Understanding Battery Feed

And Building a Simple Tester



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 audio, 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.



Typical 24 Volt 700 Ohm Relay     Contacts 4 Pole Double Throw (4PDT)



Basic Battery Feed Tester With 24 Volt 700 Ohm Relay




Relay (RLY) contacts are not shown. Relays are 4PDT, 650 or 700 Ohms.


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.


Each time you go off hook on the old telephone the LED will light, it will blink when you dial.


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.


Don’t worry about adding a long line cord or two or three, the tester will still be functional with hundreds of feet of line cord.


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 and listening for noise or noise when you operate the hook switch (should just be a single click), put the speaker phone on mute and turn up the volume so you can hear all the noise. Some older 2500 telephone sets did not have a polarity guard, if the tone dial does not work, reverse the leads on the mod jack.


Happy testing!





QD 2- The Two Line Tester – AC Wall Wart




There are two parts to this, the power supply (shown next) and the battery feed circuit, you can  build each in a separate case or combine them into one case. When I built mine I put it all in one case and used a compact jack with two separate jacks in one case.


If you are going to be doing a lot of testing and have several telephones you may want to build this tester. It uses a wall transformer rated at 24 Volts AC and 2 each 24 Volt relays and a few other parts.


This is the power supply:


If you find a 30 Volt DC wall transformer you can use that. A 24 Volt DC transformer will work and the telephone line current will be adequate for testing. Sources for surplus parts are listed in the Section Three. All Electronics or MPJA is your best source. Jameco charges a bit more. The 24 VAC transformer is preferred, the line current will be a little higher on the test phone and perfect for Pay Phones.


Be careful and do not short circuit the output of the power supply. You can add a 3300 Ohm resistor and an LED across the output if you like. This will show that you have power and will also discharge the capacitor when you unplug the power supply. What I built is a crude method of construction, the parts are glued to the box with contact cement. When I closed the box I put electrical tape over the bridge rectifier. I added two binding posts for the DC output, just in case I wanted the DC to test something.


The second part is the battery feed circuit. This is the 24 Volt relays and jacks in one box.



[Schematics are shown separately, but are built in one box]





Schematic for 2 Battery Feed Circuits


I have added a 1000 Ohm resistor in this version. This will simulate an actual telephone connection, Phone A calling Phone B and some loss because of the distance of the simulated telephone wire between the two phones. This is optional. If you have  a 1 mFd capacitors, you can use it, change the 1000 ohm resistor to 680 Ohms. For louder volume between the two phones use a 100 ohm resistor.


You may also add the LED Resistor combination to the telephone jack for the 2500 Tel Set if you want. I have two jacks on the sample and made up a “Test Cord”, just a mod cord with alligator clips on the end.


Use of the 24 VAC transformer, which will provide about 33 VDC, gives a bit higher line current as compared to the 24 VDC unit. The higher Voltage results in slightly higher line current and is better if you are going to test coin telephones as they require a bit higher line current for good operation.


The 24 VDC relay will not be harmed by operating on 33 VDC, the telephone set has some resistance so the relay will operate at 24 VDC with the rest of the Voltage found across the telephone line or phone under test.


There are 3 LED’s in the cover, one for power and one for each line. The Jack is a dual jack for two mod cords.





The White box on the cover is a 2 line Jack





The test cords in the above picture come in handy as not all old telephones have modular cords.





The diagram for the Mod Test Jack is shown above. You can also use this with any telephone line to show when a telephone is off hook.





(Excerpt from “Testing Old Telephones © 2008)