GB2083310A - Voltage detector circuits - Google Patents

Abstract

A method of and apparatus for monitoring first and second voltages in an electric circuit is suitable for use in earth leakage protection devices. A capacitor is charged from the first of the voltages (being the larger) through a first resistor and is discharged through an impedance across which the second voltage is applied and a resistor in series with it. Oscillations are generated in a circuit comprising an oscillating four- layer device, such as a programmable unijunction transistor, and a bias voltage applied to the anode-gate of the programmable unijunction transistor is related to the voltage existing across the capacitor. Oscillations are supressed below and above lower and upper cut-off points on the bias voltage characteristic of the programmable unijunction transistor, and an appropriate monitor, such as a switching diode, responds to the disappearance or abrupt reduction in amplitude of the oscillations. <IMAGE>

Description

SPECIFICATION Voltage detector circuits This invention relates to apparatus for, and a method of, monitoring the presence of, or changes in the value of, voltages in electric circuits. More especially, but not exclusively, it relates to a technique for use in electric protective circuits that provides a fail-safe response to earth-leakage (ground current) and/or undervoltage.

In accordance with one aspect of the invention, apparatus responsive to at least one of a first voltage and a second voltage, the first being larger than the second comprises: a capacitor; a charging circuit for the capacitor comprising a first resistor and connected to the said first voltage; a discharge circuit for the said capacitor comprising an impedance across which the said second voltage may appear or be applied and a second resistor in series with it; an oscillator circuit comprising an oscillating programmable unijunction transistor (which may be powered by the said first voltage or by an independent source); means for applying to the anode gate of the programmable unijunction transistor a voltage related to the voltage existing across the said capacitor; and means for monitoring the presence of oscillations in said oscillator circuit.

When the first voltage and/or the second voltage is an alternating voltage, the charging circuit and/or the discharge circuit, as the case may be, will include a diode or other rectifier (which may also serve as the resistor in some cases) so that a unidirectional voltage is maintained across the said capacitor; when both circuits include rectifiers, they need to be correspondingly polarised.

The invention includes a method of monitoring either or both of first and second voltages in an electric circuit, the first voltage being larger than the second, comprising: charging a capacitor from the first voltage through a first resistor; discharging the capacitor through an impedance across which the second voltage is applied and and a resistor in series with it; generating oscillations in a circuit comprising an oscillating programmable unijunction transistor; applying to the anode gate of the programmable unijunction transistor a bias voltage related to the voltage existing across the capacitor; and monitoring the presence of the said oscillations.

The means for monitoring oscillation may be of any appropriate type, depending on the application, and may for example include impedance networks, isolating transformers and/or capacitors, electronic and/or electromechanical switches, amplifiers, meters, light-emitting diodes etc.

The characteristics of the programmable unijunction transistor are such that oscillation can be maintained only between steeply bounded lower and upper cut-off points on the anode gate voltage characteristic, the lower cut-off point being relatively insensitive to the anode voltage and the upper cut-off point approximately proportional to it.

The apparatus is more sensitive to the second voltage than to the first, and the voltage, or the more critical of the voltages, to be monitored should accordingly be applied as the second voltage (in the event that the more sensitive of the two voltages to be monitored is the larger, a potential divider, transformer or the like can be used to derive a voltage proportional to it and less than the other voltage to be monitored).

The polarity of the second voltage, or of the rectifier in the discharge circuit if the second I voltage is alternating, is preferably such than an increase in the second voltage (from zero or from its normal value) tends to increase the current in the discharge circuit, and component values will normally be chosen, and adjusted if necessary, so that in normal conditions the voltage applied to the anode-gate is just above the lower cut-off point.

Any increase in the second voltage then rapidly brings the oscillator circuit to the lower cut-off point and thereby stops any output or at least produces an abrupt and easily detectable reduction in amplitude; a short circuit in the discharge circuit has the same effect, and immediate complete loss of output also results, of course, on loss of the first voltage or of power to the oscillator circuit, if separate. The second voltage may decrease, within limits, but a large decrease will result in the capacitor accumulating charge to the point at which the anode-gate voltage reaches the upper cut-off point and oscillation stops or at least abruptly reduces in amplitude.

The first voltage may vary within limits in either direction, but a substantial decrease will of itself bring the system to the upper cut-off point and a substantial incresase will lead to an accumulation of charge in the capacitor with the same result. Major malfunctions in any of the main circuit components will normally also suppress oscillation, and so the system is almost entirely fail-safe.

The invention also includes power circuits, especially in mines and quarries, protected by the techniques described The invention will be further described, by way of example, with reference to the drawings in which Figure I is a circuit diagram of a monitor module for an earth-leak (ground fault) circuit breaker suitable for protecting alternating-current power circuits to the safety standards required for use in mines, and the remaining figures illustrate various ways of applying the module of Fig. 1.

In Fig. 1, dotted lines represent connections made externally to the module.

In the form shown in Fig. 1, the first voltage to be monitored is that of the A.C. supply, connected to terminals B and D, and the second voltage to be monitored is that existing across a core balance transformer CBT, which is close to zero in the absence of any earth-leak current.

The supply voltage is rectified by a diode bridge BR and charges capacitor C4 through resistors R2, R3, R6, of which R6 is a pre-set to adjust sensitivity. Zener diode ZD and capacitor C2 form a voltage-limiting circuit.

The capacitor C4 discharges through the core balance transformer CBT and the impedance of diode D1. C5 is simply a harmonic filter, and can be omitted if the supply is reasonably free of harmonics.

By the external connection between terminals 4 and 5, the output of the bridge BR is utilised also as a power supply for the oscillator circuit comprising the programmable unijunction transistor PUT, feedback resistor R5, frequency-setting components R4 and C3, and the primary of pulse transformer T.

The voltage present between R3 and R6, which is in fixed ratio with that across C4 while R6 remains constant, is connected to the anode-gate of the programmable unijunction transistor.

In normal operation, the oscillator injects pulses into the secondary of transformer T; unwanted half-cycles are shunted by diode D and the surviving unidirectional pulses fed to the gate of a silicon-controlled rectifier SCR, so causing it to draw current through the main circuitbreaker relay RL/2. If, for any of the reasons described above, oscillations cease or are reduced in amplitude to an extent depending on the characteristics of the silicon-controlled rectifier, it ceases to conduct and RL/2 opens in a time determined (and adjustable, if desired) by the capacitor C6.

In Figure 2the module M/E is the same as the module of Fig. 1, except that the diode D1 has been placed physically close to a remote core balance transformer, as this gives a more secure protection against short-circuit faults in the connecting cable. This figure illustrates the provision of simple lock-out facilities (i.e. facilities preventing re-closure of the circuit breaker while an earth fault persists). In this case voltage supply for the oscillator circuit is taken not from terminal 4 but from a potentiometer VR1 connected across the regulated voltage at terminals 7 and 8. The voltage se derived is taken to the oscillator (terminal 5) via two voltagedivider resistors R21 and R22. Any earth-leak ground impedance shuts R21 and sufficiently alters the voltage at terminal 5 to prevent oscillation and so prevent RL/2 for closing.An indicator LED1 displays the condition of RL/2.

Figures 3 and 4 illustrate how two modules of the kind described can be used in basically conventional lock-out circuits to provide further fail-safe character, for example by continuously monitoring the earth (ground) connections El and E2.

Figure 5 illustrates the use of the module M/E in conjunction with a conventional lock-out module L/O operating independently of it.

Figures 6-7 illustrate the use in conjunction with the invention of conventional latching and indicating circuits.

Figures 8-12 illustrate subsidiary uses of the standard module of Fig. 1.

Fig. 8 to provide low-voltage remote control for a contactor; Fig. 9 the same with undervoltage detection facility; Figs. 10-11 as low-voltage control circuits to monitor drop-off and pick-up of contactors; and Fig. 1 2 as a control circuit for electric motors with thermistor-actuated over-temperature cutout provision.

In the major application of earth-leak detection, the technique of the invention has the merits of having a controlled fast response time (but not so fast as to respond to switching transients), simplicity, and immunity to noise that is better than conventional circuits.

With the component values given in Table I, the sensitivity is about 1 00mA, and a sensitivity of 20-30mA can easily be achieved if required. This is especially beneficial in installations in accordance with North American practice in which the neutral point of the circuit is earthed via a current-limiting resistor; with increased sensitivity the value of this resistor can be increased to the extent that the maximum earth-leak current can be reduced from conventional values of around 1 5 A to around 750mA; the maximum earth-circuit resistance (set by the maximum voltage permitted between equipment items) is greatly increased.

TABLE I COMPONENT REFERENCE TWE s NO. MAKER Silicon Controlled SCR BTX X 1 8 - 400 Mullard Rectifier Diode D 1N4001 R.S. Components Ltd Diode D1 1N4003 ,, Zener Diode ZD 12V, 1.3W Mullard Bridge Rectifier BR 0.9A 200V R.S. Components Ltd Programmable Uninjunction Transistor PUT BRY 39 Mullard Pulse Transformer P 1:1 Ratio R.S. Components Ltd Resistors R2 1kOhm 1W " " ,, R3 R3 120kOhm SW ,, ,, R4 47kOhm or " ,, 68kOhm *W RS R5 8k0hm -41W ,, Capacitors C2 22 nF 250V dc " ,, C3 100 nF 250V dc ,, C4 471lF 63V dc ,, Power Control Circuit Resistor R1 1000hm 2.5W w/w ,, Capacitor C 100of Min. 220 Max. ,, Relay RL/2 12V.-205 Ohm coil ,, (minature PCB) Additional and When Necessary Capacitor C5 10 F 63V ac ,, (non-polarised)

Claims (1)

1. Apparatus responsive to at least one of a first voltage and a second voltage, the first being larger than the second, comprising: a capacitor; a charging circuit for the capacitor comprising a first resistor and connected to the said first voltage; a discharge circuit for the said capacitor comprising an impedance across which the said voltage may appear or be applied and a second resistor in series with it; an oscillator circuit comprising an oscillating programmable unijunction transistor; means for applying to the anode-gate of the programmable unijunction transistor a voltage related to the voltage existing across the said capacitor; and means for monitoring the presence of oscillations in the said oscillator circuit.

2. Apparatus as claimed in Claim 1 in which the charging circuit and the discharge circuit include correspondingly polarised rectifiers.

3. Apparatus as claimed in Claim 1 or Claim 2 in which the means for monitoring oscillation includes an electronic switch.

4. A method of monitoring either or both first and second voltages in an electric circuit, the first voltage' being larger than the second, comprising: charging a capacitor from the first voltage through a first resistor; discharging the capacitor through an impedance across which the second voltage is applied and a resistor in series with it; generating oscillations in a circuit comprising an oscillating programmable unijunction transistor; applying to the anode-gate of the programmable unijunction transistor a bias voltage related to the voltage existing across the capacitor; and monitoring the presence of the said oscillations.

6. A method as claimed in Claim 5 in which under normal conditions the voltage on the anode-gate of the programmable unijunction transistor is just above its lower cut-off point.

7. Voltage detector apparatus substantially as described with reference to and as shown in Fig. 1.

8. Voltage detector apparatus substantially as described with reference to Fig. 1 and any one of Figs. 2 to 12.

9. A method of detecting voltages substantially as described with reference to at least Fig. 1 of the drawings.

10. A power circuit protected by the apparatus or method claimed in any one of the preceding claims.