US 3518489 A
Description (OCR text may contain errors)
June 30, 1970 J. R. MUSHAM 3,518,489
VOLTAGE SUPPRESSION CIRCUIT Filed April 5. 1968 T TREE. %R A FIG. 2
United States Patent 3,518,489 VOLTAGE SUPPRESSION CIRCUIT John Robin Musham, Lincoln, England, assignor to Associated Electrical Industries Limited, London, England, a British company Filed Apr. 3, 1968, Ser. No. 718,544 Claims priority, application (9 ;eat Britain, June 2, 1967,
Int. (31.110213/22, 9/04 US. Cl. 317-16 Claims ABSTRACT OF THE DISCLOSURE This invention relates to electrical circuit arrangements for protecting a load from damage due to over-voltage. It is frequently found that over-voltages in the form of transient spikes are superimposed on the voltage of a source particularly on an AC. source. These spikes may have a maximum voltage very much greater than the nominal voltage of the source and may damage a load fed from the source.
Some degree of suppression of transient voltages may be obtained by connecting a non-linear resistor across the supply to the load. A non-linear resistor is chosen to have a resistance which is high for applied voltages up to the nominal voltage of the supply but which falls rapidly as the applied voltage is raised above the voltage of the supply. Thus the greater the amplitude of the spike (in the absence of the resistor), the more heavily it is loaded by the resistor. The result is to limit the amplitude of the spikes to a value somewhat greater than the nominal value of the source voltage. This arrangement is satisfactory so long as the difference V between the limit level V, and the nominal voltage V of the source is fairly large, for example, in the order of 50% of the nominal voltage since it is then possible to choose a non-linear resistor which has a very low resistance for an applied voltage equal to or greater than V +V while having high resistance for an applied voltage equal to V The non-linear resistor thus draws little current from the source. If an attempt is made to limit at a voltage level only just in excess of the nominal voltage the resistor must have a relatively low resistance at the nominal voltage and it draws considerable current from the source..
The power loss appears as heat in the resistor and may be sufiicient to destroy the resistor.
Accordingly thegpresent invention provides an overvoltage protection circuit for connection in shunt with a load (which is to be protected against over-voltage), said circuit comprising a fixed resistor connected in series with a non-linear impedance which is such that its impedance falls sharply when the voltage thereacross exceeds a predetermined value, and a voltage breakdown device connected across said fixed resistor, the circuit being such that breakdown of said device occurs when the voltage across the circuit exceeds a given maximum permissible value, thereby causing the voltage applied to the non-linear impedance to exceed said predetermined value so that said circuit forms an efiectively low resistance protective shunt.
3,518,489 Patented June 30, 1970 ice The non-linear impedance is conveniently a resistor such as a sintered carbide resistor in which the current I therethrough varies with the voltage V thereacross according to the equation: I=kV where k is a constant and a is between 3 and 8. Such resistors are known under the trade name Metrosil.
When an over-voltage in the form of a spike is applied across the circuit, the breakdown device breaks down and is rendered conductive, thereby connecting the nonlinear resistor directly across the supply. When the supply is an alternating voltage, the breakdown device once rendered conductive is automatically extinguished at the next current zero of the source, supply, or when the holding current is reached. The Metrosil resistor draws current only during a spike and the remainder of the half cycle during which the spike occurs and the Metrosil therefore may be chosen to have a resistance/voltage characteristic which permits limiting at a voltage only slightly in excess of the nominal voltage of the supply.
In order that the invention may be more readily understood it will now be described, by way of example, with reference to the accompanying drawing, in which:
FIG. 1 is a circuit diagram of an arrangement in accordance with one embodiment of the invention; and
FIG. 2 is a circuit diagram of a voltage suppression circuit which is an alternative to the one shown in FIG. 1.
Referring to FIG. 1 a fixed linear resistor R is connected electrically in series with a non-linear Metrosil resistor M across a load L which is fed from an AC. source S through supply lines 1 and 2. The load L may include semi-conductor devices which have little capacity to withstand over-voltage. A semi-conductor breakdown device in the form of a thyristor T is connected in shunt with the resistor R. There is no firing or triggering circuit provided for the thyristor T which breaks down when the voltage developed across the resistor R exceeds the forward blocking voltage of the thyristor. For a supply voltage of 240 volts R.M.S. a thyristor T having a forward blocking voltage of the order of 200 volts could, for example, be used.
When the thyristor T is not conductive, resistor R and the non-linear Metrosil resistor M form a potentiometer across the supply. The value of resistor R is chosen so that the voltage V developed across it when the supply voltage is at its nominal peak value V is just less than the thyristor breakdown voltage. The voltage across the Metrosil resistor M under these conditions is arranged to be less than half V and the resistance of the resistor M will then be so high that the power dissipated in the series connected resistor M and resistor R is small.
When, however, the supply voltage is such that the voltage developed across tthe resistor R causes the thyristor T to break down the full supply voltage is applied to the Metrosil resistor M, which then has a low resistmice. The Metrosil resistor M and the thyristor T then provide a low resistance path in shunt with the load L to absorb the over-voltage and thereby protect the load.
The arrangement shown in FIG. 1 employing a single thyristor T is effective in protecting the load only against over-voltage spikes of one polarity and in FIG. 2 a protection circuit employing a semi-conductor bi-directional breakdown device is shown which is capable of suppressing spikes of either polarity applied across the circuit. The bi-directional device may be a Triac TR or a two-way switching bridge circuit constituted by thyristors or thyristors and diodes.
1. An overvoltage protection circuit comprising in combination:
a fixed resistor,
a non-linear impedance having an impedance which falls sharply when the voltage thereacross exceeds a predetermined value,
means connecting said fixed resistor and said nonlinear impedance in series, and
a voltage breakdown device connected in shunt across the fixed resistor,
the circuit being such that said voltage breakdown device breaks down when the voltage across the series connected fixed resistor and non-linear impedance exceeds a given maximum permissible value, to cause the voltage across the non-linear impedance to exceed said predetermined value and thereby to form a low resistance protective shunt.
2. A circuit as claimed in claim 1 wherein the nonlinear impedance comprises a resistor such that the current therethrough is proportional to the ath power of the voltage thereacross, where a is between 3 and 8.
3. A circuit as claimed in claim 1 wherein the voltage breakdown device comprises a semiconductor device.
4. A circuit as claimed in claim 1 wherein the voltage breakdown device is bi-directional, that is, it breaks down when the voltage thereacross exceeds a breakdown value in either direction.
5. A circuit as claimed in claim 1 in combination with a load to be protected and power supply means across which the load is connected, said series-connected fixed resistor and said non-linear impedance being connected in shunt 'with the load.
References Cited UNITED STATES PATENTS 3,246,206 4/1966 Chowdhuri 31716 X 3,418,530 12/1968 Chcever 317-16 3,435,293 3/1969 Bodge 317--31 JAMES D. TRAMMELL, Primary Examiner US. Cl. X.R.