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Is it possible for one to connect a telephone cable to light up a light-bulb?

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3 Answers 3

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  • Theoretically - sort of yes.

  • In practice, not really.

  • The telephone system operator would be unhappy.


It is possible to connect a telephone cable to light up a light-bulb (assuming that you mean the cable from a "Central Office" telephone exchange or a PBX system) BUT

  • Doing so will probably disrupt telephone operation

    • Doing so will be against the terms of service for the line involved

    • The amount of power obtainable will be very very very small. An LED would be better and even that will be limited.

Central Office telephone systems use 50 volt supplies. Anything below about 100,000 ohms may cause problems and even that is marginal.

Available current at 100k would be 50/100k = 0.5 mA.
An LED would glow dimly at that current.

At 0.5 mA the available energy is V x I = 50 x 0.5 mA = 25 mW.
If you used a buck converter at say 80% efficint to power an LED you'd get about 25 mW x 80% = 20 mW.
For a white LED at say 3V that's about 7 mA
A high efficiency modern LED would be "quite bright" to the eye but not much good for illumination. ie 1 person could read a paperback book at good light level with this.


If you are prepared to draw as much power from a line as possible it will depend on how far you are away - but you may get tens of mA. Central Office systems are typically 50V. PBX may be 25V and possibly other voltages.

I forget typical feed resistances by a figure of 600 ohms comes to mind. You get maximum load power when you load to half voltage (maximum power transfer theorum) so at say 600 ohm load, 25V Power = V^2/R = 625/600 or about 1 Watt.
Quite possible less in practice and much less with distance.

Not so good for a bulb - good for many things with an LED.

BUT telephone circuits may shut the line circuit down if it sees this sort of loading.

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  • \$\begingroup\$ Thanks for answering. I was looking for a source of power that can at least light up a light-bulb from a telephone cable from a "Central Office" telephone exchange or a PBX system but as you mention, it may not be powerful enough to do it. \$\endgroup\$ Commented Oct 3, 2011 at 5:52
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    \$\begingroup\$ This is, of course, to say nothing of what happens when the 100 V P-P ring signal comes through. It is designed to make a mechanical bell move loudly. Anyway, if you want to know what you can get away with on actual phone lines, look up FCC Part 68. \$\endgroup\$ Commented Oct 3, 2011 at 6:16
  • \$\begingroup\$ @Larry Morries - see addition to answer at bottom \$\endgroup\$
    – Russell McMahon
    Commented Oct 3, 2011 at 9:42
  • \$\begingroup\$ @RussellMcMahon, my objective is to light a light-bulb, not a LED. Since it cannot light a light-bulb, then no point using that telephone cable. It will still remain its own role - power up my telephone. \$\endgroup\$ Commented Oct 3, 2011 at 9:47
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There was a convention, used by certain Princess Phones, that provided power on the non-tip-and-ring pair. This was not central office power, but provided by a local transformer, that provided DC power for a light. Your phone has to be specifically wired this way to expect 12V power on the black and yellow wires, leaving red/green for tip and ring.

In the modern era, this is difficult, because of two-line phones being more common, as they use the black/yellow wires for a second phone line.

http://www.oldphones.com/servlet/Detail?no=48 shows an example of this kind of power arrangement.

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Entirely different story when you refer to a VOIP environment with Power over Ethernet, which can deliver roughly some 35 - 60VDC at several hunderds of mA.

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