Telephonic communications – Subscriber line or transmission line interface – Protective circuit
Patent
1985-05-06
1989-04-11
NG, Jin F.
Telephonic communications
Subscriber line or transmission line interface
Protective circuit
361119, 361193, 307544, 307257, H04B 300, H03K 1700
Patent
active
048213152
DESCRIPTION:
BRIEF SUMMARY
The invention relates to electronic contacts enabling to establish a low or high impedance between a first and a second terminal under the control of a circuit providing a control signal between a third and a fourth terminal.
Such electronic contacts are for instance used in the Belgian Pat. No. 896,388 particularly relating to a controlled capacitive charge circuit enabling to positively or negatively charge a capacitance which, in accordance with the sign of this charge, opens or closes an electronic contact constituted by two DMOS transistors in series opposition, in such a way that their drains respectively constitute the two terminals of the electronic contact while their sources are each tied to the same terminal of the capacitance and their gates both connected to the other terminal of the capacitance, with the possibility for this last to be constituted by the parasitic capacitance between these paired terminals. In such a way, by using transistors able to withstand relatively high voltages, one obtains an electronic contact which can be inserted in a circuit where one or the other polarity may appear at the contact terminals. Indeed, when the polarity of the charge on the control capacitance for the contact is such that it does not offer a low resistance path, i.e. that the two transistors are blocked, the parasitic diodes which appear for this transistor state between the source and the drain are thus connected also in series opposition which maintains a high impedance whatever the polarity applied by the circuit in which the contact is inserted.
One of the objects of the present invention is to enable the use of a more advantageous type of electronic contact able also to be controlled by the polarity of the charge of a capacitance, and particularly thyristor type device able to work with high breakdown voltages (300 Volts for instance) as envisaged in the above mentioned patent but which can only pass current in one direction while they can block voltages of one or the other polarity, the transistors of the above mentioned patent having inverse properties, i.e. they can conduct the current in one or the other direction but block only one voltage polarity.
The general object of the present invention is to enable the use of such electronic contacts while avoiding a complication of the control circuit.
In accordance with a first characteristic of the invention, the electronic contact defined above is characterized in that two auxiliary electronic contacts are foreseen and enable to establish a low or high impedance between the first and the third terminals and between the second and the third terminals, the impedance conditions of these two auxiliary contacts being opposed.
Such an arrangement offers the advantage that two electronic contacts of the thyristor type can be connected top against bottom as a triac and controlled with the help of the same control circuit and particularly that of the above mentioned patent using a positive or negative charge for a capacitance to close or open the electronic contact. Indeed, with the help of the auxiliary electronic contacts, in accordance with the polarity of the voltage applied to the terminals of the electronic contact constituted by the two biassed contacts connected in shunt opposition, it will automatically be possible to obtain a connection between a terminal of the control capacitance and the terminal of the main electronic contact having a given polarity. In this manner, the same capacitance charge circuit, i.e. the voltage doubler AC/DC converter described in the above mentioned patent, shall always be used to close or open that of the two polarized contacts which is effectively inserted in a load circuit and depending upon the polarity of the voltage appearing at the terminals of these contacts connected in shunt opposition.
On the other hand, the advantage of thyristor electronic contacts able to be controlled in the indicated manner, and with respect ot the DMOS transistors connected in series opposition as in the above mentioned Belgian pate
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Remmerie Guido P. T. C.
Van den Bossche Luc J. L.
ALCATEL N.V.
Ng Jin F.
Vaas Randall S.
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