GB2080643A - Resistance monitoring circuit - Google Patents

Abstract

A monitoring circuit is disclosed for monitoring an electrical device e.g. a heating tape 1 which is periodically turned on and off. The circuit detects whether the monitored device is on or off, monitors an electrical characteristic of the monitored device when turned on, and monitors an electrical characteristic of the monitored device when turned off. A switch controlled in dependence upon whether or not the monitored device is turned on switches between two circuits which perform respective monitoring functions. When a characteristic monitored exceeds a preset limit, a fault is indicated. The monitoring utilizes resistance bridges e.g. 8,13,12,1, and a signal processing circuit for connexion to Figure 1 is described in detail. Figure 2 (not shown). <IMAGE>

Description

SPECIFICATION Resistance monitoring circuit The present invention relates to a monitoring circuit which is particularly suited to monitoring the condition of an electrical device which is periodically turned on and off.

Electrical heaters controlled by thermostats to turn on and off as required are widely used to protect industrial plant and other installations against poten tiallydamaging low temperatures. For example, exposed water pipes must be prevented from freezing up. In such applications the heating circuit comprises a heating tape which is wound around the pipe and covered with thermal insulation. One type of heating tape suitable for this purpose is described in our British Patent No. 1,523,129.

Heating tapes and any other type of resistive heating device can fail. The repeated heating cycles to which they are subjected can result in an initially unsignificant weakness caused by faultv design or manufacture or physical damage developing into a major fault leading to the tape burning out or otherwise failing completely.

It is important to detect when a heating device fails so that repairs can be made before the installation protected by that device is damaged. One known system monitors the operation of a heating device when it is energised by detecting the heating current supplied to the device and indicating a fault when the detected current rises above or falls below present thresholds. The problem with this known system is that if the heating device fails when de-energised the failure is only indicated when an attempt is made to re-energise it. In the case of a circuit protecting against frost damage, a fault will be indicated when the protected installation begins to freeze up. Thus the fault must be rapidly repaired or damage will result. Often repairs cannot be made sufficiently quickly and therefore knowledge that there is a fault is of little practical use.

It is an object of the present inventior to provide a monitoring circuit which can continuously monitor the condition of an intermittently energised device such as an electrical heating tape.

According to the present invention, there is provided a monitoring circuit for monitoring an electrical device which is periodically turned on and off, characterised by means for detecting whether the monitored device is on or off, first means for monitoring an electrical characteristic of the monitored device when turned on, second means for monitoring an electrical characteristic of the monitored device when turned off, switching means controlled by the detecting means for connecting the first means to the monitored device when on and for connecting the second means to the monitored device when off, and means for indicating when a characteristic monitored by the first or second means identifies a fault in the monitored device.

Preferably the first and second monitoring means comprise two resitance bridge circuits each including in one of its arms the device to be monitored, the switching means being operative to connect the device to be monitored in one or other of the bridge circuits under the control of the detecting means.

The two resistance bridges may share a common output which is connected to a signal level monitoring circuit.

Preferably the indicating means comprise contacts controllable by the signal level monitoring circuit to switch when the monitored signal level exceeds a predetermined threshold.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa circuit diagram of a power supply for an electrical heating device; and Figure 2 is a circuit diagram of signal processing circuitry for connection to the circuit of Figure 1.

Referring to Figure 1, the heating device to be energised, for example a heating tape, is represented by load 1 connected to terminals 2 and 3 of the power supply, and a thermostat which controls the energisation of the load 1 is represented by a switch 4 connected to terminals 5 and 6. The thermostat switch 4 is arranged to close if the sensed temperature falls to a preset level. The power supply is connected to the mains via live, neutral and earth terminals L, N and E.

Assuming that the thermostat switch 4 is closed (not as shown) to indicate a heating demand, power is supplied to the load 1 via fuse 7, switch 4 and resistor 8. A relay coil 9 is energised with the result that contacts 10 switch from the position shown, thereby connecting a resistor 11 to the live supply via switch 4.

The resistors 8 and 11 form the upper arms of a resistance bridge, the lower arms of the bridge being formed by resistor 12 and the load 1. The bridge balance voltage is picked off by the wiper of a potentiometer 13, adjustment of the potentiometer enabling null adjustment when the circuit is set up.

Any signal appearing at the wiper of potentiometer 13 is applied to a transformer 14 and a resultant output signal appears at terminal 15. As described below, this output signal is monitored and a fault is indicated in the event of the signal exceeding a preset limit.

Assuming now that the thermostat switch 4 is open as shown, the load 1 is connected in series with resistors 8, 16 and 17 across the supply. The resistance of resistors 16 and 17 is sufficiently high that only a trickle current passes through the load resulting in only a small power consumption. The coil 9 is also connected in series with resistors 16 and 17 and thus carries a current which is insufficient to switch contacts 10 which therefore assume the position shown.

A resistor 18 forms one of the upper arms of a further resistance bridge, the other upper arm being formed by resitors 8, 16 and 17. The lower arms of the bridge are formed by resistor 11, now connected to the neutral terminal via contacts 10, and the load 1. As before, any out of balance voltage generated across the bridge results in an output signal appearing at output 15. Thus a signal representative of the resistance of the load appears at output 15 regardless of whether or not the thermostat switch 4 is open.

Figure 1 shows a conventional transformer 19 and full wave rectifier bridge 20 which provide positive, negative and zero volts outputs to the circuit of Figure 2.

Referring to Figure 2, the terminal 21 is connected to the output 15 of Figure 1 and thus receives any out of balance voltage generated by the bridge circuitry.

A first integrated circuit operational amplifier 22, the gain of which can be adjusted by potentiometer 23, and a second integrated circuit opertional amplifier 24, provided an amplified out of balance signal to the input of a third integrated circuit 25 conneted to appropriate components so as to operate as a rectifier. The third integrated circuit 25 and its associated components provide a rectified input to a forth integrated circuit 26 connected as a Schmitt trigger. When the input to the circuit 26 exceeds a preset threshold voltage, its output goes from negative to positive, turning off transistor 27. When transistor 27 is on, it energises a relay coil 28 which switches contacts 29 to disable on alarm (not shown) connected to terminals 30.As soon as transistor 27 turns off, either because of a power failure or because of the appearance of an out of balance voltage, the alarm is enabled. Thus the circuit provides a warning in the event of a power failure as well as a load fault. A light emitting diode 31 is energised when the transistor 27 is turned off. Thus in the event of the alarm sounding, if the diode 31 is energised this indicates a fault other than a power failure or a failure in the relay 28, 29.

Although the described embodiment of the invention is concerned with a heat trace application designed to provide protection against low temperatures, the invention is applicable to any application in which a load is turned on and off intermittently.

For example, the invention could find application in industrial processing equipment such as ovens, furnaces and heated moulds.

The invention enables the detection of the complete failure of a load in either its energised or deenergised states. In addition it can detect a change in the resistance of a load which is indicative of a partial failure (e.g. failure of one heating device of a series of such devices or a resistive fault which may be damaging but does not cause total failure of the load). The detection of such partial failures means thatthe necessary repairs can be made before the operation of the system is noticeably impaired.

Claims (5)

1. A monitoring circuit for monitoring an electrical device which is periodically turned on and off, characterised by means for detecting whether the monitored device is on or off, first means for monitoring an electrical characteristic of the monitored device when turned on, second means for monitoring an electrical characteristic of the monitored device when turned off, switching means controlled by the detecting means for connecting the first means to the monitored device when on and for connecting the second means to the monitored device when off, and means for indicating when a characteristic monitored by the first or second means identifies a fault in the monitored device.

2. A monitoring circuit according to claim 1, characterised in that the first and second monitoring4 means comprise two resistance bridge circuits each including in one of its arms the device to be monitored, the switching means being operative to connect the device to be monitored in one or other of the bridge circuits under the control of the detecting means.

3. A monitoring circuit according to claim 2, characterised in that the two resistance bridges share a common output which Is connected to a signal level monitoring circuit.

4. A monitoring circuit according to claim 3, characterised in that the indicating means comprise contacts controllable by the signal level monitoring circuit to switch when the monitored signal level exceeds a predetermined threshold.

5. A monitoring circuit substantially as hereinbefore described with reference to the accompanying drawings.