METHOD AND APPARATUS FOR INDICATING AN ALTERNATING VOLTAGE

Abstract: The invention pertains to an alternating voltage indicator placed next to an electrical conductor. Indication is based on the detection of an electric field around a live conductor. Energy is taken capacitively (100) from an alternating electric field in order to bring a resonant circuit (200) tuned to the frequency of the alternating voltage into oscillating state. The voltage of the resonant circuit (200) is amplified (300) and converted into an on/off type (400) signal indicating the presence of an alternating voltage. The indicator according to the invention is electrically safe and it indicates the presence of an alternating voltage regardless of whether the alternating voltage source is loaded at that particular point or not.

Description: Method and apparatus for indicating an alternating voltage The invention relates to a method for indicating an alternating voltage primarily in an electrical conductor as defined in the preamble of claim 1. The method makes it possible to obtain information about whether an alternating voltage, such as mains voltage feeding an apparatus, is on, without touching live conductor and independent of the load caused by the apparatus. The invention also relates to an apparatus for indicating an alternating voltage in an electrical conductor as defined in the preamble of claim 2. It is known from the prior art apparatus to indicate the electrical service mains voltage in particular. Their operation is based on one of the following principles: 1) A conductor is sensed inductively by means of a coil placed in its vicinity. 2) Galvanic coupling is made to the conductor to be monitored. Principle 1) produces compact and safe indicators. They, however, suffer from the disadvantage that they indicate an alternating voltage only if the conductor has current flowing through it. If the alternating voltage is unloaded, there is no current and thus no magnetic field, resulting in that the apparatus incorrectly indicates that no voltage is present. Principle 2) produces indicators that correctly indicate the existence of an alternat- ing voltage both in the loaded and unloaded case. They, however, have the disadvantage that galvanic coupling to a live conductor is a safety hazard. Also, installation of the apparatus is difficult in cases where a cord has to be peeled or a plug has to be opened. The cost of manufacture and installation of an indicator that meets the electrical safety regulations is relatively high. It is, among other things, an object of the invention to eliminate the aforementioned disadvantages. The method according to the invention is characterized by what is expressed in claim 1. The apparatus according to the invention is characterized by what is expressed in claim 2. The basic idea of the method is as follows: An electrical conductor to be monitored is capacitively sensed. The alternating electric field of a live conductor charges the pair of plates, which is used as sensor and has a small capacitance. In parallel with the pair of plates is connected a resonator tuned close to the mains frequency, which resonator is made to oscillate by the energy coming from the pair of plates. The energy of oscillation is remarkably higher than the energy existing in the field of the pair of plates. If in the vicinity occurs electric fields having another frequency, the same kind of amplification doesn ' t happen relating to those fields. The signal of the resonator is then further amplified and shaped e.g. such that its has two states (on/off). State "on" means an alternating voltage is present, and state "off means it is missing. It is an advantage of the invention that an alternating voltage is indicated regardless of whether a current flows through the conductors or not. In both cases, there is an electric field around the conductors. It is a further advantage of the invention that the indicator is safe as no galvanic connection is required to live parts. The invention is advantageous in a system where the operating voltage of an apparatus is a direct voltage generated by means of an AC/DC power supply from the mains alternating voltage. If the direct voltage disappears, one cannot immediately know the reason for it if one does not have some auxiliary equipment in use. The reason may be e.g. a short-circuit in the apparatus fed, fault in the stabilizer of the AC/DC power supply, blown fuse in the AC/DC power supply, or missing mains voltage. If it is the latter, a mains voltage indicator reveals it immediately. If the system includes a back-up power supply, the indicator can also switch it on auto- matically. The invention will now be described in more detail. Reference will be made to the accompanying drawing wherein Figure 1 is a block diagram of the alternating voltage indicator according to the invention, Figure 2 illustrates the principle according to the invention for capacitive sensing of an alternating voltage, Figure 3 shows an exemplary circuit diagram of the indicator, Figure 4a shows an exemplary realization of the sensor as foil pattern on a printed circuit board, Figure 4b shows a mains electric plug to which is fitted the indicator according to Figure 4a and Figure 5 shows another exemplary placing of the indicator. Figure 1 shows a capacitive sensor 100 which, when in an alternating electric field, causes a resonator 200 to oscillate at the frequency of the alternating voltage. The resonator is connected to a processing circuit 500. This comprises an amplifier 300 and a detector circuit 400. The alternating voltage of the resonator 200 is amplified by an amplifier 300. The amplified alternating voltage is converted into an on/off type direct voltage in the detector circuit 400 in order to indicate the presence or absence of an alternating voltage. Figure 2 shows conductors 97, 98 of an AC line viewed from the end such that the conductors are shown in cross section. Small conductive plates, or electrodes 101 and 102 are placed near the conductors. When there is a voltage present in the line, an electric flux exists in its vicinity. Part 99 of the flux flows between the electrodes 101, 102 serving as sensing elements, causing a low voltage between them. As the line voltage is an alternating voltage, the voltage between the plates is also an alternating voltage of the same frequency. The circuit further comprises a parallel resonator 200 having an inductive unit L and a capacitive unit C. The electrode 101 is connected to the upper end of the resonator 200 and the electrode 102 to its lower end. This alternating voltage of the pari of the plates affects across the resonator 200, causing it to oscillate. The resonator 200 is a parallel resonant circuit. In order for the arrangement according to the invention to be able to make it oscillate at a sufficient amplitude, the losses of the resonator have to be small and, on the other hand, its reactances have to be big. The latter prerequisite means small capacitance and high inductance. High inductance can be realized in a small space only with active circuits. In the example of Figure 3, inductance 201 is produced by means of gyrator circuit 201 comprising operational amplifiers Al and A2. The value of inductance L is the product of component values R2, R3 and Cl. If R2 and R3 are 332 kΩ, and Cl is 1 μF, is the value of inductance 1 10 kH. If the resonator capacitance C2 is 80 pF, the resonant frequency is 54 Hz, which is right close to the mains frequency 50 Hz. If an AC field from the vicinity is directed to the sensor 100, the frequency of which field is other than the mains frequency, the resonator does not react. Only the frequencies which correspond to the main frequency are accpted to the subsequent processing. The diodes Dl and D2 across the resonant circuit are for protection. The amplitude of the oscillation is so small that the diodes are insignificant in the primary operation. In Figure 3 the voltage at the lower end of the resonator 200 is fixed to zero level by means of an operational amplifier A8. Let Uj represent the alternating voltage at the upper end of the resonator 200. The circuit of operational amplifier A3 produces the voltage Ui as amplified and the circuit of operational amplifier A4 produces the voltage U\ nearly as much amplified as the circuit of A3 buy inverted. So, U3 « -U2. The circuit of operational amplifier A5 produces in theory the voltage U4 = 2-(R10/R8)-U 2 at low frequencies. The component values chosen in this case give at 50 Hz frequency an amplification of such magnitude that voltage U4 is clipped, i.e. it is a square wave limited by the operating voltages ± 7.5V. A high-pass filter C7, R12 is used to drop the average level of said square wave and round its shape. The result is the voltage U 5 . The circuit of operational amplifier A6 charges a capacitor C8 so that voltage U rises to the upper level of voltage U5, i.e. a little below zero. It also stays there while U5 is in the lower state because of a high time constant R14 C8. If the alternating voltage monitored disappears, the waveform of voltage U5 disappears, too, and voltage Ug settles at -7.5V through a resistor R14. The voltage U 6 is lead to a comparator realized by an amplifier A7. The comparator gives the whole indicators output voltage U 7 , the levels of which are approximately +6.5 V (ON) and -6.5V (OFF). There is a hysteresis located approximately in the middle of the fluctuation range of voltage Uβ in order to prevent unnecessary variation of voltage U7 in the transition state of the mains voltage. Figure 4a shows an example of the realization of the sensor 100. The sensor is realized on a printed board. The sensing electrodes 101, 102 of the alternating electric field are formed directly on a printed circuit board 45 as conductive areas S I, S2. Figure 4b shows an example of placing of the indicator according to the invention. In Figure we can see a mains voltage plug 41 as a side view partially open cutted, and the mains wire 42 with protecting rubber. In the plug we see one mains voltage wire 43 and one mains voltage contact 44. The printed circuit board 45 in Figure 4a is placed into the plug 41. To the circuit board is connected an other wire 46 to supply power to the indicator and to getting out the indicate result. Figure 5 shows another example of placing of the indicator according to the invention. In Figure we can see a rectangular plug 51 and the mains voltage wire 52, the device 59 supplied from the mains and the indicator 55 of alternating voltage. In this case the indicator 55 is placed outside the mains voltage plug, between said plug and device 59. Above it was described an example of the circuit indicating a mains voltage and examples of the practical implementations. The invention is not limited to the embodiments described here, nor to indicating a mains voltage. The same inventional idea can be applied in all cases where it is desirable to obtain knowledge about the presence of a cyclically alternating voltage.

Claims: Claims 1. A method for indicating an alternating voltage in electrical conductors, characterized in that in the vicinity of conductors (97, 98) an electric field is sensed by means of electrodes (101, 102) capacitively coupled to said conductors, the voltage generated between the electrodes in an electric field is lead to a resonant circuit (200) tuned substantially to the frequency of the alternating voltage in order to produce oscillation, and the resonant circuit voltage is processed to produce a signal indicating the presence of said alternating voltage. 2. An apparatus for indicating an alternating voltage, characterized in that it comprises - electrodes (101 and 102) for capacitively sensing an electric field around said conductors, - a resonant circuit (200) to which the electrodes (101, 102) are connected and which is tuned substantially to the frequency of the alternating voltage to produce oscillation when an electric field exists, and - a processing circuit (500) to shape the voltage obtained from the resonant circuit such that it indicates said alternating voltage. 3. The apparatus of claim 2, characterized in that the electrodes are formed of conductive areas (SI, S2) arranged on a circuit board (45). 4. The apparatus of claim 2 or 3, characterized in that the inductance (L) of the resonant circuit (200) is formed by means of an active circuit (201). 5. The apparatus of any one of the preceding claims 2 to 4, characterized in that said processing circuit (500) comprises a high-impedance differential amplifier (300). 6. The apparatus of any one of the preceding claims 2 to 4, characterized in that said processing circuit (500) comprises a circuit (400) which detects an alternating voltage having a large amplitude.

Inventors: RIDAL TOIVO, SKOGBERG JOAKIM, RIDAL, TOIVO, SKOGBERG, JOAKIM
Applicants: NOKIA TELECOMMUNICATIONS OY, RIDAL TOIVO, SKOGBERG JOAKIM, RIDAL, TOIVO, SKOGBERG, JOAKIM
Application year: 1998
Main classification: G01R19
IPC codes: G01R19