AU716232B2 - Switchable isolating capacitor - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "SWITCHABLE ISOLATING CAPACITOR" The following statement is a full description of this invention, including the best method of performing it known to us:- Technical Field This invention relates to isolating capacitors.

Capacitors are useful in providing DC isolation in electrical circuits where both DC and alternating signals occur. One application where both DC and alternating signals occur is in the telephone and the invention will be described with reference to the telephone ring circuit.

Background Art The telephone ring circuit is connected across the line while the phone is ON hook and isolating capacitors are used to prevent the ring circuit drawing DC current which may be interpreted by the exchange as a looped line. The capacitors must be capable *010 of passing the ring signal which is typically 16Hz, 75V. Sufficient power must be drawn by the ringer circuit to produce an audible ring sound so the capacitors are relatively large. Some telephone utilities specify a lIkF capacitance for line testing.

""'"Where there are a pair of isolating capacitors in series to provide isolation to both line Sterminals, each capacitor should be 2gtF. Where 1 iF capacitors are used as standard 15 components, each 2gF capacitance would need to be made up of a pair of 1,uF capacitors in parallel. Thus a total of 4 x 1 F capacitors are required to produce a net 1 gF capacitance.

Such an arrangement may require a large surface area on a printed circuit board, and in modern telephones with increasing component densities, this space may 20 not be readily available.

Disclosure of the Invention This specification discloses an isolating capacitor arrangement which uses a single isolating capacitor to provide DC isolation to both line terminals and to pass alternating signals.

Accordingly there is disclosed an isolating arrangement for isolating a load from a DC voltage while permitting a time varying signal to be applied to the load, the arrangement including; a capacitor; a switching arrangement to provide a first path between first and second terminals through the capacitor and the load during at least part of the period during which the time varying signal applied across the first and second terminals is positive with respect to a reference voltage, the switching arrangement providing a second path between the first and second terminals during at least part of the period during which the time varying signal is negative with respect to the reference voltage.

Brief Description of the Drawings Figure 1 is a schematic diagram of the use of a pair of DC isolating capacitors.

Figure 2 illustrates DC isolation of a telephone ringer circuit from both input terminals using two capacitors, C1, C2.

Figure 3 shows a first embodiment of the invention.

Figure 4 shows an embodiment of the invention using a single switched 10 isolating capacitor.

S' Figure 5 shows a further embodiment of the invention.

S'Best mode of carrying out the invention Figure 1 shows an exemplary circuit illustrating the use of two blocking ooo•• 0 capacitors.

15 Two impedances Z1 and Z2 are connected across lines 1 and 2 to which a DC supply 8 and alternating signal generator 7 are applied across lines 1 and 2.

Z2 is connected to line 1 via capacitor C1 and to line 2 via capacitor C2.

Capacitors C1 and C2 serve to block steady state DC from Z2, while permitting alternating signals to flow through Z2.

Figure 2 shows schematically the relevant parts of a telephone subset connected to lines 1 and 2. Capacitor C1 is connected from line 1 via resistor 25 to diode bridge 21, made up of diodes, 211,212, 213 and 214. The other side ofthe diode bridge connects to line 2 via capacitor C2. The junction of diodes 211 and 212 is connected to ring circuit 20 and its associated storage capacitor C3. The circuit through ring circuit 20 is completed via connexion to diodes 213, 214.

Line 1 is also connected to second diode bridge 22 made up of diodes 221 to 224. The junction of diodes 221 and 222 is connected via hook-switch 23 to speech circuit 24. The path through speech circuit 24 is completed by a connexion to the junction of diodes 223 and 224, and thence via diode 223 to line 2.

To illustrate the function of the isolating capacitors, suppose C2 to be replaced by a short circuit. There is then a DC path via line 2 Diode 212, ring circuit 20, diode 224, line 1.

Similarly if C1 were replaced by a short circuit the DC path would be, line 1, resistor 25, diode 211, ring circuit 20, diode 223, line 2.

Figure 3 shows a first embodiment of a load 10 isolated by a single line capacitor C1.

The switching elements 11, 12 and 13 are closed and provide a first path from line 1 to line 2 via C1 and load 10 when alternating signal on line 1 is more positive than line 2. Switches 14, 15 and 16 are open during this half cycle.

When the polarity between line 1 and line 2 reverses, the path is from line 2 via switch 14, capacitor C1, switch 15, load 10 and switch 16 to line 1. Switches 11, 10 12 and 13 are open.

Thus the current flows through C1 in opposite directions during the two half cycles, giving a net average current of zero.

The control circuitry to operate the switches 11, 12, 13, 14, 15 and 16 is not oeooo shown and will be described in relation to subsequent embodiments.

15 Figure 4 schematically represents a telephone circuit embodying the invention.

In Figure 4 there is one isolating capacitor C1 connected via switches 1 a, S1 b, S2a and S2b. The switches are synchronized to the ring signal to provide a first path from line 1, via S1 a, C1, S1 b, ring circuit, diode 223, and line 2, during a first o (positive) half cycle of the ring signal, and a second path from line 2, via S2a, C1, 20 S2b, ring circuit, diode 224, and line 1, during the second (negative) half cycle.

Because the current flows in the first and second half cycles are equal and in opposite directions the average current through C1 is zero.

When no ring signal is present, all switches are open.

The control circuitry to operate the switches in synchronism with the ring signal is not shown in Figure 4.

In practice the switches S2b, S1 b would be transistor switches. S1 a and S2a could be replaced by diodes.

Figure 5 shows a circuit diagram of an embodiment of the invention using diodes and transistor switches.

In Figure 5, a positive half cycle path is from line 1, via diode 41, resistor 42, capacitor C1, diode 48, transistor 47, capacitor C3, diode 223 and line 2. Transistor 47 is driven by the rising edge on line 1 via capacitor 46 and resistor 45. Transistor 51 is held off by current flowing through diode 52, capacitor 53 and resistor 54.

By symmetry, the path via transistor 51 is active during the negative half cycle.

Although the circuit provides full wave rectification of the ring signal, transistors 47 and 51 each only conduct for a maximum of 900 due to the fact that their base drive current is derived by capacitors 46 and 53.

Resistors 42, 43, 45 and 54 limit the current during a lightning surge.

Diodes 48 and 50 prevent transistors 47 and 51 from operating in reverse mode and discharging C3.

Typical values for the base current components are calculated below for 10 2N6517 transistors used for transistors 47 and 51

S.

The value of capacitors 46 and 53 is calculated below: Base current for transistors 47 and 51 must be sufficient to hold the transistors in saturation, even for min hfe and max ring.

SValue of capacitors 46 and 53 should be small to keep current drawn from C3 via diodes 49 and 52 small and also to keep the size and cost of capacitors 46 and 53 down.

Peak collector current (Vring x V

Z

SWhere V ring 75VRMS ZT min AC loop impedance 1.6k 1Wc 1 5499 Q For resistors 42, 43 1.6k, C1 1.1 gF (1 :UF± w 2nf, f= 27.5Hz (25Hz =134 24.4 mA Cpk 5499 For V ring 95 VRMS hfe min for 2N65] 7 Ic 3OmA, Vce 1 hfe, min Ibk=24.4 mA -0 1m bk 30 .8m a a.

a V P lZbl where Vpk 1 34V (see above) IZb I impedance of base circuit 0..

10 z Vk I bpk 134 =165k 0. 81 mA a.

a a a aa a..

a Base circuit impedance is dominated by capacitors 46 and 53. Therefore R25, R26 can be ignored.

xC

I

=165k jWC

C-

WiX

C

1 2ic(25 10%) 165k c The base current capacitors are thus much smaller than the line capacitor C1.

In the telephone ringer application shown in Figure 5, the invention enables the diode bridge 21 of Figure 2 to be eliminated.

Claims (7)

1. An isolating arrangement for isolating a load from a DC voltage while permitting a time varying signal to be applied to the load, the arrangement including; a capacitor; a switching arrangement to provide a first path between first and second terminals through the capacitor and the load during at least part of the period during which the time varying signal applied across the first and second terminals is positive, the switching arrangement providing a second path between the first and second terminals during at least part of the period during which the time varying signal is negative. An arrangement as claimed in claim 1 wherein the time varying signal is substantially symmetric with respect to a reference voltage and wherein the net current through the capacitor is substantially zero. S3. An arrangement as claimed in claim 1 or claim 2 wherein the first path 15 includes: a series connexion of a first switching element, the capacitor, a second switching element, the load and a third switching element, connected between the first terminal and the second terminal; and wherein *eo:o* the second path includes: a series connexion of a fourth switching element, the capacitor, a fifth switching element, the load and a sixth switching element, connected between the second terminal and the first terminal.

4. An arrangement as claimed in claim 3 wherein the first switching element is connected to the opposite side of the capacitor to the side of the capacitor to which the fourth switching element is connected. An arrangement as claimed in claim 3 or claim 4 wherein the second and fifth switching elements each connect opposite side of the capacitor to load.

6. An arrangement as claimed in any one of claims 3 to 5 wherein the third and sixth switching elements connect the load to the second terminal and the first terminal respectively.

7. An arrangement as claimed in any one of claims 3 to 6 wherein the third and sixth switching elements are diodes.

8. An arrangement as claimed in any one of claims 3 to 7 wherein the first and fourth switching elements are diodes.

9. An isolating arrangement substantially as herein described with reference to Figures 3 to 5 of the accompanying drawings.

10. A telephone subset including an isolating arrangement for the ring circuit as claimed in any one of claims 1 to 9. 10 DATED THIS TWENTY-FOURTH DAY OF FEBRUARY 1997 ALCATEL AUSTRALIA LIMITED 000 005 363) e 0 0 **o