Composite Power-Signal Cables

As CAT-5 wiring and long signal cable runs have become popular. It has resurrected questions and techniques previously explored in the audio world. The bundling of low voltage DC power sources within twisted pair signal cable is a popular concept that has many shortcomings in practice.

We recently serviced requirements for operating 5 VDC, 0.9 Amp, web cameras over powered CAT-5 cable. It presented similar problems to those experienced by our band when we needed to situate the guitar effects a good distance from the amplifiers and AC power. We created special cables to handle the guitar signals and and the 9 VDC power within the same bundle. While this technique was successful, it had significant drawbacks that generally made it more convenient to just run a 110 VAC power cord out to the effects. Some of the issues were:

  1. To combat the problem of DC resistance over a run of 15-20 feet, we were forced to use very high gauge wire for the current carrying cable. This made the cable big, stiff, and expensive.

  2. Our guitar players used a large number and variety of effects from different of manufacturers, sometimes pulling as much as 2 amps, necessitating yet a second heavy cable to minimize resistance to adequately carry the current.

  3. There is no standard for effects devices when it comes to voltages, polarity, or connector size. Some devices use 6 VDC, some use 9 VDC. Some have a positive center pin, some have a negative center pin. There are a whole host of DC power connector sizes, the mismatch of which can lead to intermittent connections. As a result, there is an issue of getting everything connected and junctioned correctly at the destination end. Failure to do so properly can lead to the destruction of some expensive devices.

  4. Tests had to be made at the destination end to confirm proper voltage. Sometimes, when current use exceeded expectations, the voltage was at an inordinately low level which damages some devices.

In regard to CAT-5, the situation is even more limiting because of the small gauge wire and relatively high current draw of LAN cameras (around 1.0 Amps).

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