GB1586495A

GB1586495A - Overload protection circuit

Info

Publication number: GB1586495A
Application number: GB2242777A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-05-29
Filing date: 1977-05-27
Publication date: 1981-03-18
Also published as: AT349087B; DK144500B; CH617806A5; AT350877B; JPS52154622A; ATA400477A; DK235977A; ATA350877A; FR2353194A1; DE2624252C3; NL7705969A; FR2353194B1; DK144500C; DE2624252B2; DE2624252A1

Abstract

The loudspeaker voltage which is applied to the terminals (1, 2) is rectified using a diode (D1), to monitor the amplitude of the loudspeaker voltage, and to protect the loudspeaker (La) from overload. If an undesired increase of the loudspeaker voltage occurs, a relay (S) is activated, and a resistor (R2) is connected in series with the loudspeaker (La), by means of a changeover contact (S2) of the relay (S). To protect the loudspeaker for longer after the arrangement has responded to overload, a previously charged capacitor (C1) is switched to the base electrode of a transistor (Tr.), through which the current to feed the relay (S) passes, by another changeover contact (S1). This circuit arrangement is suitable for overload protection of both individual loudspeakers and loudspeaker combinations. <IMAGE>

Description

(54) OVERLOAD PROTECTION CIRCUIT (71) I, RUDOLF GOEBEL, a German citizen of Batzenbaumweg 15, 6382 Friedrichsdorf 4, Federal Republic of Germany, do hereby declare the invention, for which I pray that a patent may be granted to me and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to an overload protection circuit.

Loudspeaker arrangements, including both single loudspeakers as well as a combination of loudspeakers, may be electrically loaded to only a limited extend determined by the construction of the arrangement. Thus, there is a risk of the destruction of one or more of the elements of the loudspeaker arrangement when connected to audio-frequency sources of high output power. electrical Known electrical safety devices, which are destroyed at a specific rated current, and thus interrupt the feeding of current, are less suitable as an overload protection for loudspeakers or loudspeaker combinations, since they frequently fail as a result of the pulse-like character of the audio-frequency signal which is fed in.

Overload protection circuits for a loudspeaker arrangement are known which protect the loudspeaker arrangement against overload, without the above frequency of failure conditioned by the pulse-like signal.

Thus, for example, German pulished patent specification No. 24 48 506 describes an overload protection circuit which, in dependence upon the temperature of the loudspeaker coil, reduces the audiofrequency power supplied thereto. However, a circuit of this type has the disadvantage that it is relatively sluggish owing to the fact that the heat activated operations take place relatively slowly and have a slow response, the loudspeaker coil being thus sometimes loaded to a relatively great extent.

British Patent Specification No. 1,407,824 describes a circuit which avoids these disadvantages. Figure 1 of this specification shows an overload protection circuit for a loudspeaker arrangement which is connected to an audio-frequency source, the overload protection circuit having a series combination which comprises a diode, a limiting resistor and a capacitor and which is connected in parallel with the output of the audio-frequency source, and a relay circuit which, in dependence upon the voltage across the capacitor, interrupts the operative connection of the loudspeaker arrangement and thereby preferably interposes a series resistor. This circuit responds very rapidly, so that the loudspeaker coil is virtually not loaded at all. Thus, a loudspeaker arrangement provided with this circuit can be operated with any known amplifier having an optionally high amplifier output or a higher amplifier output than the specified rated loading capacity. The latter circuit thus constitutes an automatic, selfclosing electrical or electronic safety device acting as an overload protection for a loudspeaker arrangement.

An object of the present invention is to improve the latter known circuit such that the overload response point is more precisely defined and, moreover, the reclosing delay after response to overload is increased.

According to the present invention there is provided an overload protection circuit for the protection of a load arrangement driven by an alternating voltage source, the circuit comprising a relay disposed in the load circuit of a switching transistor, a first switching path controlled by the relay and adapted to operatively connect the load arrangement in parallel with the output of the voltage source when the relay is not actuated and to interrupt the operative connection when the relay is actuated, a circuit combination connected, in use, in parallel with the output of the voltage source and including a diode and a resistor which, when the relay is not actuated, are connected in series with a capacitor via a second switching path of the relay, a resistance voltage divider which is connected in parallel with the capacitor and resistor and which has a tapping connected to the trigger electrode of the transistor, and a Zener diode connected in parallel with a first portion of the voltage divider, the arrangement being such that under normal conditions, the Zener diode is non-conductive and the transistor and relay are held in a de-actuated state by means of the voltage divider but when the voltage of said source exceeds a predetermined level corresponding to an overload condition, the Zener diode conducts so as to bypass said first portion of the voltage divider and thereby actuate the transistor and relay, whereby said first switching path of the relay interrupts said operative connection of the load arrangement and the voltage source and the second switching path connects the capacitor so that it lies in parallel with a portion of the voltage divider not bridged by the zener diode so as to hold the transistor in a conductive state for a period determined by the time constant of the capacitor and the latter portion of the voltage divider.

The point at which the transistor and thus the relay respond in the event of an overload condition can be set in a relatively precisely defined manner. The reclosing delay may be rendered relatively high by choosing appropriate values for the elements of the parallel combination comprising the capacitor and the divider resistor. In this case, reclosing of the overload protection circuit, which acts as a type of safety device or "fuse", is possible virtually only by removing the input voltage, so that the person using the circuit learns not to apply voltages equal to or greater than that necessary to actuate the circuit.

The present invention will be described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Figure I is a diagram showing an overload protection circuit disclosed in British Patent Specification No. 1,407,824, for a loudspeaker arrangement: and Figure 2 is a diagram showing an improved overload protection circuit in accordance with the invention for a loudspeaker arrangement.

An audio-frequency voltage is applied to the terminals 1, 2 in Figure 1. One terminal of a loudspeaker La to be protected is connected directly to the terminal 2, and its other terminal is connected to the terminal 1 by way of a normally-closed contact r of a relay S. The relay S is also connected to the terminals 1 and 2, a Zener diode ZD and a variable series resistor R1 being located in the lead connected to the terminal 1. A capacitor C is connected in parallel with the relay. A series resistor R2 is located in the lead which connects the normally-open contact a of the relay S to the loudspeaker La.

The known circuit shown in Figure 1 operates in the following manner: When the breakdown voltage of the Zener diode ZD is reached, the Zener diode becomes conductive to act as a rectifier and charges the capacitor C by way of the variable series resistor R1. When the specific switching voltage of the relay is reached, the relay is pulled in.

The audio-frequency voltage is then fed to the loudspeaker by way of the series resistor R2. The resistor R2 is intended to prevent overloading of the loudspeaker.

Alternatively, instead of interposing the series resistor, the loudspeaker can be fully switched off, i.e. isolated from the voltage source.

A pull-in and release delay can be obtained by correct tuning of R1 and C, thus preventing chattering of the relay in the switching range.

If the energy fed to the terminals 1 and 2 drops, the relay automatically returns to its normal and initial state. The loudspeaker is now re-connected directly to the terminals 1 and 2.

The electronic transistor-controlled embodiment of an automatic and self resetting safety device in accordance with the invention, illustrated in Figure 2, switches more accurately and is universally adjustable to any optional switching ranges or load values of the loudspeaker. The desirable release delay of the relay, which is as long as possible, prevents premature switchingback to the initial state.

The audio-frequency voltage is again applied to the terminals 1 and 2, and, in the normal operating state, the loudspeaker La is again connected directly to the terminals 1 and 2 by way of the normally-closed contact of the relay switch S2. The series resistor R2 is located in the connection lead of the normally-open contact a of the switch S2.

A capacitor C2 is connected in parallel with the relay S which is located as a load in the emitter circuit of a transistor Tr. A series combination comprising the limiting resistor R1 and the capacitor C1 (R1 and Cl can be isolated from one another by means of the normally-closed contact r of the relay switch S1) is located in the control circuit of the transistor Tr and is connected in parallel with a voltage divider comprising variable resistors R4 and R3 for the purpose of adjusting the working point, and a Zener diode ZD, the resistors R4, R3 and the Zener diode ZD being connectible to the capacitor C1 by way of the normally-open contact a of the relay switch S1.

The circuit of Figure 2 operates in the following manner: The audio-frequency voltage applied is rectified by means of a diode D1. When the relay S is in its normal, non-actuated state, the capacitor C1 is charged with a direct voltage by way of the resistor R1.

The Zener diode ZD becomes conductive when the direct voltage across the voltage divider R4, R3 attains, on R4, the breakdown voltage of the Zener diode ZD. R4 is thereby shunted and the transistor is switched on. The relay S is pulled in and the relay switches S1 and S2 thus assume their working positions (a).

The capacitor C1, carrying the potential of the operating voltage of the switching transistor, is now connected to the base and maintains the switching transistor in its conductive state in dependence upon the time constant determined by the capacitor C1 and the resistor R3.

Furthermore, in this condition, the capacitor C1 also acts as a charging capacitor when subsequent voltage pulses on the Zener diode again exceed the breakdown voltage.

The capacitor C2 acts to additionally delay release during intervals when voltage pulses occur which lie below the breakdown voltage of the Zener diode.

When the relay S is in its operated state, the loudspeaker La is connected to the audio-frequency voltage by way of the series resistor R2 for the period of time during which the transistor is in its conductive state.

When the power supplied to the terminals 1 and 2 diminishes, the Zener diode ZD becomes non-conductive, and the switching transistor becomes non-conductive after the discharge operation of C1 has terminated.

The relay then returns to its normal state and the loudspeaker is again connected directly to the terminals 1 and 2.

The relays in Figures 1 and 2 are electomagnetic relays. In principle, electronic relays or electronic relay switches S1 and S2 (transistors, thyristors etc) having correspondingly modified control circuits may also be used.

WHAT WE CLAIM IS: 1. An overload protection circuit for the protection of a load arrangement driven by an alternating voltage source, the circuit comprising a relay disposed in the load circuit of a switching transistor, a first switching path controlled by the relay and adapted to operatively connect the load arrangement in parallel with the output of the voltage source when the relay is not actuated and to interrupt the operative connection when the relay is acutated, a circuit combination connected, in use, in parallel with the output of the voltage source and including a diode and a resistor which, when the relay is not actuated, are connected in series with a capacitor via a second switching path of the relay, a resistance voltage divider which is connected in parallel with the capacitor and resistor and which has a tapping connected to the trigger electrode of the transistor, and a Zener diode connected in parallel with a first portion of the voltage divider, the arrangement being such that under normal conditions, the Zener diode is non-conductive and the transistor and relay are held in a de-actuated state by means of the voltage divider but when the voltage of said source exceeds a predetermined level corresponding to an overload condition, the Zener diode conducts so as to bypass said first portion of the voltage divider and thereby actuate the transistor and relay, whereby said first switching path of the relay interrupts said operative connection of the load arrangement and the voltage source and the second switching path connects the capacitor so that it lies in parallel with a portion of the voltage divider not bridged by the Zener diode so as to hold the transistor in a conductive state for a period determined by the time constant of the capacitor and the latter portion of the voltage divider.

2. A circuit as claimed in claim 1 in which said first switching path of the relay is operative, in the event of an overload condition, to interpose a series resistor in the connection between the alternating voltage source and the load arrangement.

3. A circuit as claimed in claim 1 in which said first switching path of the relay is operative, in the event of an overload condition, and disconnect entirely the load arrangement from the alternating voltage source.

4. A circuit as claimed in claim 1, 2 or 3, in which the relay is located in the emitter circuit of the transistor.

5. A circuit as claimed in claim 1, 2, 3 or 4, in which the resistors of the voltage divider are variable resistors.

6. A circuit as claimed in any preceding claim in which the relay is an electromagnetic relay, a normally-closed contact and a normally-open contact of which relay provide said first switching path by means of which the load arrangement may be connected operatively to the alternating voltage source.

7. A circuit as claimed in any preceding claim in which a further capacitor is connected in parallel with the relay for additionally increasing the release delay of the relay following an overload condition.

8. A circuit as claimed in claim 6, in

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Zener diode ZD being connectible to the capacitor C1 by way of the normally-open contact a of the relay switch S1. The circuit of Figure 2 operates in the following manner: The audio-frequency voltage applied is rectified by means of a diode D1. When the relay S is in its normal, non-actuated state, the capacitor C1 is charged with a direct voltage by way of the resistor R1. The Zener diode ZD becomes conductive when the direct voltage across the voltage divider R4, R3 attains, on R4, the breakdown voltage of the Zener diode ZD. R4 is thereby shunted and the transistor is switched on. The relay S is pulled in and the relay switches S1 and S2 thus assume their working positions (a). The capacitor C1, carrying the potential of the operating voltage of the switching transistor, is now connected to the base and maintains the switching transistor in its conductive state in dependence upon the time constant determined by the capacitor C1 and the resistor R3. Furthermore, in this condition, the capacitor C1 also acts as a charging capacitor when subsequent voltage pulses on the Zener diode again exceed the breakdown voltage. The capacitor C2 acts to additionally delay release during intervals when voltage pulses occur which lie below the breakdown voltage of the Zener diode. When the relay S is in its operated state, the loudspeaker La is connected to the audio-frequency voltage by way of the series resistor R2 for the period of time during which the transistor is in its conductive state. When the power supplied to the terminals 1 and 2 diminishes, the Zener diode ZD becomes non-conductive, and the switching transistor becomes non-conductive after the discharge operation of C1 has terminated. The relay then returns to its normal state and the loudspeaker is again connected directly to the terminals 1 and 2. The relays in Figures 1 and 2 are electomagnetic relays. In principle, electronic relays or electronic relay switches S1 and S2 (transistors, thyristors etc) having correspondingly modified control circuits may also be used. WHAT WE CLAIM IS:

1. An overload protection circuit for the protection of a load arrangement driven by an alternating voltage source, the circuit comprising a relay disposed in the load circuit of a switching transistor, a first switching path controlled by the relay and adapted to operatively connect the load arrangement in parallel with the output of the voltage source when the relay is not actuated and to interrupt the operative connection when the relay is acutated, a circuit combination connected, in use, in parallel with the output of the voltage source and including a diode and a resistor which, when the relay is not actuated, are connected in series with a capacitor via a second switching path of the relay, a resistance voltage divider which is connected in parallel with the capacitor and resistor and which has a tapping connected to the trigger electrode of the transistor, and a Zener diode connected in parallel with a first portion of the voltage divider, the arrangement being such that under normal conditions, the Zener diode is non-conductive and the transistor and relay are held in a de-actuated state by means of the voltage divider but when the voltage of said source exceeds a predetermined level corresponding to an overload condition, the Zener diode conducts so as to bypass said first portion of the voltage divider and thereby actuate the transistor and relay, whereby said first switching path of the relay interrupts said operative connection of the load arrangement and the voltage source and the second switching path connects the capacitor so that it lies in parallel with a portion of the voltage divider not bridged by the Zener diode so as to hold the transistor in a conductive state for a period determined by the time constant of the capacitor and the latter portion of the voltage divider.

8. A circuit as claimed in claim 6, in

which said second switching path is provided by a further normally-closed and a further normally-open contact of the relay.

9. A circuit as claimed in any of claims 1 to 5 in which the relay is an electronic relay or electronic relay switch, such as a transistor or thyristor.

10. A circuit as claimed in any preceding claim in which the load arrangement is a loudspeaker arrangement and the alternating voltage operates at audio frequencies.

11. A circuit constructed and adapted to operate substantially as hereinbefore particularly described with reference to and as illustrated in Figure 2 of the accompanying drawings.