Design of car audio speaker protection circuit

First, the working principle

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The speaker protection circuit is shown in Figure 1. Mainly composed of midpoint potential detection circuit, delay circuit and relay. The circuit working process is:

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Figure 1 speaker protection circuit

1. At the moment when the audio power is turned on, the voltage across the capacitor C3 cannot be abruptly changed, which can be regarded as a short circuit. The potential of the 2nd and 6th pins of the time base circuit 555 is higher than 2/3 Vcc, so the 555 is in the reset state, 3 feet. The output is low, the transistor VT2 is turned off, and the relay JK normally open contact does not operate. At the same time, the +12 V voltage is charged to the capacitor C3 through the resistor R4. After a delay of about 5 s (seconds), the potential of the 2 and 6 pins of the 555 is reduced to 1/3 Vcc, the 555 is triggered to be set, and the 3 pin is changed from the low level to the high level. Level, transistor VT2 is turned on, relay JK is energized, normally closed contact is closed, thus achieving a delay for a period of time to connect the speaker to the power amplifier, completely eliminating the impact of large current on the speaker when starting up;

2. When the audio power is turned off, the +12 voltage disappears quickly, but the output signal of the power amplifier does not disappear immediately, and the impact noise generated during the shutdown process is also avoided;

3. When the power amplifier works abnormally or accidentally damaged and the midpoint potential is too high (higher than 1.8 V), the DC voltage is limited by R1 and R2, and sent to C1, C2 filter and D1~D4 rectifier, about 1~2s ( Second), transistor VT1 is turned on, 555's 4 pin is changed from high level to low level, 555 is directly reset, 3 pin output is low level, transistor VT2 is cut off, relay JK is de-energized, normally open contact is tripped, The speaker is disconnected from the amplifier circuit, effectively protecting the speaker from damage.

By changing the parameters of R4 and C3, you can adjust the length of the delay time of the speaker protection circuit, which is usually set to 5 s (seconds).

Second, component selection

The 555 must use a low-power CMOS time-base circuit. VT1 and VT2 use 9014 and C1815 low-power plastic-sealed transistors, which require current amplification factor β>100.D1~D5 are all 1N4148 type silicon switch diodes, D6 is used for power supply reverse protection, and 1N4001~1N4007 type silicon rectifier diodes can be selected. R1~R5 are all made of 0.5 W five-color ring metal film resistors. C1, C2 use high-quality aluminum electrolytic capacitors, C3 should use tantalum electrolytic capacitor with small leakage and high precision, otherwise it will affect the delay precision. JK selects 12 V/7 A dual-type relay (one for each of the left and right channels), such as JZC-22F.

Third, production and debugging

Figure 2 is a PCB diagram of the speaker protection circuit. Its actual size is about 50 x 43 mm.

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Figure 2 PCB board diagram

The specific production is as follows: first cut off the output end of the power amplifier and the lead of the speaker, according to the circuit shown in Figure 2, the lead heads a and b are respectively connected to the two sets of output terminals of the power amplifier, c and d are respectively connected to the left and right channel speakers: speaker protection The power supply of the circuit can only be obtained from the radio and the radio, and can not be directly obtained from the 12 v power supply of the car. Otherwise, it cannot be opened/closed. If it is equipped with a diesel car (using 24 V power supply), it is required to increase. A buck circuit (shown in Figure 3) supplies power to the speaker protection circuit to prevent the power supply voltage from damaging the IC.

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Figure 3 step-down circuit

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