GB2217026A - Voltage sensor - Google Patents

Abstract

A sensor arrangement for checking whether a conductor (not shown) is live comprises a sensor (20) for sensing the presence of a voltage and means IC1b for inhibiting output at an LED (1) if the sensed voltage is below a predetermined voltage. A relaxation oscillator IC1C provides test signals to the sensor 20 to test the operation of the arrangement. <IMAGE>

Description

DEAD LINE CHECKER This invention relates to a dead line checker.

Dead line indicators are devices which are used to determine whether an electric conductor is live or not.

They are of particular use in hazardous areas such as coal mines. Examples of such dead line indicators are described in our United Kingdom Patents 1410100 and 1603408.

In circumstances where two conductors are in close proximity and one conductor only is live it can happen that the dead conductor still reacts to the dead line checker as if it were live. This problem can lead to serious difficulties in that when efforts are made to switch off current supply to the conductor it is found that the supply is already switched off. That in turn may lead to an assumption that the switch is faulty.

It may take a considerable time to establish the true state of affairs.

Dead line indicators are normally powered by disposable batteries. It is important that users remember to replace the battery before it becomes exhausted since otherwise the power supply may fail while the checker is in use.

The invention has been made with these problems in mind.

According to the invention there is provided a dead line checker comprising a sensor, and means controlling the sensor to prevent operation thereof below a predetermined voltage.

In a preferred embodiment of the invention means are provided for producing a signal which indicates that the checker is operative. Such means makes the dead line checker failsafe.

In a particularly preferred embodiment of the invention a rechargeable battery, preferably a nickel/cadmium battery, is provided as the power source for the dead line checker. The checker circuitry including the battery is contained in a sealed housing. Recharging of the battery is accomplished inductively by providing a charging circuit in the housing, said charging circuit including a coil. The housing is adapted to be removably received in another housing containing a second coil connected to a pulsed power supply having a high peak current but low mean current, this being a particularly favourable mode for recharging nickel/cadmium batteries.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig.l is a circuit diagram of the dead line indicator; and Fig.2 is a circuit diagram of an inductive charging circuit.

Referring to Fig.l the circuit consists of four main sections as follows: Transconductance amplifier Level detector Self test oscillator Low voltage detector All the above functions are implemented using a quad op-amp IC1-LP324 in association with other components as shown in the circuit of Fig.l.

If an AC field lies between sensor 20 and earth, i.e. there is capacitive coupling, then there will be a current flow into the virtual earth input of ICla pin 2.

The op-amp in conjunction with the resistor network R1, R2, R15 and VR1 will convert this current into a voltage at ICla pin 1. VR1 gives a degree of gain control and C1 rolls the frequency off from about 100 Hz. The output of ICla pin 1 will take the form of a periodic oscillation but clipped on the negative half cycle due to the unipolar power supply.

ICld is arranged as a form of Schmitt trigger.

R3 and D1 provide a stable voltage reference. With no signal on pin 12 the inverting input pin 13 will be positive hence the output, pin 14, will sit at Ov holding the transistor TR1 off. This means there is no current flow through R10 hence pin 13 of the ICld will be at the reference voltge of 1.2v. For the output voltage of ICld to rise the input on pin 12 must exceed 1.2v. Any AC signal on pin 12 with a peak voltage of 1.2v or more will cause the output pin 14 to rise sharply positive turning on TR1 hence pulling current through resistor R10 thus reducing the voltage on pin 13 from 1.2v to 1.2 x (R10/R4 + R10]). R4 and R10 have been arranged to give about 5% hysterisis.

The light emitting diode LED1 is also tied to ICld pin 14 and will illuminate each time the voltage threshold is exceeded. For AC signals of 50Hz the flash rate is not perceived by the human eye.

IClb is arranged as a 1Hz relaxation oscillator.

If C2 is taken as being discharged and IClc pin 8 is at a positive voltage so that by a potential divider a positive voltage appears on pin 5 of IClb, then the output of IClb will also be positive. C2 can, therefore, charge at a rate given by the time constant C2 x R6. The voltage on C2 will rise until it exceeds that on pin 6 at which point the output on pin 7 will switch to zero volts. This effectively shunts R8 with R9 reducing the voltage on pin 5.- At the same time C2 begins to discharge through R6 until it reaches the new lower voltage on pin 5, the process then repeats itself.

The result of this is that a LF square wave appears on pin 7. This is connected via the differentiator network C3 and R5 to the sensor. This produces a current pulse through the input of the transconductance amplifier hence a voltage pulse to the input of the level detector which exceeds its threshold. If there is no signal input at the sensor due to external fields the LED will flash indicating a high probability that the unit is operational.

IClc is arranged as a simple voltage detector.

Pin 9 is fixed at 1.2v by the reference diode D1.

Pin 10 connects to a voltage divider, R13 and R14. -These are chosen such that the voltage on pin 10 will be 1.2v when the battery voltage has fallen to 5v. When this happens the output pin 8 falls to Ov and inhibits the oscillator, IClb, from running and hence the LED from flashing.

In order to recharge the battery an inductive charging circuit as shown in Fig.2 can be used. In operation Capacitor C1 is charged via R1, at appropriate voltage, DlAC triggers thyristor 8T151 which in turn discharges C1 via L2 producing a high current pulse.

The flux coupled to L1 producing a pulse of energy to be fed to the battery in the deadline checker R1, may be varied to allow for different input voltages.

Claims (6)

1. A dead line checker comprising a sensor for inductively sensing the presence of a voltage and means controlling the sensor to prevent operation thereof below a predetermined voltage.

2. A dead line checker as claimed in Claim 1, wherein means are provided for producing a signal which indicates that the checker is operative.

3. A dead line checker as claimed in Claim 1 or Claim 2, wherein a rechargeable battery is provided as the power source.

4. A dead line checker as claimed in Claim 3, wherein the battery is contained in a sealed housing, inductively operable charging means being provided in the housing.

5. A dead line checker as claimed in Claim 4, wherein the sealed housing is adapted to be received in another housing containing inductively operable charging means.

6. A dead line checker substantially as described herein with reference to the accompanying drawings.