Bat Feed GO
#telephone
#DC current
#AC current
#carbon transmitter
#battery feed
#voice signal
#telephony
#line feed
Description: DC current from a battery is required by the carbon transmitter to operate the telephone. The telephone company supplies battery power to the telephone line using a device that permits DC (Direct Current) to flow while preventing the AC (Alternating Current) or voice signals from shorting out. The voice can also be referred to as audio, the talk path, or speech. In telephone applications, an inductor is a coil of wire with an iron core that introduces impedance to AC signals. For example, if a coil of wire is created by wrapping 200 feet of copper around several nails as in a school project to make an electromagnet, several principles apply. When connected to a battery, a magnet is formed, and current flows from the battery. If a meter measures the DC resistance, it may read around 10 Ohms. However, when using this coil with AC, both resistance and reactance are present, resulting in a combination known as impedance. At power line frequency, this impedance could be estimated at 40 Ohms, while at voice frequencies, it may increase to 200 Ohms. A telephone typically has an impedance ranging from 600 to 900 Ohms. To supply battery power, an inductor must allow DC to flow, typically around 35 milliamps (0.035 Amps), while having an impedance several times greater than that of the telephone to ensure that voice signals remain unaffected. For a coil, this means an impedance of approximately 5,000 Ohms or more at voice frequencies. Line losses, such as cable resistance, should also be considered, though this discussion will not cover loading coils, as they are generally not used with DSL lines. For further information, it is recommended to refer to "Principles of Electricity and Electronics as Applied to Telephone and Telegraph Work" by AT&T, which can be found on eBay for $6 as a CD or $15 for a hardbound copy.
Considering the aforementioned criteria, a simple 24 Volt DC relay (RLY) can fulfill these requirements. This relay has a DC resistance of about 700 Ohms and allows approximately 30 milliamps of current to flow at 24 Volts, which is sufficient for talk power. The impedance is also appropriate for this application. This common type of 24 Volt relay can be connected to a 24 Volt DC power supply to provide battery power to an old telephone. The preferred power supply is a 1A2 Key System Power Supply, which typically offers 24 VDC Talk (filtered, with minimal hum), 24 VDC Signal (unfiltered), and 10 VAC. If this specific power supply is unavailable, a generic 24 VDC power supply may be used, although it may introduce hum or noise into the phone line, necessitating the addition of a filter capacitor.
It is crucial to avoid using a switching power supply, as these are generally lighter than iron core transformer supplies and may not provide the necessary performance. Telephones can remain connected and off-hook indefinitely. When a telephone is off-hook, the associated relay operates, and a rotary dial will produce a clicking sound as it is used. It may be necessary to adjust the relay contacts or spring tension to ensure proper operation during dialing and on-hook/off-hook transitions. During testing, it is advisable to wiggle all wires and listen for noise; any loose connections should be tightened, damaged wires replaced, and dirty contacts cleaned. Leaving a 2500 telephone set off the hook during testing will not affect the results. It may be possible to use a speakerphone-type set for hands-free testing of the 2500 telephone set. However, compatibility may vary, and finding a suitable speakerphone that works in this manner may be necessary. This approach facilitates noise detection by allowing movement of components while monitoring for unwanted sounds.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 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 loading coils, but then you probably have a DSL line so those are not used.
For more explanation on this purchase Principles of Electricity and Electronics as applied to Telephone and Telegraph Work by AT&T. On E Bay you can buy a CD of the book for $6 or a hard bound copy for $15. After taking the above criteria into consideration a simple 24 Volt DC relay (RLY) meets these requirements.
It has a DC resistance of about 700 Ohms and allows about 30 milliamps of current to flow at 24 Volts, adequate for talk power or current, and an impedance of. well it is about right. This is a very common type of 24 Volt relay. Connect a 24 Volt DC relay (RLY) to a 24 Volt DC power supply and feed battery to your old telephone, that`s it.
The power supply of choice is a 1A2 Key System Power Supply. Typically this has 24 VDC Talk (filtered, no hum), 24 VDC Signal (unfiltered) and 10 VAC. Lacking all this, you can try a 24 VDC power supply, but it will probably have hum or noise that you will hear in the phones. You may need to add a filter capacitor. DO NOT use a switching type supply, these are usually very light in weight compared to an iron core transformer supply.
You can leave telephones connected to this and off hook, as long as you like. When a telephone is off hook, the relay associated with that telephone will operate, if you dial with a rotary dial the relay will click as you dial. It may be necessary to slightly bend some of the relay contacts or adjust the relay spring to make it operate and release as you dial the phone and as you go on hook and off hook.
With a telephone connected, wiggle all the wires and listen for noise, if you find a loose connection tighten the screw. If you find a bad wire, replace it. If you find a dirty contact clean it. Leaving the 2500 telephone set off the hook when testing the old telephone will have no effect. It may be possible to use a speaker phone type of set for the 2500 telephone set allowing you hands free testing.
Some types of speakerphones may not work this way, but perhaps you can find one that will. This makes an easy way to listen for noise by wiggling things an
Considering the aforementioned criteria, a simple 24 Volt DC relay (RLY) can fulfill these requirements. This relay has a DC resistance of about 700 Ohms and allows approximately 30 milliamps of current to flow at 24 Volts, which is sufficient for talk power. The impedance is also appropriate for this application. This common type of 24 Volt relay can be connected to a 24 Volt DC power supply to provide battery power to an old telephone. The preferred power supply is a 1A2 Key System Power Supply, which typically offers 24 VDC Talk (filtered, with minimal hum), 24 VDC Signal (unfiltered), and 10 VAC. If this specific power supply is unavailable, a generic 24 VDC power supply may be used, although it may introduce hum or noise into the phone line, necessitating the addition of a filter capacitor.
It is crucial to avoid using a switching power supply, as these are generally lighter than iron core transformer supplies and may not provide the necessary performance. Telephones can remain connected and off-hook indefinitely. When a telephone is off-hook, the associated relay operates, and a rotary dial will produce a clicking sound as it is used. It may be necessary to adjust the relay contacts or spring tension to ensure proper operation during dialing and on-hook/off-hook transitions. During testing, it is advisable to wiggle all wires and listen for noise; any loose connections should be tightened, damaged wires replaced, and dirty contacts cleaned. Leaving a 2500 telephone set off the hook during testing will not affect the results. It may be possible to use a speakerphone-type set for hands-free testing of the 2500 telephone set. However, compatibility may vary, and finding a suitable speakerphone that works in this manner may be necessary. This approach facilitates noise detection by allowing movement of components while monitoring for unwanted sounds.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 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 loading coils, but then you probably have a DSL line so those are not used.
For more explanation on this purchase Principles of Electricity and Electronics as applied to Telephone and Telegraph Work by AT&T. On E Bay you can buy a CD of the book for $6 or a hard bound copy for $15. After taking the above criteria into consideration a simple 24 Volt DC relay (RLY) meets these requirements.
It has a DC resistance of about 700 Ohms and allows about 30 milliamps of current to flow at 24 Volts, adequate for talk power or current, and an impedance of. well it is about right. This is a very common type of 24 Volt relay. Connect a 24 Volt DC relay (RLY) to a 24 Volt DC power supply and feed battery to your old telephone, that`s it.
The power supply of choice is a 1A2 Key System Power Supply. Typically this has 24 VDC Talk (filtered, no hum), 24 VDC Signal (unfiltered) and 10 VAC. Lacking all this, you can try a 24 VDC power supply, but it will probably have hum or noise that you will hear in the phones. You may need to add a filter capacitor. DO NOT use a switching type supply, these are usually very light in weight compared to an iron core transformer supply.
You can leave telephones connected to this and off hook, as long as you like. When a telephone is off hook, the relay associated with that telephone will operate, if you dial with a rotary dial the relay will click as you dial. It may be necessary to slightly bend some of the relay contacts or adjust the relay spring to make it operate and release as you dial the phone and as you go on hook and off hook.
With a telephone connected, wiggle all the wires and listen for noise, if you find a loose connection tighten the screw. If you find a bad wire, replace it. If you find a dirty contact clean it. Leaving the 2500 telephone set off the hook when testing the old telephone will have no effect. It may be possible to use a speaker phone type of set for the 2500 telephone set allowing you hands free testing.
Some types of speakerphones may not work this way, but perhaps you can find one that will. This makes an easy way to listen for noise by wiggling things an
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