What is the maximum DC voltage a Local batt phone can handle?

[RB]

been lookin at various projects, and each one uses a different power supply.
voltages are all over the place.
What is the max voltage a lb phone can operate at? without burnin up?

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Local Battery Phones with their typical DC voltages ^**

German WW-II FF33 field telephone with green "X" on the front of transmitter capsules, 1.5V battery

Military Field Phones, 3 VDC is commonly used

Western Electric 1317s, designed for 3.0 VDC (2-cell "C" models) - 4.5 VDC (3-cell models)

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^**  Carbon transmitters in phones are current dependent. The ideal current is about 20mA and more than 50 mA will fry the carbon in most transmitters.  A "good" working range of current would between 20mA and 35 mA. Ohm's law, V=IR, is used to calculate the current through a transmitter based on its resistance (ohms) and the DC voltage being used. The table below shows current drawn based on measured transmitter resistance and the DC voltages to stay within the 20 to 35 mA range (voltages being "C," "D," and Dry Cell multiples).

[Key2871]

That's a good question, I'm only familiar with key systems, and those do it with a maximum of 35 vdc. Pots lines go from 50- to 5 when taken off hook.

[poplar1]

Western Electric 1317s were designed for 3.0 VDC (2-cell "C" models) - 4.5 VDC (3-cell models). Note that the cells are in series with the transmitter and the primary of the induction coil when the receiver is off-hook. The transmitters -- including the 635B with F2 transmitter unit -- are designed for this lower voltage.

[dsk]

It is depending on the current delivered, a 9V battery may work because of its small current carrying capacity, but usually the voltage should not force a current of more than 50 milli-amps trough the carbon transmitter.

dsk

[RB]

Ok, thanks for the replys.
I figured it was something like that.

[Key2871]

I knew the very old phones were only able to work well on 3-to 6 volts, and key power unit's I have worked on had rather high or what seemed high in the thirty range.
I used to make intercom sets using two phones and a 9 volt battery, worked great even with fifty feet of 18 gauge wire.

[TelePlay]

been lookin at various projects, and each one uses a different power supply.
voltages are all over the place.
What is the max voltage a lb phone can operate at? without burnin up?

     GLOBAL MODERATOR POST

The original question, quoted above, asked for voltages on Local Battery phones. The only Local Battery phones I am familiar with, and others can correct me if there are more, are magneto based phones such as wood wall phones (and a possibly a few desk/wall Bakelite sets) which used battery boxes that would hold two or three dry cell batteries, or 1.5 to 4.5 VDC talk voltage.

Good information to that effect has been provided above but any discussion of Common Battery phones is taking the original question off topic and adding confusion to the question's answer.

Any member who knows Local Battery phones well could come up with a short list of phones including their acceptable range for DC talk circuit voltages that would answer the original question, what is the maximum DC voltage that could be used for the Local Battery talk circuit without frying the carbon transmitter, a list that would be a great reference list for others who may be looking for this information at some point in the future.

I've not seen such a "technical" list on the forum and such a list would be a very good contribution to the forum. If several members provide information, I will create a summary list of all information provided so it is all in one place, most likely the first post, and easy to find.

[rdelius]

CB current is about 20ma.I would assume that same figure would work on a LB transmitter .Use the 20ma figure and the transmitter restiance to find the voltage drop.I collect field phones  and 3v is commonly used

[dsk]

The nature of the carbon transmitters used in phones are current depending, The ideal current is about 20mA more than 50 will fry the carbon in most transmitters.  When I ended my POTS connection the line had a self adjusting circuit trying to get 35mA or less trough the phone.  Depending on the circuitry all or just a part of this current travels trough the transmitter capsule.  Rdelius use to try to get close to 20 mA, I use 25 as my goal, and I guess that within that range you will have the optimal current for a phone transmitter.  Voltage are actually something who follows of this design, and may vary within a large range.

dsk

[dsk]

The table in the first topic here is a good idea.

The ohmic resistance in the induction coil are usually pretty low, 1-3 ohms are the most common, but I have seen up to 10 ohms.  Cords, wires, voltage drop in hook switch etc are usually less than 2 ohms (total) and the carbon transmitter has a typical nominal value of 50 ohms, even when up to 200 has been used.

The normal resistance in a LB phone will be about 55 ohms theoretical, all transmitters I have tried Except those used in German FF-33 has actually a considerable higher resistance, and 3V has usually been fine. That makes a total not far from 100 ohms reasonable.

The German WWII field telephone named FF33 are the only one I know giving great sound quality with only 1.5V battery.  All these transmitter capsules has a green X on the front. 

When you have a CB phone used on local battery, the resistance has been less important, and 3V and more has been used.a
Analyzing the circuit of the US field telephone TA 312 TP will show The this circuit is a modified CB circuit, but is working perfect with 3V.  Actually this circuit is almost equal to the W.E. 500   

dsk

[dsk]

This is actually a difficult to measure, and answer. At the moment I am on vacation at our cabin. Here we have an old modell 1918 (probably made not to long after 1925 since Kristiania changed to Oslo that year, and the phone is marked Kristiania, and the diagram is marked Oslo)  phone with a 40 ohm (stamped on) transmitter, and a 3V battery.  I tried to measure with an analog multi-meter, Current was about 25-30 mA but could be as low as 20, and as high as 60 mA.  (I would call this 25-30mA)


The primary winding has a resistance of 2.5 ohms.  The rest is less than 1 ohm. so the nominal should have been 48.5 ohms. 


dsk

[RB]

Thanks for the info DSK!!!
much help in understanding.