GB2066484A - Single Pole Voltage Detector - Google Patents

Abstract

The detector includes a contact probe 1 connected via a discharge tube 3 and capacitor 3/1 to an earth 2 which co-operates capacitively with the ground, a power source 4 supplying a voltage converter, and a luminous discharge tube 6. Main electrodes 6/1 and 6/2 of tube 6 are subject to a DC voltage stored in capacitor 5/5 and a control electrode 6/3 is connected to a relaxation circuit 7/1-7/4. The voltage converter charges capacitors 5/5 and 7/1 and comprises an oscillator having switching transistors 5/1 and 5/2 and a step-up transformer 5/3 5/4 giving an output rectified by diode 5/6. The oscillator is only triggered by input voltage to probe 1 thereby to trigger tube 6 and provide a light signal which also blocks the oscillator until a new cycle of operation. Current consumption when quiescent is negligible. An additional acoustic warning device may also be provided. <IMAGE>

Description

SPECIFICATION Single Pole Voltage Detector The present invention relates to the field of electricians' checking instruments, especially instruments for use in preventing electrical accidents. More particularly, the invention concerns a single-pole voltage detector for use in checking for the presence of alternating voltages, especially alternating industrial voltages as in AC mains supply systems, the detector being of a kind that emits a light signal (possibly supplemented by a sound signal) to indicate a voltage presence, the light signal being produced by a luminous electrical discharge in a tube containing rarefied gas which has two main electrodes connected to an in-built power source and one control electrode.

In respect of their general principle, so-called single-pole voltage detectors are instruments which comprise, in essence, on the one hand an input terminal or probe and on the other hand conductive elements acting as an earth capable of co-operating capacitively with the ground. When the probe is brought into contact with a conductor carrying an alternating voltage with reference to the ground, a very small current, of the order of 10 or 100 microamps depending upon the voltage, is set up in the space between the said earth and the ground. The voltage detector also includes a warning device which is so arranged as to emit a signal when the magnitude of the current reaches a predetermined level which defines the sensitivity threshold of the instrument.

To enable contact to be made with a conductor the state of voltage on which it is desired to check, these instruments are carried at the end of an insulating rod the length of which may be as much as several metres in the case of the highest voltages. With lengths of this order, the detector needs to be of very low weight (a few hundreds of grammes) as otherwise it would be virtually impossible to handle. As a result, many detection systems which are feasible in theory must in practice be ruled out due to the impossibility of producing them within acceptable limits on weight and size.

In their oldest and simplest form, such voltage detectors make use of a tube employing luminescent discharge in a gas, such as neon for example, which tube is connected in series between the probe and the earth. Due to the very low value of the capacitive current, the energy of the discharge is very small. Attempts have been made in various ways to improve their output but the possibilities are restricted and do not allow a light to be obtained which is sufficiently bright to be clearly seen when the ambient light level is high, particularly out-of-doors.

Attempts have also been made to increase the light emission of a discharge tube by using a power source incorporated in the detector. The tube then has two main electrodes and one control electrode. Betweenathe main electrodes is applied a voltage supplied by the source which is lower than the voltage which would be capable of triggering a discharge between the said electrodes. The voltage picked up from the probe is applied to the control electrode; when it reaches a predetermined value (the threshold) it causes the gas in the tube to become sufficiently ionised for the discharge between the main electrodes to be triggered.

This discharge draws the major proportion of its energy from the in-built power source and thus the emission of light is much brighter than would be obtained solely from the small amount of energy picked up between the probe and the earth.

Another way of increasing the brightness of the light signal consists in rectifying the alternating current flowing between the probe and the earth by means of rectifying components in order to store its energy in one or more capacitors which are discharged periodically into a luminiscent tube. What are obtained in this way are short but bright flashes which are far more satisfactorily seen by the eye than a continuous discharge involving the same amount of power.

Another solution to the problem of providing a satisfactory light signal is achieved by combining the above two solutions; it consists in charging a piurality of capacitors via rectifying components using the current flowing between the probe and the earth and connecting them to a discharge tube having two main electrodes and one control electrode. The voltage from at least one capacitor is applied between the main electrodes as in the first case. Another capacitor is contained in a relaxation circuit to produce periodic discharges which are applied to the control electrode and each time cause the tube to be triggered.

The various solutions proposed represent considerable advances over detectors with a simple luminescent tube but none of them gives entirely satisfactory results. In particular, in the case of the solutions where the energy picked up from a conductor and stored in one or more capacitors is discharged periodically, the time required to charge the capacitor or capacitors (and thus the interval between discharges) is inversely proportional to the voltage applied to the probe. Consequently, if the said voltage is gradually reduced, the interval between two light signals is increased indefinitely but these instruments do not have an exact operating threshold, which is a major disadvantage. For the same reason the detectors can only be used at fairly high voltages.

In the light of experience, the main characteristics or requirements for a detector which would give users the best possible service and the greatest possible safety can be identified as follows: it must be possible for the selected principle of operation to be embodied in a device of small bulk and very low weight.

-The instrument must have a precise operating threshold above which a regular and clearly perceptible signal is emitted and below which no signal is emitted.

power consumption in the quiescent state must be virtually zero so that the instrument can remain permanently in a working state without need to provide a switch in the supply circuit.

-The light signal which indicates the presence of a voltage at the probe must be of the greatest possible brightness but must draw the smallest possible amount of power from the source, which source must be of low weight but which must nevertheless remain serviceable for a long period.

-The component parts of the instrument must be simple, few in number and low in price, to enable production to be economical.

It is also desirable for it to be possible for the light signal to be supplemented by a sound signal so that persons in the vicinity who are in a position where they cannot see the light signal are nevertheless warned that voltage is present in the installation being tested.

The present invention provides voltage detectors which, at least in preferred embodiments, can substantially meet the above practical requirements.

More specifically, in a basic aspect the present invention provides a single-pole voltage detector for use in checking for the presence of alternating voltages such as in AC mains supply systems, said detector comprising a contact probe, an earth which co-operates capacitively with the ground when the detector is in use, an in-built electrical power source, and a luminous discharge tube having two main electrodes subject to a DC voltage stored in a capacitor and having a control electrode connected to a relaxation circuit, wherein the electrical energy for charging said capacitor and for supplying the relaxation circuit is derived from a voltage converter powered by said in-built electrical power source said voltage converter comprising an oscillator arranged to be triggered so as to start operating to generate sustained oscillations in response to an external alternating voltage applied to the probe, said oscillator being such that it is unable to start operating spontaneously until it is so triggered from an external input voltage.

Thus, voltage detectors in accordance with the invention make use of a discharge tube having two main electrodes and one control electrode.

The DC voltage of several hundred volts required to obtain discharges between the main electrodes is supplied by a voltage converter fed by a built-in source and comprising an oscillator which will generally operate in conjunction with a voltage step-up transformer and a rectifying element. The power put out by this system is accumulated in a capacitor during the interval between two discharges.

A particular feature of the arrangement in accordance with the invention, however, is that the oscillator is so designed or adjusted that it cannot or will not begin operating spontaneously.

It only comes into action when a first oscillation, which may be of very low power, is applied to the input of the oscillating circuit; from this moment on sustained oscillations are maintained by feedback during an indefinite period.

Moreover, the control electrode of the discharge tube is connected to a relaxation circuit, capable of emitting pulses of a suitable size to trigger the discharges between the main electrodes, which is also supplied by the voltage converter powered by the in-built source. By this means, the interval between two pulses may be set as desired and remains completely independent of the voltage applied to the probe.

In accordance with another important feature of preferred embodiments, the discharge of the capacitor into the light tube acts to block the operation of the oscillator.

Also, in preferred embodiments, the circuit lying between the probe and the earth is so arranged as to produce oscillations when an alternating voltage at least equal to a predetermined threshold is applied to the probe.

The said circuit is connected to the input of the oscillating circuit and is thus capable of bringing it into action.

The detector may also include a sound warning member supplied by the in-built power source and controlied by, for example, a thyristor whose gate is connected to the relaxation circuit.

By the combination of the above features, the following effects are obtained: The voltage converter circuit, whose consumption in the quiescent state is virtually zero, is permanently connected to the source and is constantly in an operating state.

If an alternating voltage at least equal to the set threshold is applied to the probe, it produces oscillations in the circuit between the probe and earth. The first of these oscillations at once brings the oscillator into action. The energy given out by the latter charges the capacitor. When the voltage at the terminals of the latter reaches a predetermined level, the relaxation circuit emits a pulse to the auxiliary control electrode of the tube and a luminous discharge takes place in the latter.

This discharge, as already mentioned, blocks the operation of the oscillator. If at this moment the probe is still subject to an alternating voltage at least equal to the threshold the resulting oscillations re-start the oscillator and the cycle is repeated. If, on the other hand, voltage is no longer applied to the probe, there is no fresh oscillation in the input circuit for triggering the oscillator so that the oscillator remains quiescent and the instrument at once ceases to drain the power from the source.

The voltage detector will generally be contained in a housing adapted for fitting to the end of an insulating rod, which housing carries the contact probe in its upper region.

By way of example, there is illustrated in the accompanying drawing the circuit diagram of a typical voltage detector instrument in accordance with the invention which is the subject of the foliowing detailed description.

Referring to the circuit diagram of said drawing, the instrument comprises essentially a probe 1 which can be brought into contact with a conductor capable of carrying a certain alternating voltage with respect to the ground. In the following explanations, it will be assumed that this probe is subject to a voltage at least equal to the sensitivity threshold of the instrument. The latter also comprises an "earth" 2 formed by one or more conductive elements and capable of cooperating capacitively with the ground. This earth may be formed by screening enclosing the circuits in order to shield them from the effect of interference fields. In the absence of such screening, the function of the earth is taken over by the group of conductive elements situated downstream of the member 3 that is referred to below.

Between the probe 1 and the earth 2 is connected a member 3 consisting of an element such as a spark gap or, preferably, a discharge tube which is characterised by a low striking voltage and which is positioned in parallel with a capacitor 3/1. The current which flows between the probe 1 and the earth 2 produces in this member 3 oscillations of a high frequency and very low power which are applied to the input of an energy converting circuit as will be explained below. The member 3 at the same time forms a voltage miter which protects the circuits described below.

The instrument also comprises an in-built power source 4, preferably formed from battery members supplying a low tension voltage of a few volts. The source 4 feeds an energy converting circuit comprising a conventional oscillator or generator of sustained oscillations formed, in this example, by two transistors 5/1, 5/2 which are connected in cascade to the primary winding 5/3 of a voltage step-up transformer whose secondary winding 5/4 is connected to the terminals of a capacitor 5/5 via a rectifying element 5/6. The base of the first transistor 5/1 is connected to the probe 1 via a resistor 5/7. One output of the secondary 5/4 is also connected to the base of transistor 5/1 via a resistor 5/8. The latter connection forms a feedback circuit which enables oscillations to be maintained.In the quiescent state, however, the base of transistor 5/1 is at the potential of the negative pole of the source 4 via the secondary winding 5/1 of the transformer and resistor 5/8. Transistor 5/1 is thus in the non-conducting state and the circuit cannot begin to oscillate spontaneously. On the other hand, if a first oscillation coming from member 3 reaches the base of transistor 5/1 via resistor 5/7, the said oscillation is amplified by transistors 5/1, 5/2 and, due to the action of the feedback circuit 5/4, 5/8 the system is maintained. The secondary winding 5/4 thus carries a current which, via rectifying element 5/6, charges capacitor 5/5.The ratio of the transformer is so calculated as to produce at the terminals of capacitor 5/5 a voltage of a few hundred volts adapted to the characteristics of a luminous discharge tube 6 which has main electrodes 6/1, 6/2 connected to the terminals of capacitor 5/5 and a control electrode 6/3 connected to one output of the secondary winding 5/4.

The instrument further includes a relaxation circuit comprising a resistor 7/2 and a capacitor 7/1 which is connected in parallel with capacitor 5/5 via the source 4, with the result that capacitors 5/5 and 7/1 are able to charge simultaneously. Capacitor 7/1 is also connected in series with the primary winding 5/3 via a resistor 7/3 and switching discharge tube 7/4 having a triggering voltage of 200-300 V. The values of capacitor 7/1 and resistor 7/2 are so selected that, when the capacitor 5/5 has charged to reach the voltage value needed by tube 6, the voltage at the terminals of capacitor 7/1 reaches the striking voltage of tube 7/4 so that the latter thus becomes conductive and capacitor 7/1 discharges into the primary winding 5/3 via resistor 7/3.The discharge current produces in the secondary winding 5/4 a pulse of a high voltage (of the order of 1000 V) which is applied to the auxiliary control electrode 6/3 of tube 6, the gas in which is thus ionised, with the result that capacitor 5/5 discharges across electrodes 6/1, 6/2, thus producing a flash of light. The effect of this discharge is at the same time to block the oscillations of the converter circuit although, as has already been explained, the latter is able to resume operation immediately provided that it is supplied with a triggering pulse by member 3 as a result of voltage being continued to be applied to probe 1.

The detector instrument of this embodiment also includes a sound or acoustic warning device which gives an indication of the presence of voltage additional to that provided by the discharge tube 6. This sound warning device may, for example, be an electromagnetic buzzer 8 connected as shown between the poles of the source 4 via a relay, such as a thyristor 8/1. The gate 8/2 of the thyristor is connected to one output of the primary winding 5/3 via a protective resistor 8/3 and a capacitor 8/4. The gate 8/2 is connected to the negative pole of the source 4 by a resistor 8/5 and a decoupling diode 8/6.

Between the terminals of the buzzer 8 are connected in series a resistor 8/7 and a capacitor 8/8 which define a time constant shorter than that of the relaxation circuit comprising capacitor 7/1, resistors 7/2, 7/3 and the switching tube 7/4.

When the relaxation circuit discharges through the primary winding 5/3, a pulse travels through capacitor 8/4 and resistor 8/3 to the gate 8/2. The thyristor 8/1 becomes conductive and a current is set up which travels on the one hand through the buzzer 8, whose electromagnetic switch contact opens and closes at a frequency of 1000-2000 Hz, and on the other hand through resistor 8/7 and capacitor 8/8. The buzzer thus comes into operation at the moment when the luminous discharge takes place in tube 5/5 and it continues to operate during the period set by the time constant 8/7, 8/8. At the end of this period capacitor 8/8 is charged and arm 8/7, 8/8 of the circuit no longer carries a current adequate to maintain the thyristor 8/1 in the conductive state during the open periods of the electromagnetic switch contact of the buzzer 8.The current is thus interrupted in the thyristor 8/1 , the contact of the buzzer 8 closes again and capacitor 8/8 discharges through it via resistor 8/7. The circuit is then ready to begin a new cycle of operations at the next luminous discharge in the tube 5/5. Each of these luminous discharges is thus accompanied by the emission of a sound signal.

By way of illustration, the orders of magnitude of the capacitances and resistances shown in the circuit diagram may be as follows: Capacitors 3/1 1 to 10 nF 5/5 2 to 4 yF 7/1 0.1 to 0.2 F 8/4 10 to 20 nF 8/8 200 to 400 of Resistors 5/7 10 to 100 my 5/8 1 to 5 MQ 7/2 Sto 20MQ 7/3 30 to 100 Q 8/3 10 to 2b 8/5 1000 S1 8/7 500 to 1500 Various modifications of greater or lesser importance in the details shown in the accompanying circuit diagram may of course be made, if desired, within the scope of the invention since the invention as set forth in the appended claims relates primarily to the functions performed by the various members and circuits and to their mutual relationship and not to the precise details of their construction and arrangement.

Claims (6)

Claims

1. A single-pole voltage detector for use in checking for the presence of alternating voltages such as in AC mains supply systems, said detector comprising a contact probe, an earth which cooperates capacitively with the ground when the detector is in use, an in-built electrical power source, and a luminous discharge tube having two main electrodes subject to a DC voltage stored in a capacitor and having a control electrode connected to a relaxation circuit, wherein the electrical energy for charging said capacitor and for supplying the relaxation circuit is derived from a voltage converter powered by said in-built electrical power source said voltage converter comprising an oscillator arranged to be triggered so as to start operating to generate sustained oscillations in response to an external alternating voltage applied to the probe, said oscillator being such that is unable to start operating spontaneously until it is so triggered from an external input voltage.

2. A voltage detector as claimed in Claim 1 so arranged that triggering of the luminous discharge tube has the effect of interrupting the generation of sustained oscillations and operation of the oscillator.

3. A voltage detector as claimed in Claim 1 or 2, wherein the probe is connected to the earth via a discharge tube connected in parallel with a capacitor.

4. A voltage detector as claimed in any of the preceding claims, wherein an acoustic warning device is incorporated which is connected in series with the in-built power source and with a relay connected to the relaxation circuit, said acoustic warning device being arranged so as to operate for a predetermined time interval when actuated.

5. A voltage detector as claimed in Claim 4, wherein the relay is a thyristor which fires when the relaxation circuit triggers the luminous discharge tube.

6. A single-pole voltage detector having a circuit constructed and arranged substantially as herein described with reference to the circuit diagram of the accompanying drawing.