Bat Feed GO


Posted on Feb 6, 2014

DC current (Battery) is needed by the carbon transmitter to make the phone work. The way the telephone company does things is to feed battery to a telephone line using a device that allows the DC (Direct Current) to flow but does not short out  the AC (Alternating Current) or voice as it is called in telephony. The voice can also be called the a


Bat Feed GO
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udio, the talk path or speech. An inductor in telephone work is a coil of wire with an iron core. This has Impedance on AC. If you took several nails and put 200 feet of copper over them, like maybe you did in school and made an electromagnet, several principles apply. If you connect this to a battery, you get a magnet and current flows from the battery. If you took a meter and measure the DC  resistance it might be 10 Ohms. But if you used this same coil on AC it has both resistance and reactance (this is going to get complicated, you might want to skip this section until you get your electrical engineering degree or you read that AT&T book) and this combination is called Impedance. A wild guess would say that at the power line frequency this might be 40 Ohms. If you used this at voice frequencies, to feed battery to a telephone circuit that Impedance might rise to 200 Ohms. A Telephone has an impedance of between 600 and 900 Ohms. To feed battery you need an inductor that allows DC to flow, typically about 35 milliamps (. 035 Amps), and has an impedance of several times the impedance of the telephone so as to allow the voice (audio or speech) to be unaffected. For a coil this means about 5, 000 Ohms impedance or more at voice frequencies. For you engineering types, the line losses (cable resistance and such) must also be considered but I am going to leave much of this out of the discussion. I am also not going to cover...




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