Patent GB-1407824-A (GB)
Title: Overload protection circuit for loudspeakers
IPC: H02H7/08
Filed: 1972-08-22
Abstract: 1407824 Protective arrangements ELEKTKOAKUSZTIKAI GYAR 22 Aug 1972 [7 April 1972] 39136/72 Heading H2K [Also in Division H4] A loudspeaker overload protection circuit responsive to reel power averaged over a period of time utilizes an indirectly heated thermistor, (Fig. 5). A rectifier D and capacitor C 1  are in series between input terminals A, B. Also connected across the input terminals A, B are a loudspeaker H and a pair of back contacts of a relay. The relay winding J 1  is in parallel with the capacitor C 1  while the indirectly heated thermistor TK 1  is in series with the relay winding J 1  and in parallel with the capacitor C 1 . A Zener diode Z 1  in parallel with the capacitor C 1  operates in the Zener direction. A filament winding of the thermistor TK 1  is connected in series with a resistor R 2  to the input terminals A, B. The Zener voltage is preferably about 10% below the voltage at which relay J 1  operates. Operation of the protection circuit depends on the heating up time of thermistor TK 1  and this is a function of current flowing in its filament and of the ambient temperature. A resistor R 3  in parallel with the thermistor TK 1  may be omitted, (Fig. 4, not shown), but, if present, prevents unwanted current flow through thermistor TK 1 , permitting greater safety and accuracy while preventing self heating of the thermistor TK 1  thereby giving it a longer life. An even simpler circuit (Fig. 3, not shown) replaces the full wave rectifier D with a half wave rectifier. To take account of frequency dependant heating up of the loudspeaker coil the resistor R 2  of Fig. 5 may be replaced by a complex impedance (Z 2 ), Fig. 7, (not shown), e.g. an inductance and resistor in parallel, with both in series with a further resistor. Alternatively, a complex impedance (Z 2 #, Fig. 8 (not shown), formed of an inductance, capacitor and resistor in parallel and, in series with these, another resistance, may be placed in parallel with the thermistor TK 1 . To ensure that after overload conditions cease the operating condition is restored without undue delay there may be provided, in parallel with relay # 1 , a capacitor (C 2 ) not shown, the parallel circuit of relay # 1  and capacitor (C 2 ) being connected in series with a further resistor (R 5 ), not shown. This resistor (R 5 ) is connected in parallel with a further pair of back contacts of relay # 1 . Alternatively three back contacts may be omitted. The resistor (R 5 ) may be a voltage dependent resistor. In another alternative arrangement the resistor R 1  is a voltage dependent resistor, or a voltage dependent resistor in series with a resistor of constant value is substituted for resistor R 1 . An alternative circuit (Fig. 10) has a diode D 2  operating in the forward direction. If overload conditions occur, current flow through diode D 2  rises rapidly and the potential built up on the poles of the diode D 2  is applied to a pulse inverter acting as a logical NO circuit. The pulse inverter is arranged to operate a relay J 2  before the relay J 1  operates. Relay J 2  is used to signal overload on the loadspeaker so that remedial action may be taken before relay J 1  disconnects the loudspeaker H. A resistor R 4 , Fig. 12, in series with the diode D 2  shortens the time between operation of relay J 2  and J 1 . Fig. 12 shows an elaboration of the circuit of Fig. 10 in which two loudspeakers H, H<SP>1</SP> are used, (e.g. a tweeter and woofer), and are controlled using a single voltage stabilizing Zener diode Z 1 . Overload of either loudspeaker cuts off both loudspeakers. The circuit including the phase inverter provides early warning of overload.