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 8O 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.

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.

Simple AC Power Current detector Circuit with LM1458

This circuit will detect power line currents of 250 mA or more without making electrical connections to the line. Current is detected by passing a line of alternating current through an inductive pickup (L1) as a screen diameter of 1 inch U-bolt wound with 800 turns # 30 – # 35 magnet wire.

The pickup could be made from rings of iron or other type of processor core that allows enough space to pass one of the AC lines through the center. Only one of the power line is the line or neutral must come from the center of the pickup to avoid the fields cancel.

I tested the circuit with a 2 wire extension cord that had separated the twin wires a small distance with an exacto knife to allow the U-bolt to encircle a single cable. The magnetic pick-up (U-bolt) produces about 4 millivolts peak for a series of alternating current of 250 mA, or AC load of around 30 watt

The signal from the pickup has grown about 200 times the output of pin op-amp 7, followed by the peak detected by diode and capacitor connected to pin 7. The second op-amp used as a reference point, which detects voltage increases above the drop pass.

The minimum mark required to cause the comparator output stage is a positive change of 800 mV peak around a power line 30-watt. The 1458 swing op-amp output in just a couple of times the ground so a voltage divider (1K/470) is used to reduce the voltage signal at about 0,7 volt.

An additional diode added in series with transistor base to ensure that off when the voltage op-amp is 2 volts. You can get a piece “of relay chatter if the AC load is close to the switching point so a larger load 50 W or higher recommended. The sensitivity can be increased by adding more becomes a pick-up