DE1763292A1 - Overvoltage protection circuit - Google Patents

Description

ASSOCIATED SIEGTRICALnTDUSTRIES Ltd., London, England

Overvoltage protection circuit

The invention relates to an overvoltage protection circuit which connected in parallel to a consumer. The ijchutζ circuit is intended to prevent the consumer from being destroyed by an overvoltage.

The output voltage of a voltage source, in particular an alternating current source, is often in the form of overvoltages pointed needle pulses superimposed. The peak voltage of these pulses can be much greater than the nominal voltage of the Voltage source and one fed from the voltage source Destroying consumers. "

The otorimpulse can be suppressed to a certain extent by connecting a non-linear resistor in parallel to the consumer. Then the resistance of the non-linear resistor is chosen so that it is large at voltages whose magnitude does not exceed the nominal voltage of the supply voltage and drops sharply when the applied voltage exceeds the value of the supply voltage. The greater the amplitude of the interference pulse (without the resistor), the more it is loaded by the resistor. As a result, the amplitude of the interference pulses is limited to a ⁽ 'ert which is slightly larger than the nominal value of the supply voltage. This arrangement is satisfactory as long as the difference V ^ between the limit value V- and the nominal value V_ " _v . Of the supply voltage is fairly large , for example in the order of 50 fo of the nominal value of the supply voltage , because

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it is then possible to choose a non-linear resistor whose resistance is very small when the voltage applied is equal to or greater than V + V., and has a large / v'ert, when the applied voltage is V. The non-linear resistance then only draws a small current from the ' Source. However, if you want to limit to a voltage value that is only slightly above the nominal voltage, must the resistor have a relatively low resistance at the nominal voltage, so that it can draw a large current from the ^ Source draws. The power loss is converted into heat in the resistor and can be sufficient to destroy the resistor.

According to the invention, this difficulty is eliminated in that the circuit includes a fixed resistor, which is in series with a non-linear resistor, the impedance of which drops when the voltage applied to it exceeds a predetermined value, and which contains a voltage-dependent breakdown device: which lies on the fixed resistor, and that the resistance of the breakdown device collapses when the voltage applied to the circuit voltage exceeds a predetermined maximum permissible value, thereby causing the power supplied to the non-linear resistor w voltage ert the predetermined 'exceeds such that the Circuit forms a low-resistance branch in parallel with the consumer.

The non-linear resistor is preferably an (ohmic) resistor, for example a resistor made of sintered carbide, in which the current I changes as a function of the applied voltage V according to the relationship I = k · V ^a , where k is a constant is and a is between three and eight. Such resistors are known under the trademark "Metrosil".

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When a pulsed overvoltage is applied to the circuit the resistance of the breakdown device collapses, so that it becomes conductive and applies the non-linear resistance directly to the supply voltage. When the supply voltage is an AC voltage, the breakdown device will when once it is conductive, the next time the AC supply voltage passes through zero or automatically deleted or blocked when the holding current is reached. The Metrosil resistor pulls in only for the duration of one pulse and the rest of the half-wave the control pulse occurs, current, so that the resistance-voltage characteristic of the Metrosil resistor can be chosen so that it is possible to limit the voltage to a value that is only slightly above the nominal value of the supply voltage lies.

The invention will now be explained with reference to the drawings of two exemplary embodiments described in more detail. .

Fig. 1 is the circuit diagram of one and

Fig. 2 shows the circuit diagram of the other embodiment.

In the embodiment according to FIG. 1, a fixed linear resistor R is connected in series with a non-linear Metrosil resistor M. at a consumer L, which is from an alternating current source S. Power supply lines 1 and 2 is fed. The consumer L can Contain semiconductor components that are sensitive to overvoltages are. A semiconductor breakdown device in the form of a thyristor T is parallel to the resistor R. Pur the Thyristor T is no ignition or trigger circuit provided because the resistance of the thyristor T collapses when the The voltage drop across the resistor R is the (positive) reverse voltage of the thyristor in the forward direction exceeds. At a

109883/0390

BATHROOM ÖBK51NAL

Supply voltage with an effective value of 240 volts can for example a thyristor with a positive reverse voltage of about 200 volts can be used.

When the thyristor T is blocked, the resistor R and the non-linear luetrosil resistor M form a voltage divider connected to the supply voltage source S. The value of the resistor R is chosen so that the voltage V ₁₃ drop across it, when the supply voltage has its peak value V, is only slightly smaller than the thyristor breakdown voltage. Under these circumstances, the voltage drop across the Metrosil resistor M is less than half of V, and the resistance of the resistor M is then so great that the power dissipation converted in the series-connected resistors M and R is small.

If, however, the supply voltage has such a value that the voltage dropped across the resistor R exceeds the resistance of the thyristor T collapses, the full supply voltage is applied to the Metrosil resistor M, which then has a low resistance. The resistor M and the thyristor T then form a low-resistance parallel branch to the consumer L, so that the overvoltage short-circuited and the consumer is thereby protected.

The arrangement shown in Fig. 1 with only a single thyristor T protects the consumer only against overvoltage peaks of one polarity, and in Fig. 2 there is a protective circuit with a bidirectional semiconductor breakdown device shown, the interfering impulses of both polarities can. The bidirectional device can be one Triac TR or a two-way switching bridge circuit made of thyristors or thyristors and diodes.

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6AD OR (OlNAL

Claims (4)

- "170292 claims 1. Overvoltage protection circuit that is connected in parallel to a consumer, because you rc hg ekenn ζ ei chn et that the circuit contains a fixed resistor (R) which is in series with a non-linear resistor (M) whose impedance drops steeply when the voltage applied to it exceeds a predetermined value, and which contains a voltage-dependent breakdown device (T, TR) which is connected to a fixed resistor (R), and that the resistance of the breakdown device (T; TR) collapses when the on the circuit applied voltage exceeds a predetermined maximum permissible value, which has the effect that the voltage supplied to the non-linear resistor (M) exceeds the predetermined value in such a way that the circuit forms a low-resistance branch parallel to the consumer (L). 2. Circuit according to claim 1, characterized in that g e k e η η drawing η e t, that the current-voltage characteristic of the nonlinear resistance (M) runs according to the relationship I = kV, where k is a constant and a is between three and eight. 3. Circuit according to claim 1 or 2, characterized g e k e η η ze i chn et that the breakthrough device a is known semiconductor device. 4. Circuit according to one or more of claims 1 - 3 » thereby ge ke η η ζ eich η et that the breakdown device (TR) is a well-known bidirectional switching device whose blocking resistance breaks down when a voltage exceeding the maximum permissible value in any direction (positive or negative) is applied to the circuit. 109883/0390 Blank page