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# electet mic

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#### evaine23

##### New Member
I was building a simple sound meter using an electet mic, audio amp, and a LM339. I purchased the mics years ago so I don't remember any of the specifications about them. I measure the resistance of the mic between the two leads and it appears to have an internal resistance of 1Kohm. I may be wrong, but I thought, to select the desired voltage for the mic, I could treat the mic as a resistor and set it up like a voltage divider. With 1K of resistance in the mic and a 5Vdc power supply, if I wanted 1V I would need a 4K resistor ((1K/[1K+4K])*5V). I measured at the node where the mic, resistor, and capacitor meet. This doesn't seem to work properly and I needed to resort to trial and error with a multimeter. Currently I'm using a 15k resistor which should only allow 0.3V at that node if my calculations are correct ((1K/[1K+15K])*5V). Am I wrong in my assumption that I could treat the mic as a resistor in this circuit? If my logic is wrong, how do I go about selecting the proper resistor to supply the proper voltage if this doesn't act like a standard voltage divider?

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It's not a resistor, and you can't treat it as such, measuring across it on ohms is a completely pointless operation and tells you nothing.

alright, but if that's the case, then how should I determine the proper resistor to set the desired voltage for my project. Mine is currently fine but I would like to know the proper way of setting the voltage, not just putting a resistor in and measuring the voltage. I'm trying to improve my understanding of circuit designing and analysis so i can work on some better projects.

An electret mic typically uses a supply of about 8V through a 10k ohms resistor. Then its current is about 0.5mA and its voltage is about 3.0V.

With a lower supply voltage then its voltage and current are reduced a little. For 2.5V across the mic its current is about 0.4mA and the resistor from 5V is 6.2k ohms.

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