GB2159285A

GB2159285A - Circuit monitor

Info

Publication number: GB2159285A
Application number: GB08412092A
Authority: GB
Inventors: Clive Reginald Steer
Original assignee: Cambridge Instruments Ltd
Current assignee: Cambridge Instruments Ltd
Priority date: 1984-05-11
Filing date: 1984-05-11
Publication date: 1985-11-27
Also published as: GB8412092D0; GB2159285B; DE3516251A1

Abstract

A circuit for monitoring whether or not a switch (2) is open or closed (or for monitoring the resistance of a resistive load) consists of a double wound inductor (8), one winding (10) of which is connected in parallel with a capacitor (16) and a switch (2) (or load). The other winding (12) may be periodically connected to a DC power supply (22) by a semiconductor switch (24). The reverse induced voltage in the winding (12) after the switch (24) has been closed and re-opened is measured by a comparator (26) to determine whether the switch (2) is open or closed (or to determine the load resistance e.g. of a resistance thermometer). <IMAGE>

Description

SPECIFICATION Circuit monitor The present invention relates to a method of and apparatus for monitoring an isolated switch or resistive circuit.

In many process control operations, it is necessary to monitor the condition of a switch, such as a pressure switch, flow switch or a safety microswitch from a remote location. For example, in equipment for controlling the operation of a gas turbine, it may be necessary to close down the turbine if the pressure or gas flow exceeds a predetermined value. A pressure or flow switch is provided which closes when the pressure or flow reaches the predetermined value. In other cases a resistive circuit may be used to measure a parameter such as temperature.

Such a switch or resistive circuit is normally located in adverse environmental conditions and therefore it is not possible to use the switch directly to operate electrical equipment to close down the process because switching large loads will increase the risks of sparks. It is therefore necessary to monitor the condition of the switch from a remote and electrically isolated location. This has been done by the use of an optoisolator including a light emitting diode and a photosensor. The diode may be connected in series with the switch to be monitored and a power supply such that when the switch is closed the diode emits light which is detected by the isolated photosensor which produces a current which can be used to control the necessary shut down operation.

It will be noted that the use of an optical isolator requires a power supply, albeit small, in the vicinity of the switch being monitored.

As this switch and therefore the power supply can be located in a hostile environment, this power supply may result in an additional safety risk.

It is therefore desirable to provide a method and of apparatus for monitoring the condition of a switch which does not require a power supply to be located in the vicinity of the switch and therefore provides safer operation particularly in hostile environments.

The present invention accordingly provides apparatus for monitoring a switch or resistive circuit, comprising a double wound inductor, a capacitor and the switch or resistive circuit to be monitored being connected in parallel across the ends of one winding of the inductor, means for charging the capacitor by applying power to the other winding, and means for monitoring the voltage induced in said other winding from said first winding after said charging means has ceased to operate.

It will be appreciated that the portion of above defined circuit including the monitored switch or resistive circuit is self-powering and there is no need for any power supply to be connected in a hostile environment. Provided a flux linkage can be provided between the two windings of the double wound inductor it is not necessary that they should be mounted very close together.

Preferably the means for measuring the reverse induced voltage is a comparator which has one input connected to the winding and another input connected to a reference voltage. The output of the comparator may be used to operate a shut down or other safetycircuit when the reverse induced voltage is such as to indicate that the switch is in a position indicative of a dangerous situation.

In a preferred embodient, the charging means comprises a power supply and switch connected across the terminals of said other winding the switch being closed and reopened to charge the capacitor.

The invention also includes a method of monitoring the condition of the switch or a resistive circuit comprising the steps of charging a capacitor in parallel with said switch or resistive circuit from a power supply through a transformer and thereafter measuring the reverse voltage induced in the transformer winding connected to the power supply in the previous step.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a circuit diagram of apparatus for monitoring the condition of a switch; and Figure 2 shows a typical waveform of the voltage at point A in Fig. 1.

The circuit illustrated in Fig. 1 is adapted to monitor whether or not a switch 2 is in an open or closed condition. The circuit is divided into two isolated circuit parts 4 and 6.

The circuit elements of circuit part 4 may be located in a hostile environment. The circuit portions 4,6 are linked by a double wound inductor 8, which includes a first winding 10 connected into circuit part 4 and another winding 1 2 connected into circuit part 6. The windings are inductively linked by a suitable magnetic flux linkage (not shown).

The circuit portion 4 includes a diode 14 having one terminal connected to one terminal of the inductor winding 10 and its other terminal connected to one terminal of a capacitor 1 6 which is connected in series with the inductor winding 10. A resistive load is connected in parallel across the capacitor 1 6 and consists of a series resistor 18, the switch 2 and a resistor 20 shunted across the switch 2.

Circuit part 6 consists o a DC power supply 22 which is be connected across the inductor winding 1 2 via a series semiconductor switch 24. One terminal of the inductor winding 1 2 is designated A and provides one input to a comparator 26. The other input to the comparator 26 is a reference voltage which may be selectively variable, the source of which is not shown. The output of the comparator 26 represents the output of the circuit and will be a logic high or low in dependence upon whether the switch 2 is open or closed respec tively.

The circuit operates as follows: The switch 24 is closed for a predetermined short time to connect and disconnect the DC power supply 22 across the winding 1 2 of the double wound inductor 8. Whilst switch 24 is closed a current will flow in winding 12 storing energy in the magnetic circuit of in ductor 8. As the switch 24 is opened the voltage induced across winding 10 will cause diode 14 to conduct and energy to be trans ferred to capacitor 1 6.

The magnitude of the voltage induced in winding 10 to cause diode 14 to conduct will depend on the voltage across capacitor 1 6.

The voltage across capacitor 1 6 will depend on the resistance of the parallel resistive load made up of switch 2 and resistors 1 8 and 20.

Thus if switch 2 is closed the resistance of this resistive load will be relatively low and most current will flow through the load rather than charge the capacitor 1 6. However if the switch 2 is open then the capacitor 1 6 will be charged to a higher voltage.

Whilst the voltage is induced across wind ing 10 a reverse voltage is induced across winding 12 due to mutual flux linkage. The magnitude of this reverse induced voltage will depend on the original voltage stored across capacitor 1 6 and therefore on whether the switch 2 was open or closed.

A typical plot of the voltage against time at point A, at one terminal of the winding 1 2 is shown in Fig. 2. At time to the semi conduc tor switch 24 is closed until time t,. This causes a reverse induced voltage of a rela tively large magnitude as shown to be produced once the switch 24 has been re opened, if switch 2 is open. At time t0, the switch 24 is closed whilst the switch 2 is also closed. Once the switch 24 is re-opened at time t,', the reverse induced voltage is of a lower magnitude than previously. The values of the resistors 1 8 and 20 may readily be arranged so that the magnitudes of the re verse induced voltage when switch 2 is open and closed respectively fall either side of the predetermined reference voltage. For example the resistor 1 8 may be 1 K.ohm whereas the resistor 20 is 100 K.ohm. The resistance of the load is therefore approximately 1 K.ohm when the switch 2 is closed and 100 K.ohm when the switch 2 is open.

In some circumstances it may be required to monitor the resistance of a load absolutely.

Such a load might be a resistance ther mometer. This variable resistance would be then connected in parallel across a capacitor 1 6 in place of the resistive load made up of switch 2, and resistors 18 and 20. In this case the reverse induced voltage would be directly proportional to the load resistance.

The reference voltage to the comparator 26 may then be set at a value indicative of a particular temperature being reached.

Claims

1. Apparatus for monitoring a switch or resistive circuit, comprising a double wound inductor, a capacitor and the switch or resistive circuit to be monitored being connected in parallel across the ends of one winding of the inductor, means for charging the capacitor by applying power to the other winding, and means for monitoring the voltage induced in said other winding from said one winding after said charging means has ceased to operate.

2. Apparatus as claimed in claim 1, wherein said means for monitoring the induced voltage comprises a comparator which has one input connected to said other winding and another input connected to a reference voltage.

3. Apparatus as claimed in claim 1 or 2, wherein said monitoring means has an output which is connected to a shut-down circuit which is operable when said induced voltage in said other winding indicates an unsafe condition of the switch or resistive circuit.

4. Apparatus as claimed in any one of the preceding claims, wherein the charging means comprises a power supply and switch connected across the terminals of said other winding, the switch being closed and reopened to charge the capacitor.

5. Apparatus as claimed in any one of the preceding claims, wherein said switch or resistive circuit, capacitor and said one winding are mounted in a hostile environment and the other winding, charging means and monitoring means are mounted externally of said hostile environment, flux linkage between said windings of said inductor being across a barrier surrounding said hostile environment.

6. A method of monitoring the condition of a switch or a resistive circuit, comprising the steps of charging a capacitor in parallel with said switch or resistive circuit from a power supply via a transformer, and thereafter measuring the reverse voltage induced in the transformer winding connected to the power supply in the previous step.

7. Apparatus substantially as herein described with reference to the accompanying drawings.

8. A method of monitoring the condition of a switch or a resistive circuit substantially as herein described with reference to the accompanying drawings.