AU623558B2 - Protective circuit for electroacoustic transducers - Google Patents

Description

23558 COMMONWEALTH OF AUSTRALIA FORM PATENTS ACT 1952 COMPLETE SPECIFICATION FOR OFFICE USE: Class Int.Class Application Number: Lodged: Complete Specification Lodged: *4 Accepted: S' Published: 0 CI o' "Priority: ,,..Related Art: Name of Applicant: HONEYWELL-ELAC-NAUTIK GmbH *,Address of Applicant: Westring 425-429, D-2300 Kiel 1, Germany 0 0 9 Actual Inventor: Reinhard Sobotta 0 .o...Address for Service: SHELSTON WATERS, 55 Clarence Street, Sydney omplece Specification for the Invention entitled: 0 "PROTECTIVE CIRCUIT FOR ELECTROACOUSTIC TRANSDUCERS" The following statement is a full description of this invention, including the best method of performing it known to us:- 1 Ia Protective Circuit for electroacoustic Transducers The invention relates to a circuit for protecting an electroacoustic transducer against thermal overload. A transformer or a loudspeaker can be protected against overload by inserting a resistor with positive temperature coefficient 4 PTC into the input circuit of such transducer.

0 0 S It is the object of the invention to provide a protective circuit for an electroacoustic transducer which alternately is used as a transmitter transducer or as a receiver transducer.

4"o' During transmitter operation, the circuit should protect the 04 1) transducer against thermal or current overload, whereas during receiving operation the transducer function should not be Simpaired by the protective circuit. During receiving operation A 4 at small amplitudes are fed by the transducer to the protective circuit. The protective circuit should not have any negative a Sinfluence on the supply of the transducer output signals to the receiver circuitry connected to the transducer and the protective circuit.

These objects are achieved by a protective circuit, whereat an 2 electroacoustic transducer being alternately used as a transmitter or as a receiving transducer is connected to a series circuit consisting of the primary winding and the secondary winding of a one-coil transformer and a PTC resistor, and whereas the transmitter/receiver circuit feeding the transducer or being fed by the transducer, respectively, is connected to the series circuit consisting of the primary winding and the PTC resistor.

Further preferred details of the invention are described in the dependent claims.

The invention will now be described with reference to several embodiments shown in the drawings, where S. Figure 1 shows a first protective circuit according o."o to the invention; Sl° 15 Figure 2 shows a second embodiment; 0 0 Figure 3 shows a third embodiment; and Figures 3a and 3b show the equivalent circuit a oo during receiving and transmitting operation, respectively.

In Figure 1 the primary winding W1 of a one-coil transformer T is connected via an anti-parallel circuit consisting of two diodes Dl1 and D2 and via a PTC resistor PTC to a common return or reference line M. Input terminal E is connected to a driver circuit (not h.own), which during transmnitter operation feeds the electroacoustic transducer EA connected to terminals A and M. The same input terminal E is connected to a receiver circuit which during receivur operation is supplied with the output signals of transducer EA. A secondary winding W2 connected at junction P to primary 3 winding W1 is used during transmitter operation for upward transformation of the input voltage supplied to terminals E and M. During receiver operation, this secondary winding W2 is fed with the output signal of electroacoustic transducer EA connected to terminals A and M. The PTC resistor works as protective means and protects during transmitter operation the primary winding Wl and the transducer EA connected to output A against thermal or current overload. If after an overload condition such thermal or current overload disappears, the transducer after some time will automatically again become operative. The PTC resistor preferably is :0 positioned in thermal contact with the component which has to be protected, i.e. the transducer EA or the 15 transformer T.

In the case of an overload condition during transmitter operation, the PTC resistor becomes a high resistance element and disconnects the common 0* primary/secondary junction P from reference line M.

20 During receiver operation, however, the proper function 0 00 0 0 of transducer EA is not impaired by PTC resistor PTC.

S: Diodes D1 and D2 forming an anti-parallel circuit disconnect the transformer from reference line M when only small signal amplitudes are applied to those diodes, i.e. during receiving operation. These diodes are rendered conducting only by larger signal amplitudes as they are applied during transmitter operation. Since the PTC resistor is not connected in series with transducer EA but is connected in series to transformer winding WI, I 4 which is common to the input circuit and the output circuit, the receiver circuit connected to terminal E is not influenced or interrupted by PTC resistor PTC.

Figure 2 shows a modified circuit in which an auxiliary winding W3 of opposite winding sense is connected in series with primary winding W1 of transformer T, whereat PTC resistor PTC is connected in parallel to auxiliary winding W3. This circuit primarily is used for reducing the open circuit voltage across the PTC resistor. During receiver operation, the transducer connected between terminals A and M feeds a low voltage to the series circuit consisting of windings W2, W1 and W3. The low resistance of the PTC resistor constitutes a shunt to auxiliary winding W3 and connects the lower SI 15 terminal of primary winding W1 to terminal M. In the 0o o: case of transmitter operation when a driver circuit (not Sshown) is connected to terminals E and M for feeding an a y electroacoustic transducer connected to terminals A and M, this driver voltage is normally converted into a higher transmitter voltage by the transformer function of windings W1 and W2, whereat the lower terminal of W1 is •a connected to terminal M via low-resistance PTC resistor PTC. The voltage across auxiliary winding W3 is almost zero. If because of the driver voltage at terminals E and M exceeding a predetermined value, the PTC resistor is heated up and its resistance is increased, the shunting action of PTC resistor PTC across auxiliary winding W3 is decreased. Therewith the input voltage at terminals E and M is divided between windings W1 and W3 having opposite winding sense so that the resultant output voltage transformed by winding W1 into secondary winding W2 is reduced and therewith the electroacoustic transducer connected to terminals A and M is protected against being overloaded by exceeding transmitter voltages fed between terminals E and M.

In the embodiment of Figure 3 the parallel circuit consisting of auxiliary winding W3 and PTC resistor PTC and the anti-parallel circuit of the two diodes D1 and D2 are connected in series with primary winding WI. The electroacoustic transmitter/receiver transducer EA is connected to this series circuit via secondary winding W2.

ft e4 0 During receiving operation as indicated in Figure 3a, the diodes D1 and D2 disconnect the primary winding 15 W1 and the auxiliary winding W3 together with PTC o o: aresistor PTC from reference line M because the small signal amplitudes supplied by transducer EA do not exceed o. the blocking voltage of diodes D1 and D2. Therefore, the transducer EA is via secondary winding W2 directly connected to terminals E and M of the receiving circuit (not shown). During transmitter operation, as shown in Figure 3b, both diodes D1 and D2 are rendered conducting so that a driver circuit connected to terminals E and M feeds the primary winding W1 of transformer T, and this transformer by means of secondary winding W2 provides an upward transformation of the transmitter voltage which then is supplied to transducer EA. This transducer then emits the acoustic signal e.g. into a body of water or other medium.

I

5a As long as the transmitter current fed by a driver circuit (not shown) to terminals E and M does not exceed a predetermined value, the PTC resistor has a low-resistance and therewith constitutes a shunt for auxiliary winding W3. The operation then is the same as described in connection with Figure 2. If, however, the transmitter voltage across terminals E and M exceeds a predetermined value so that an inicreased current flows through the PTC resistor, the shunting action of this resistor is decreased so that auxiliary winding W3 becomes effective therewith reducing the net voltage induced in secondary winding W2. The reason for this is that the winding sense of auxiliary winding W3 and primary winding WI are opposite so that an increase of 15 the voltage across auxiliary winding W3 results in a decrease of the voltage induced in secondary winding W2.

Therewith electroacoustic transducer EA is protected against being overloaded by an excess voltage applied to e o terminals E and M.

9 4 4Q.

Claims (4)

1. Circuit for protecting an electroacoustic transducer against thermal overload by means of a PTC resistor, characterized in that: a) the electroacoustic transducer alternately used as transmitter or receiver transducer is connected to a series circuit consisting of the primary winding and the secondary winding of a one-coil transformer and the PTC resistor, and b) the transmitter/receiver circuit is connected to the series circuit consisting of primary winding and the PTC resistor.

2. Circuit according to claim 1, characterized in that an auxiliary winding of opposite winding sense with respect to the sense of the primary winding is connected in series with the primary winding and is connected in parallel to the PTC resistor.

3. Circuit according to claim 1 or 2, characterized by an anti-parallel circuit consisting of two diodes connected in series with the primary winding.

4, Circuit according to claim 1, 2 or 3, characterized in that the PTC resistor is positioned in thermal contact with the component which is to be protected. A protective circuit substantially as herein described with reference to Figure 1, Figure 2, or Figures 3 to 3b of the accompanying drawings. DATED this 21st day of February 1992 HONEYWELL-ELAC-NAUTIK GmbH Attorney: PETER HEATHCOTE S Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS