Help with an electrical problem please

  bumpkin 18:45 01 May 2015
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Hi everyone, I have reasonable knowledge of electrics but this has me baffled. A 15Mtrs long Cat5e cable, if I connect one end to a 12V 2Amp power supply I get 12V at the other end which I connect to a camera but it does not work. Now if I connect the same cable to a PC PSU it gets extremely hot in a few seconds making me turn it off instantly. Even more confusing is that it does it even when not connected at the camera end.

Obvious answer to me would be a faulty cable by why does it not do it when using a different power source. Replacing the cable is extremely difficult for various reasons and I still don't know if that is the answer anyway.

I know it is not a PC question but would appreciate any suggestions as what is happening.

  bumpkin 19:02 01 May 2015

To complicate it further, no shorts show with a continuity test between all combinations although there is a small resistance reading which I put down to resistance in the cable, it only seems to happen when a higher power supply is used.

  bumpkin 19:43 01 May 2015

Hi Beta, cable is cat5, meter shows no voltage drop. I was using 3 of the four pairs for power and the other pair for signal. In the end I just tried one pair at a time for power in case one was shorting out, no power drop even just on a single pair using the 2A power supply. Using the higher power supply I dont have a chance to check it or the cable will start to smoke or trip the overload or worse.

  bumpkin 19:53 01 May 2015

It would appear that 2amps are not enough for the fault to occur but a PC supply delivering 20amp does, must be some kind of leakage somewhere triggered by the extra current available but it baffles me.

  wee eddie 20:54 01 May 2015

Why on earth do you want to use a cable designed to carry signals measured in fractions of a milliamp to carry loads of up to 20 amps is beyond me. Particularly as a standard 2 core cable will do the job for pennies per metre without heat loss.

  bumpkin 21:29 01 May 2015

Beta, Do you know the maximum current carrying capacity of 24awg cable?

No I do not but I am led to believe that Cat5 data cable will easily handle 1amp at 12v on one pair.

wee eddie, I don't think you have understood my meaning, I do not wish to put 20A up a data cable obviously, the device will draw the current that it needs irrespective of the available input whether 2A or 20A. The device in question requires 1A at 12V. A standard 2 core cable will do the job but there is already a suitable cable in place and I see no point in running another one.

  bumpkin 21:56 01 May 2015

To add there is a joint in this cable but soldered and heat shrinked if that can give any further ideas.

  rdave13 22:11 01 May 2015

I think wee eddie means that a Cat5 data cable deals with mA not AMPS. hence THE BURNING and camera not quite dead. Just an assumption.

  bumpkin 22:49 01 May 2015

Thanks rdave, just done a bit more research and device requires 500mA @ 12V so one twisted pair in a Cat5 should handle that but I cannot get it to work even if using 3 pairs. It has to be something in the join I think. The odd thing is that it will only get hot (and I mean hot to the point of melting the insulation) when it has a high output power supply available. I would have thought that 2A would be enough to do some damage if there was a short somewhere.

  robin_x 00:25 02 May 2015

Whatever it is, if you have seen insulation melting, the cable should be considered buggered and not used for anything now.

Can you tie on a new cable and pull the old one out, pulling the new one through?

Wire up the new one first to power supply and camera and check it does work before installing.

(presumably you have modified connections at each end. ie is the camera expecting power on 3 pairs?)

  robin_x 00:30 02 May 2015

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