GB2299238A - Loudspeaker with means to prevent drawing excessive current - Google Patents

Abstract

A public address system has a thermistor 20 or other device with a positive resistance/temperature characteristic in series with each loudspeaker 12 to limit current in the event of a short circuit fault, thereby enabling the other speakers to continue to operate. A diode pump 22-26 across the thermistor 20 provides voltage for a LED fault indicator 30.

Description

LOUDSPEAKER AND LOUDSPEAKER SYSTEM This invention relates to a loudspeaker, and to a system embodying a number of such speakers, for example a public address system.

In a public address system the audio signal from a power amplifier normally drives a large number of loudspeakers, sometimes in excess of 100.

The speakers are connected in parallel across a distribution cable pair, or line, operating at a standard voltage level. This is usually 100V rms, although 50V and 70V are also standard line voltages.

Each speaker is connected to the line via a transformer designed to draw the power requirement of the particular speaker from the 100V nominal level of the speaker line.

This is satisfactory until a single loudspeaker or its transformer develops a fault in such a way that it causes low impedance or a short-circuit across the 100V line. In this event the remaining loudspeakers on the circuit will receive little or no signal because the power amplifier output is current-limited. In effect the failure of a single loudspeaker unit causes the failure of the whole circuit. We have concluded that in any public address system, in particular one used to broadcast alarm messages, it would be of great benefit if this did not occur, such that announcements could still be heard on the remaining speakers on the circuit.

On systems used for voice alarm there are other safety requirements, these are defined in British Standard 5839 and concern the monitoring of loudspeaker circuits. It is important in the UK that any automatic protection circuits should not conflict with the requirements of this standard, and at least the preferred embodiment of the invention provides for this.

According to the invention there is provided a loudspeaker comprising means for connecting the loudspeaker to an audio frequency power source and means for preventing an excessive current being drawn from the source by the loudspeaker in the event of failure thereof to a low impedance state.

The invention also includes a loudspeaker system comprising a plurality of such speakers connected in parallel across a distribution line for providing a source of audio frequency power to the speakers.

The term loudspeaker as used herein is intended to include not only the loudspeaker itself but also a transformer or other matching or conditioning circuitry associated therewith. Thus, when the loudspeaker embodies a matching transformer, the prevention means is best located on the primary (input) side thereof.

The prevention means may be a device with a high positive temperature coefficient of resistance such as a thermistor.

Other devices with a suitable positive resistance/temperature characteristic may be used. For public address systems1 especially those which may broadcast a warning or alarm message, it is preferred that the temperature coefficient of the prevention means and its thermal inertia is such that it will operate within about a second of the start of an announcement (which is often a chime or tone) and will remain active for perhaps one to one and one half seconds if the audio signal level falls. Then little of the beginning of the message is likely to be lost from the operative speakers1 and the faulty speaker remains effectively isolated during naturally-occurring pauses in a spoken message.

There may be means for indicating that the prevention means has operated.

The indicating means may comprise means responsive to a voltage developed across the device when it is in a high-resistance state.

Thus, the indicating means may comprise an LED and means for deriving and applying to the LED a unidirectional voltage from the voltage developed across said device.

The system may comprise means for testing the integrity of the system by applying thereto a low-level signal to which the prevention means of a failed loudspeaker is substantially non-responsive.

Alternatively, or in addition, there may be means for applying a test signal to which the prevention means is sufficiently responsive to activate the indicating means in the event of failure of the loudspeaker.

The invention will now be described merely by way of example with reference to the accompanying circuit diagram.

A public address system has a pair of 100 volt rms distribution lines 10 to which are connected a number of loudspeakers, one of which is shown at 12, and its connection to the distribution lines at 14. The loudspeaker includes within its housing a matching transformer 16.

In series with the primary winding of the transformer is a protection circuit 18 comprising a device 20 (in this case a thermistor) having a high positive temperature coefficient of resistance. In parallel with the thermistor 20 is a diode 22, resistor 24, a zener diode 26 oppositely directed to diode 20, and a capacitor 28. A LED 30 is in parallel with the capacitor.

The protection circuit is small enough to be easily accommodated within the loudspeaker housing1 the LED 30 being installed in the housing wall so as to be externally visible as a failure indication when lit.

Typical component values and types for a 4 watt loudspeaker presenting via the transformer a 2500 ohm load to a 100V distribution line are: Thermistor 20 B59890-C160-A70 Diode 22 IN 4002 Resistor 24 10KC2, 1 watt Zener Diode 26 2V4 Capacitor 28 100 nF, 63V working LED 30 5mm dia yellow In normal operation under no-fault conditions the thermistor presents only a nominal resistance to the audio signal, resulting in an insertion loss of less than 0.4 dB.

If a short-circuit fault occurs in the loudspeaker or transformer, then when an audio signal is applied via lines 10, the excessive fault current heats the thermistor 20 and within a very shorttime its resistance increases and limits the fault current to approximately the current normally taken by the loudspeaker. Thus the current supply to the other loudspeakers in the system is not reduced, and they continue to function.

When the system is operated with a faulty speaker, a short delay occurs in the audio output from the other speakers as the protection device detects and responds to the audio signal. This should be taken into account in the use of the system so that the start of a message is not lost. In a practical system a pre-announcement tone, or chime, provides the ideal means to trigger the protection device.

The effect heard by listeners is that the tone starts quietly with some distortion, and rapidly builds to normal volume level as the protection device operates.

In the fault condition when the resistance of the thermistor 20 is high, a significant alternating voltage is developed across it. This voltage is half-wave rectified by diode 22 and is sufficient to overcome the reverse or breakdown voltage of zener diode 26. A DC voltage thus is built up across the capacitor 28, which is sufficient to illuminate the LED 30 and indicate that the fault exists.

The thermistor resistance remains high for so long as the audio signal is supplied from lines 10. Under no-signal conditions the thermistor returns to its low-resistance state and this enables a low-level circuit-checking signal to be applied.

For example, in BS 5839 systems,a low-level 20KHz surveillance pulse may be periodically applied and this will produce only a minimal change in the resistance of the thermistor The short-circuit fault in the loudspeaker or transformer thus can be detected by equipment monitoring the response of the system to the 20KHz pulse.

Upon detection of a fault in this manner, the speaker in which it is located can be identified by applying a test signal in the form of a 20 or 30V AC voltage to the line 10. A sufficient proportion of this voltage appears across the capacitor 28 to illuminate the LED 30 and identify the faulty speaker.

The test signal may be of a non-audio frequency, or alternatively (since in practical systems operating nominally at 1 OOV AC a speech signal is typically of 20 to 30V), a recorded spoken message may be used to invite those hearing the message to check that their nearest loudspeaker is working and that its LED is not illuminated.

The invention thus enables the system to continue to operate with one or more faulty speakers whilst permitting fault monitoring to BS5839 still to be effected.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

The appended abstract as filed herewith is included in the specification by reference.

Claims (9)

CLAIMS:

1. A loudspeaker comprising means for connecting the loudspeaker to an audio frequency power source and means for preventing an excessive current being drawn from the source by the loudspeaker in the event of failure thereof to a low impedance state.

2. A loudspeaker as claimed in Claim 1, wherein the prevention means is a device with a high positive temperature coefficient of resistance.

3. A loudspeaker as claimed in Claim 1 or Claim 2, comprising means for indicating that the prevention means has operated.

4. A loudspeaker as claimed in Claim 2 and Claim 3, wherein the indicating means comprises means responsive to a voltage developed across the device when it is in a high-resistance state.

5. A loudspeaker as claimed in Claim 4, wherein the indicating means comprises an LED and means for deriving and applying to the LED a unidirectional voltage from the voltage developed across said device.

6. A loudspeaker system comprising a plurality of loudspeakers as claimed in any preceding claim, connected in parallel across a distribution line for providing a source of audio frequency power to the speakers.

7. A system as claimed in Claim 6, comprising means for testing the integrity of the system by applying thereto a low-level signal to which the prevention means of a failed loudspeaker is substantially non-responsive.

8. A system as claimed in Claim 6 or Claim 7, comprising speakers as claimed in Claim 3 and means for applying a test signal to which the prevention means is sufficiently responsive to activate the indicating means in the event of failure of the loudspeaker.

9. A loudspeaker or a loudspeaker system substantially as herein described with reference to the accompanying drawings.