Electrical transmission or interconnection systems – Nonlinear reactor systems – Parametrons
Patent
1987-05-15
1990-05-08
Miller, Stanley D.
Electrical transmission or interconnection systems
Nonlinear reactor systems
Parametrons
307116, 307246, 307308, 307547, 307564, 307554, 361179, 361181, H03K 1730, H03K 1726, H03K 1728, H03K 3013
Patent
active
049241222
DESCRIPTION:
BRIEF SUMMARY
This invention relates to proximity switches and particularly, but not exclusively, to the type in which an induction coil is driven by an oscillator and the presence of a metallic object in proximity to the switch causes a change in the amplitude of oscillation which is detected by a detector circuit to close a switching element such as a thyristor or transistor. Such devices are well known in principle, one example being described in our British Pat. No. 1531217.
Another device of this general type is described in our PCT/GB/83/00149, which is particularly concerned with ensuring low power dissipation in and minimum voltage drop across the device in the "on" state.
It is desirable in such switches that the "off" state residual current be minimal, and one object of the present invention is to provide a proximity switch circuit which furthers this aim, and which is applicable to switches utilising an induction coil sensor as described above as well as other sensor types.
A problem associated with switches hitherto known is that, upon initial application of the supply voltage, false "on" signals may be generated prior to stabilisation of the sensing circuitry. It is a further object of the present invention to obviate or mitigate the occurrence or erroneous signals under these conditions.
According to one aspect of the invention, a proximity switch for use with an A.C. supply to control a load connected in series with the switch comprises:
a power supply including a storage capacitor;
and
a controllable switching element connected, in use, across said AC supply and having a control input;
characterised in that said power supply further includes means for limiting the current charging said storage capacitor to a predetermined maximum value when the switch is "off".
Preferably, said limiting means includes a variable resistance device connected in series with said storage capacitor and means responsive to the level of the charging current for controlling the resistance of said device.
Preferably also, said variable resistance device is a MOSFET and said charging current responsive means controls the MOSFET gate voltage.
Preferably also, said means for controlling the gate voltage of said MOSFET includes a transistor, the base-emitter voltage of said transistor being proportional to the charging current of said storage capacitor.
Preferably also, the switch includes means for bypassing said transistor when the switch is "on".
Preferably also, said sensor means comprises an induction coil, an oscillator connected to drive the coil, and a level detector connected to the oscillator to provide a detector output signal in response to reduction in the amplitude of oscillation caused by proximity of a metallic object to the coil.
According to a second aspect of the invention, a proximity switch for use with an AC supply to control a load connected in series with the switch comprises:
a power supply including a storage capacitor;
an induction coil;
an oscillator connected to drive the coil;
a level detector connected to the oscillator to provide a detector output signal in response to reduction in the amplitude of oscillation caused by proximity of a metal object to the coil; and
a controllable switching element connected, in use, across said A.C. supply and having a control input;
characterised by time delay means for overriding said level detector upon initial application of said A.C. supply so as to prevent generation of a detector output signal until a predetermined period of time has elapsed.
Preferably, said time delay means includes a controlled conduction device connected so as to clamp the output of said level detector to a predetermined level and thereby prevent the generation of an output signal.
Preferably also, said controlled conduction device is a transistor whose base-emitter voltage is controlled such that the transistor switches off after a predetermined period of time, enabling the level detector to operate normally.
Preferably also, said controlled conduction device is responsive to timing means co
REFERENCES:
patent: 4138709 (1979-02-01), Colwill
Paul Richman, "Characteristics and Operation of MOS Field-Effect Devices", McGraw-Hill, 1967, p. 116.
"Commutating and Interfacing with Junction and MOS FETs", by D. Hargrave, Electronic Engineering, pp. 56-59, Dec. 1969.
Bertelson David R.
Femal Michael J.
Miller Stanley D.
Square D Company
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