GB1591870A - Telephone line isolating circuit - Google Patents

Description

(54) TELEPHONE LINE ISOLATING CIRCUIT (71) We, PYE (ELECTRONIC PRODUCTS) LIMITED, a British company, of St. Andrew's Road, Cambridge, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to a circuit arrangement providing electrical isolation between a telephone line and equipment coupled thereto.

Equipment coupled to a telephone line is frequently connected through an isolating transformer the primary winding of which provides a d.c. path across the line.

However, such a transformer has to be relatively large and expensive since it has to be constructed so that the direct current flowing in the line does not saturate its core.

It is known, to a.c. couple the transformer to the line by connecting a capacitor in the line and to connect an inductive choke across the line to provide the required d.c.

path. However, an inductive choke is a relatively expensive and bulky component.

It is an object of the invention to provide a circuit arrangement providing electrical isolation between a telephone line and equipment coupled thereto, the circuit arrangement having an isolating transformer a.c. coupled to the line and a d.c. path provided by means other than an inductive choke.

The invention provides a circuit arrangement for providing electrical isolation between a telephone line and equipment coupled thereto, the circuit arrangement comprising an isolating transformer a.c. coupled to the line and a d.c. path connected across the line, the d.c.

path comprising the collector emitter path of a transistor wherein the collector emitter path of the transistor is arranged to have a low impedance to direct currents and a high impedance to alternating currents. The collector emitter path of the transistor may be connected across the line via a bridge rectifier.

To protect the transistor from large voltage transients on the line the voltage across the collector emitter path of the transistor may be limited by a voltage reference diode.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, the sole Figure of which shows a circuit arrangement for electrically isolating equipment from a telephone line.

The circuit arrangement shown in the drawing has two input terminals 1 and 2 to which, in operation, the telephone line is connected. The input terminals are coupled via a capacitor Cl to the primary winding of a transformer Tl. The secondary winding of the transformer Tl is connected between earth and an output terminal 3 to which equipment to be connected to the telephone line is coupled. Terminal 1 is connected to the anode of a diode Dl and the cathode of a diode D2.The cathode of diode Dl is connected to the cathode of a diode D3, the collectors of two transistors TRI and TR2, one end of a resistor Rl, and to the cathode of a voltage reference diode VRl. The anode of diode D2 is connected to the anode of a diode D4, one end of a resistor R3, the negative plate of an electrolytic capacitor C2 and the anode of the voltage reference diode VRl. The other end of resistor Rl is connected to the positive plate of capacitor C2, the base of transistor TRI, and to one end of a resistor R2, the other end of which is connected to the emitter of transistor TR2 and the other end of resistor R3. The emitter of transistor TRI is connected to the base of transistor TRI.

The junction of the anode of diode D3 and the cathode of diode D4 is connected to terminal 2.

In operation d.c. line currents flow through the bridge rectifier made up of diodes Dl to D4 which causes the polarity of the supply to the transistors TRI and TR2 to be constant regardless of changes in polarity of the supply to terminals 1 and 2, i.e. if terminal I is positive with respect to terminal 2 diodes Dl and D4 conduct but if terminal 2 is positive with respect to terminal I diodes D2 and D3 conduct. The bridge rectifier would not be necessary if the supply to terminals 1 and 2 always had the same polarity. A direct current on the line will cause the transistor pair TRI and TR2 to become conductive when capacitor C2 has charged to a potential set by the biassing resistor chain Rl, R2 and R3 and thus a direct current path across terminals I and 2 will exist via the bridge rectifier, the collector-emitter path of transistor TR2 and resistor R3.However, a.c. signals on the line will not affect the current flow through the collector-emitter path of transistor TR2 because of capacitor C2 and hence this path has a high impedance to a.c. signals.

Consequently a.c. signals on the line will pass via capacitor Cl and transformer Tl to any equipment connected to terminal 3. As shown in the drawing one end of the secondary winding of transformer Tl is connected to earth this is, however, not essential and the two ends of the transformer secondary may be brought out to provide a. balanced or non-earthed output.

The voltage reference diode VRI is included to protect the transistors from high voltage surges on the line. In order to limit the current through diode VRI a resistor should be included in one . leg of the telephone line or alternatively between terminals I or 2 and the bridge rectifier.

WHAT WE CLAIM IS: 1. A circuit arrangement for providing electrical isolation between a telephone line and equipment coupled thereto, the circuit arrangement comprising an isolating transformer a.c. coupled to the line and a d.c. path connected across the line, the d.c.

path comprising the collector-emitter path of a transistor wherein the collector-emitter path of the transistor is arranged to have a low impedance to direct currents and a high impedance to alternating currents.

2. A circuit arrangement as claimed in Claim I in which the collector-emitter path of the transistor is connected across the line via a bridge rectifier.

3. A circuit arrangement as claimed in Claims 1 or 2 in which the voltage across the collector-emitter path of the transistor is limited by a voltage reference diode.

4. A circuit arrangement for electrically isolating equipment from a telephone line substantially as described herein with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. terminal 2 is positive with respect to terminal I diodes D2 and D3 conduct. The bridge rectifier would not be necessary if the supply to terminals 1 and 2 always had the same polarity. A direct current on the line will cause the transistor pair TRI and TR2 to become conductive when capacitor C2 has charged to a potential set by the biassing resistor chain Rl, R2 and R3 and thus a direct current path across terminals I and 2 will exist via the bridge rectifier, the collector-emitter path of transistor TR2 and resistor R3. However, a.c. signals on the line will not affect the current flow through the collector-emitter path of transistor TR2 because of capacitor C2 and hence this path has a high impedance to a.c. signals. Consequently a.c. signals on the line will pass via capacitor Cl and transformer Tl to any equipment connected to terminal 3. As shown in the drawing one end of the secondary winding of transformer Tl is connected to earth this is, however, not essential and the two ends of the transformer secondary may be brought out to provide a. balanced or non-earthed output. The voltage reference diode VRI is included to protect the transistors from high voltage surges on the line. In order to limit the current through diode VRI a resistor should be included in one . leg of the telephone line or alternatively between terminals I or 2 and the bridge rectifier. WHAT WE CLAIM IS:

1. A circuit arrangement for providing electrical isolation between a telephone line and equipment coupled thereto, the circuit arrangement comprising an isolating transformer a.c. coupled to the line and a d.c. path connected across the line, the d.c.

path comprising the collector-emitter path of a transistor wherein the collector-emitter path of the transistor is arranged to have a low impedance to direct currents and a high impedance to alternating currents.

2. A circuit arrangement as claimed in Claim I in which the collector-emitter path of the transistor is connected across the line via a bridge rectifier.

3. A circuit arrangement as claimed in Claims 1 or 2 in which the voltage across the collector-emitter path of the transistor is limited by a voltage reference diode.

4. A circuit arrangement for electrically isolating equipment from a telephone line substantially as described herein with reference to the accompanying drawing.