What are the constituent characteristics of an electret microphone and how to detect it?

Last Update Time: 2018-12-29 13:15:51

l  Constituent characteristics

The electret microphone consists of two parts : the acoustoelectric conversion and the impedance transformation. When the electret strand meets the acoustic vibration, the alternating voltage varies with the change of the acoustic wave. Its output positive value is very high.


It can not be directly matched with the audio amplifier, so a junction field effect transistor is connected to the microphone to carry out the impedance transformation is  an electret microphone commonly used connection mode.

There are three end and two ends of the corresponding microphone. Common active pole output and drain output are two kinds. The source output has three leading-out lines, and the drain D is connected to the positive pole of the power supply. A resistor is connected between the source and the ground to provide the source voltage, and the signal is output by a capacitor from the source. Braided wire grounding shielding effect. The output impedance of the source output is less than 2kΩ, the circuit is relatively stable, the dynamic range is large, but the output signal is smaller than the drain output. The drain output has two leading-out lines, the drain D is connected to the positive pole of the power supply through a resistance, and then is output by a capacitor as a signal, and the source S is directly grounded. The drain output has a voltage gain, so the microphone sensitivity is higher than the source output, but the circuit dynamic range is slightly smaller.

 

A diode is connected between the gate and the source of the field-effect transistor to prevent the damage of the field-effect transistor when it is impacted by a strong signal. Therefore, the drain D of the electret microphone can be distinguished by the positive and negative resistance characteristics of the diode. Transfer the multimeter to the RX1kΩ gear, the black watch takes over one pole, and the red pen is connected to the other pole. Then adjust the two pens and compare the results of the two measurements. The black pen is connected with the source, the red pen is connected with the source, and the red pen is connected with the drain. Most electret microphone has a bias resistor, and a few electret microphone products are not equipped with field effect transistors. Two output contacts can be connected to the circuit arbitrarily, but it is best to connect a point of the shell to the ground and the other from the field effect. The transistor is composed of a high impedance input preamplifier.

 

l  Detect electret microphone

 ( 1 ) Breaking and short circuit faults place the multimeter in RX 100, and the red stylus connect the core wire of the electret microphone or the signal output point and the metal sheath of the black dial leads or a metal shell of a microphone. The measured resistance value should be in the range of 500 ~ 3kΩ. If the measured resistance is infinite, then the microphone is broken; if the measured resistance is close to zero, it indicates that the microphone has short circuit fault. If the resistance is much smaller or larger than the normal value, the performance of the measured microphone becomes worse or has been damaged.


( 2 ) Sensitivity measuring method puts the multimeter in RX 100, connecting the red pen to the negative pole of the microphone ( usually the core of the microphone lead wire ), the positive electrode of the black stylus connected to the microphone ( usually the shield of the microphone lead wire ), at this time, the multimeter indicates that there should be a certain resistance value, which is blowing on the microphone, the pointer of the multimeter should have a large swing. The greater the amplitude of the swing of the multimeter is, the higher the sensitivity of the microphone is; if the pointer swing range of the multimeter is very small, the sensitivity of the microphone is very low, and the use effect is not good. If the pointer of the multimeter is found not to swing, the position of the meter pen can be exchanged again for blowing test; if the pointer is still not oscillating, the microphone is damaged. In addition, if the resistance of the pointer indicates a drift, the thermal stability of the microphone is very poor, should not continue to be used.


For an electret microphone with three elicitors, the black stylus is connected to the positive power supply terminal as long as the polarity of the three leads is correctly distinguished, the performance of the microphone can be detected by the method mentioned above, and the output end of the red pen is connected with the output end of the output terminal and the ground terminal is suspended. Note : for some microphone with lead plug, can be measured directly on the plug. However, it should be noted that some microphones are equipped with a switch, when testing, the switch should be set on the " ON " position. The switch can not be set to" OFF " position, otherwise, it will not be able to conduct normal tests, resulting in misjudgment.


( 3 ) The common failure and overhaul of electret microphone


1 Low sensitivity.

Most of the field effect transistor performance becomes poor or the microphone itself is subjected to violent vibration so that the diaphragm displacement. The new same type electret microphone should be replaced.

 

2)open circuit or short circuit fault. This kind of failure is caused by the broken of internal lead wire or the damage of internal field effect transistor electrode. The short circuit fault is caused by the short circuit of the metal braided wire of the outer layer and the breakdown of the internal field effect transistor. When detecting circuit breakdowns and short circuit faults, the external lead wire of the microphone should be cut off, and the resistance value between the residual leads of the microphone should be measured with a multimeter, check to see if there are still open circuit or short circuit. If there is no open circuit or short circuit, it is explained that there is a problem with the cut lead, and the two ends of the residual lead can be reconnected with the new soft wire; if there is still a circuit or short circuit, it is necessary to check whether the internal field effect transistor is abnormal and if so should be replaced.


This article is from Allicdata Electronics Limited.