting voltage is lowered, and the remaining power in the voltage cutter is de¬ creased in proportion to the level of the interference voltage by cutting the input of the interference voltage to the voltage cutter and the protected circuit. The apparatus according to the invention is characterized in that the voltage cutter comprises means for monitor¬ ing the duration and level of the interference voltage and decreasing the level of the cutting voltage and re¬ ducing the remaining power in the voltage cutter in pro¬ portion to the level of the interference voltage so as to activate the fuse component when the interference voltage persists longer than a predetermined period. In the following the invention will be de¬ scribed in more detail with reference to examples ac¬ cording to the accompanying drawings, in which

Figure 1 shows the above described example of a prior art interference voltage protecting device, and Figure 2 shows the circuit diagram of an inter¬ ference voltage protecting device according to the in¬ vention.

According to the invention, the interference voltage protecting device can monitor the voltage over the protecting device, and if the interference voltage persists too long, the protecting device can cut the voltage to prevent the protecting device from being destroyed. In the following the operation according to the invention will be illustrated by means of an appar- atus according to the invention shown in Figure 2.

A fuse component PTC2 is series connected in the voltage line at the input of the protected circuit 21. A voltage cutter 20 is connected in parallel with the input. The voltage cutter comprises a forward-dir- ected diode D12 connected in series between the input of the protected circuit and the ground potential, a low ohmic resistor R13 and an NPN transistor Q6 in the col¬ lector-emitter direction. The base of the transistor Q6 is connected to the collector of a PNP transistor Q10 by way of the series connection of resistors R4 and R3, which collector is connected to the emitter of the tran¬ sistor Q6 and to the ground potential by way of the par¬ allel connection of a resistor R5 and a capacitor Cl. The base of the transistor Q10 is connected directly to the collector of the transistor Q6. The cathode of a reverse-directed Zener diode Zl is connected to the junction of diode D12 with resistor R13, and the anode to the junction of resistors R3 and R4. The cathode of the Zener diode Zl is also connected to the emitter of the transistor Q10 through a resistor R2.

When an interference voltage occurs in the input of the protected circuit 21, the voltage cutter

20 limits the voltage applied to the electric circuit

21 to a value determined by the Zener voltage of the Zener diode Zl and the base-emitter voltage of the tran¬ sistor Q6. If the interference voltage continues, the capacitor Cl is charged through the transistor Q10. When the voltage of the capacitor Cl exceeds the value of the base-emitter voltage of the transistor Q6, the tran- sistor Q10 becomes conductive and starts to feed base current to the transistor Q6. As a result of this, the transistor Q6 goes to a saturation state and the cutting voltage of the voltage cutter 20 and the remaining power in the cutter are reduced. Thus the remaining power in the fuse component PTC2 increases and the fuse component

cuts the incoming interference voltage. In this way the voltage cutter 20 and the Zener diode Zl therein are not destroyed even if the overload persists for an extended period. In a conventional voltage protecting device this would not be possible. The solution according to the in¬ vention can be used in all electronic devices in which protection against interference voltages is needed.

In the above the invention has been described by means of an example according to the accompanying drawing, but it is to be understood that the invention is not limited to that embodiment only, but it can be varied in many ways without departing from the scope defined by the attached claims.