GB1574805A - Methods of protecting electrical circuits - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO METHODS OF PROTECTING ELECTRICAL CIRCUITS (7) We, INTERNATIONAL COMPU TERS LIMITED, a British Company, of ICL House, Putney, London S.W.15, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statment: This invention relates to the protection of electric circuits and components from excess current conditions.

It is sometimes necessary to prevent excessive current flowing in a circuit on initial energisation of the circuit. For example this arises in power supply circuits in which the alternating current is applied to the input of a rectifier and the output from the rectifier consists of a train of unidirectional pulses having the form of half sinewaves and, in order to provide a substantially uniform DC output voltage, the train of pulses is smoothed by providing a reservoir capacitor connected across the rectified output. When the alternating current input is applied initially, the capacitor is uncharged and hence a relatively large current passes through the rectifier into the capacitor to charge the capacitor to a voltage substantially equal to the peak of the pulses passing through the rectifier. During operation of the power supply, the capacitor is charged by each half cycle pulse and is discharged by current drawn by a load supplied by the power supply circuit. Hence the DC output has superimposed on it a ripple waveform of which the amplitude is determined by the amount by which the capacitor is discharged. Therefore in order to obtain a DC output with a relatively small ripple component, a capacitor is required which has a capacitance value sufficiently high in relation to the current drawn from the capacitor by the load, that only an acceptable decrease of voltage across the capacitor occurs between each charging pulse. When it is desired that the power supply circuit be capable of providing a substantially ripple free DC output voltage at high values of current it becomes necessary to use a relatively large value of capacitance in the reservoir capacitor and consequently when the alternating current input is applied, a very large current is drawn through the rectifier by the initially uncharged reservoir capacitor. The rectifiers commonly used are semi-conductor devices and these have a low internal forward impedance. In consequence the circuit contains no impedance to limit the value of the initial charging currents and the current easily may attain a value which is sufficiently high to interfere with the functioning of other equipment using the same mains supply circuit. In order to limit the initial "switch-on" current a resistor may be wired in series with the rectifier and after the initial charging of the capacitor the resistor is shorted by a switch. The switch is arranged to be operated automatically after termination of the initial charging of the capacitor but if a fault should occur and the switch fails to operate the resistor is left in circuit and will carry the current drawn by the load connected to the power supply circuit. Under these conditions the resistor will dissipate a large amount of heat and may be a fire hazard.

It has been proposed to use a resistor having a negative temperature coefficient so that the resistor provides a high series resistance initially which then decreases, due to current flow heating the resistor, to a relatively low resistance for steady state running. This permits the shorting switch to be omitted but even this resistor may become a fire hazard in the event of a fault occurring within the power supply.

According to a first aspect of the invention a method of preventing the drawing of excessive current from a current supply by a circuit including a capacitor connected across the supply to be charged thereby, consists of providing a resistor in series with the current supply to the capacitor circuit so that upon initial energisation of the circuit the resistor limits the initial energisation current to a desired value and shorting said resistor after the initial energisation for the duration of subsequent operation of the circuit, the resistor having a characteristic whereby as its temperature rises its resistance increases at an increasing rate so that, in the event of failure to short the resistor, the continuing current flow through the resistor raises its temperature to thereby increase its resistance to a value sufficient to limit the current flow therethrough to a desired steady state value.

According to a second aspect of the invention apparatus operative to prevent the drawing of excessive current from a current supply by a circuit including a capacitor connected across the current supply to be charged by the current supply, includes a resistor connected in series with the current supply to the circuit so that upon initial energisation of the circuit and the capacitor the resistor limits the initial energisation current to a desired value; and shorting means effective to short said resistor after the initial energisation for the duration of subsequent operation of the circuit; the resistor having a characteristic whereby as its temperature rises its resistance increases at an increasing rate so that, in the event of failure of the shorting means, the continuing current flow through the resistor raises its temperature to thereby increase its resistance to a value sufficient to limit the current flow therethrough to a desired steady state value.

An embodiment of the invention will now be described with reference by way of example to the accompanying drawing which is a circuit diagram of a power supply circuit.

Referring to the drawing, an AC voltage input is applied to a bridge rectifier 1 from terminals 2, 3 via a resistor 4. A DC output is obtained from the bridge rectifier 1 at terminals 5. 6. A reservoir capacitor 7 is connected across the DC output between the terminals 5. 6.

Upon energisation of the circuit by the AC voltage, the bridge rectifier 1 passes a train of unidirectional half sinewave pulses to the output terminals. These pulses initially charge the capacitor 7 to a voltage approaching the peak value of the pulses.

Between the peak of each pulse the capacitor discharges through a load connected across the output terminals 5 and 6 and hence the voltage across the capacitor decreases until receipt of a charge from the next pulse. The capacitance of the capacitor is chosen to have a value sufficiently large to limit the fall in voltage between pulse peaks to an acceptable value. If the resistor 4 were not connected in series with the input to the rectifier 1, the initial charging current drawn by the capacitor could rise to a value which would interfere with the functioning of other equipment using the same AC supplies, particularly if switch-on coincided with a high instantaneous voltage on the AC input.

The resistor 4 has a positive temperature coefficient, i.e. the resistance of the resistor increases with increase of temperature. The resistance value of resistor 4 when it is passing no current and hence is at ambient temperature is chosen so that the initial surge of energisation current from the AC voltage input which charges the capacitor 7 is limited to a value which will not interfere with other equipment using the same AC supply. After a predetermined time delay from switch-on the resistor 4 is shorted by a switch 8, which may be a pair of relay contacts, so that there is no limiting resistance in the input to the rectifier 1. The time delay is so chosen that the current drawn by the capacitor 7 has fallen to an acceptable value even without the resistor 4 in circuit.

However if the switch 8 fails to operate and the resistor 4 remains in series with the AC input, the current passing therethrough will cause a rise in temperature of the resistor and hence the resistance will rapidly increase to a higher value and thereby result in a decrease in the input current passing to the rectifier 1.

It is preferred that the resistance/temperature characteristic of the resistor 4 is non-linear and that the slope of the characteristic increases with increasing temperature. A particularly desirable characteristic is one having a knee so that there is little change in resistance for temperatures below this knee temperature and that above this knee temperature the characteristic has a steep slope so that the resistance increases rapidly for small increases of temperature.

By using a resistor 4 having this latter described characteristic, during initial switch-on conditions when the capacitor 7 is being charged the current flow will cause the temperature of the resistor to rise towards the knee temperature but the time for which the resistor is in circuit will not be long enough for the resistor to reach the knee temperature. Hence the resistance of resistor 4 will only increase by a small amount.

However if the switch 8 fails to operate the heating of the resistor will continue for sufficiently long to raise it to above the knee temperature which will result in a sharp increase in its resistance. Therefore the resistor will limit the current flowing therethrough to a value which will maintain the resistor at a temperature in the region of the knee temperature. The precise temperature at which the resistor is maintained under these fault conditions would be determined by its ability to dissipate heat into its surroundings.

It will be appreciated that by choosing a resistor having a resistance/temperature characteristic in which the knee temperature is sufficiently low, the resistor will not constitute a fire hazard in the event of it remaining in circuit for an appreciable time after the initial charging period.

While the method of protecting circuits and components for excess current conditions has been described in relaton to power supply circuits in which a capacitor initially draws a high charging current it should be realised that this method may be utilized for protection of other circuits in which it is necessary to limit initial switch on current and subsequently to remove the limiting resistor from the circuit.

WHAT WE CLAIM IS: 1. A method of preventing the drawing of excessive current from a current supply by a circuit including a capacitor connected across the supply to be charged thereby, the method consisting of providing a resistor in series with the current supply to the capacitor circuit so that upon initial energisation of the circuit the resistor limits the initial energisation current to a desired value and shorting said resistor after the initial energisation for the duration of subsequent operation of the circuit, the resistor having a characteristic whereby as its temperature rises its resistance increases at an increasing rate so that, in the event of failure to short the resistor, the continuing current flow through the resistor raises its temperature to thereby increase its resistance to a value sufficient to limit the current flow therethrough to a desired steady state value.

2. Apparatus operative to prevent the drawing of excessive current from a current supply by a circuit including a capacitor connected across the current supply to be charged by the current supply, the apparatus including a resistor connected in series with the current supply to the circuit so that upon initial energisation of the circuit and the capacitor the resistor limits the initial energisation current to a desired value; and shorting means effective to short said resistor after the initial energisation for the duration of subsequent operation of the circuit; the resistor having a characteristic whereby as its temperature rises its resistance increases at an increasing rate so that, in the event of failure of the shorting means, the continuing current flow through the resistor raises its temperature to thereby increase its resistance to a value sufficient to limit the current flow therethrough to a desired steady state value.

3. Apparatus as claimed in Claim 2, in which the capacitor is connected to be charged through a rectifier.

4. Apparatus as claimed Claim 2, in which the resistance of the resistor increases at a first rate until the resistor has attained a predetermined temperature level and after the attainment of this level increases at a second rate higher than said first rate.

5. Apparatus as claimed in Claim 2 or 3, in which the resistor is shorted by a pair of relay contacts after a predetermined time delay.

6. A method of preventing a circuit from drawing excessive current substantially as described herein with reference to the accompanying drawing.

7. Apparatus operative to prevent a circuit from drawing excessive current constructed and described herein with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. resistor at a temperature in the region of the knee temperature. The precise temperature at which the resistor is maintained under these fault conditions would be determined by its ability to dissipate heat into its surroundings. It will be appreciated that by choosing a resistor having a resistance/temperature characteristic in which the knee temperature is sufficiently low, the resistor will not constitute a fire hazard in the event of it remaining in circuit for an appreciable time after the initial charging period. While the method of protecting circuits and components for excess current conditions has been described in relaton to power supply circuits in which a capacitor initially draws a high charging current it should be realised that this method may be utilized for protection of other circuits in which it is necessary to limit initial switch on current and subsequently to remove the limiting resistor from the circuit. WHAT WE CLAIM IS:

1. A method of preventing the drawing of excessive current from a current supply by a circuit including a capacitor connected across the supply to be charged thereby, the method consisting of providing a resistor in series with the current supply to the capacitor circuit so that upon initial energisation of the circuit the resistor limits the initial energisation current to a desired value and shorting said resistor after the initial energisation for the duration of subsequent operation of the circuit, the resistor having a characteristic whereby as its temperature rises its resistance increases at an increasing rate so that, in the event of failure to short the resistor, the continuing current flow through the resistor raises its temperature to thereby increase its resistance to a value sufficient to limit the current flow therethrough to a desired steady state value.

2. Apparatus operative to prevent the drawing of excessive current from a current supply by a circuit including a capacitor connected across the current supply to be charged by the current supply, the apparatus including a resistor connected in series with the current supply to the circuit so that upon initial energisation of the circuit and the capacitor the resistor limits the initial energisation current to a desired value; and shorting means effective to short said resistor after the initial energisation for the duration of subsequent operation of the circuit; the resistor having a characteristic whereby as its temperature rises its resistance increases at an increasing rate so that, in the event of failure of the shorting means, the continuing current flow through the resistor raises its temperature to thereby increase its resistance to a value sufficient to limit the current flow therethrough to a desired steady state value.

3. Apparatus as claimed in Claim 2, in which the capacitor is connected to be charged through a rectifier.

4. Apparatus as claimed Claim 2, in which the resistance of the resistor increases at a first rate until the resistor has attained a predetermined temperature level and after the attainment of this level increases at a second rate higher than said first rate.

5. Apparatus as claimed in Claim 2 or 3, in which the resistor is shorted by a pair of relay contacts after a predetermined time delay.

6. A method of preventing a circuit from drawing excessive current substantially as described herein with reference to the accompanying drawing.

7. Apparatus operative to prevent a circuit from drawing excessive current constructed and described herein with reference to the accompanying drawing.