Hi, to contrast the frequent "Sending data over power line" threads I
thought I would start this one...
Seriously, what are the implications of using remaining pairs in an STP
low-cap cable for carying 24Vdc over fairly long distances (200 feet) ? I
plan on using something similar to Alpha 5272 or Belden 9842. The low
impedance rating (120 Ohms) of these cables is my main concern. I presume
this figure means the cable's impedance per 1000 feet ? Will I be "choking"
my power-suplly with such a low impedance ?
The 120 Ohm impedance is a function of inductance and capacitance of the
cable and is independent of length. It also has no relevance to
performance at DC. What you need to know is wire gauge or DC
resistance. Insulation resistance and withstand voltage are no a
problem at 24VDC. If you know your max current demand and the wire
resistance (not AC impedance) you can find the voltage drop and decide
if it is acceptable.
If the load end is a switching power supply you can also have problems
with oscillation. The input of a switching supply is a negative
resistance. You may need a damping RC if your wire is very long. We
sometimes have this problem with 10,000 meter submarine cables, but 200
feet should not be a problem.
AFAIK impedance in this type of case has no bearing on DC, it only comes
into play when you are putting AC into the cable (or when you first turn the
power supply on). More important in this sort of situation is the resistance
of the cable, at that length the resistance of the cable may become a factor
if you are planning to draw alot of current. TTYL
> Seriously, what are the implications of using remaining pairs in an STP
> low-cap cable for carying 24Vdc over fairly long distances (200 feet) ? I
> plan on using something similar to Alpha 5272 or Belden 9842. The low
> impedance rating (120 Ohms) of these cables is my main concern. I presume
> this figure means the cable's impedance per 1000 feet ? Will I be
> my power-suplly with such a low impedance ?
The cable impedance rating has nothing to do with its DC resistance. The
transmission line impedance is what the cable looks like to step voltage
source before the step bounces back from the end of the cable back to the
source. Note that this is independant of the cable length. You need to
find resistance per length of cable, then multiply that by your distance.
The cable will look like a resistor at low frequencies, and this resistance
is proportional to length. Don't forget to consider the resistance of the
return wire too.
The cable impedance at ac has nothing to do with its resistivity/meter at
dc which you can measure directly if you have a 100m spool near you. Then
use a table to find out how much current you are allowed to put into it.
The rating is different for buried or in-wall cables and otherwise. Then
compute how much voltage you will loose at that current and your length.
If it still looks good then you can use it.
Don't expect that cable to carry more than about 0.5A per conductor (maybe
1A if suspended in air - but the cable was not made for that). You will
HAVE to fuse the cable at the source side with fuses rated for your max.
cable current. 24V and 1A is plenty enough to start an in-wall fire given
some time and a few 'accidents'.