Loudspeaker Protector Monitors Current

This circuit uses a 0.1O
1W resistor connected in series with the output of a power amplifier.
When the amplifier is delivering 100W into an 😯 load, the resistor will
be dissipating 1.25W. The resulting temperature rise is sensed by a
thermistor which is thermally bonded to the resistor. The thermistor is
connected in series with a resistor string which is monitored by the
non-inverting (+) inputs of four comparators in an LM339 quad
comparator. All of the comparator inverting inputs are connected to an
adjustable threshold voltage provided by trimpot VR1. As the thermistor
heats up, its resistance increases, raising the voltage along the
resistor ladder.

Circuit diagram:

Loudspeaker Protector Circuit

Loudspeaker Protector Circuit Diagram

When the voltage on the non-inverting input of each comparator
exceeds the voltage at its inverting input, the output switches high and
illuminates the relevant LED. NOR
gate latches are connected to the outputs of the third and fourth
comparators. When the third comparator switches high, the first latch is
set, turning on Q1 and relay 1. This switches in an attenuation network
(resistors RA & RB) to reduce the power level. However, if the
power level is still excessive, comparator 4 will switch, setting its
latch and turning on Q2 and relay 2.

This disconnects the loudspeaker load. The thermistor then needs to
cool down before normal operation will be restored. The values of R1-R4
depend on the thermistor used. For example, if a thermistor with a
resistance of 1.5kO at 25°C is used, then R1 could be around 1.5kO and
R2, R3 and R4 would each be 100O (depending the temperature coefficient
of the thermistor). The setup procedure involves connecting a sinewave
oscillator to the input of the power amplifier and using a dummy load
for the output. Set the power level desired and adjust trimpot VR1 to
light LED1. Then increase the power to check that the other LEDs light at satisfactory levels.
Author: David Devers – Copyright: Silicon Chip Electronics

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