Time for some ohm law here guys. The radio you are powering is a load in
series with a transmission line that has resistance. The ohms of the
line and the ohms of the load add together to determine the effective
load for determining the amount of current flow that will be drawn. If
you already know the number of amps that the radio draws then this gets
easier to calculate. 24 AWG wire has a resistance of roughly .0302 ohms
per foot. If you are going 300 foot to the load then you actually have
300 foot on the positive side and 300 foot on the negative side of the
load. This yields 600 X .0302 = 18.12 ohms. Now here is where those with
no electronics background may start to glaze over a bit. We actually
have two lines for positive and two lines for negative when using two
pairs of a 24 AWG cat 5 cable. When you have two identical loads in
parallel the resistance gets cut in half. So the effective resistance of
the transmission line for 300 feet of Cat 5 24 AWG cable is 9.06 ohms.
Let's assume the radio being powered has a current draw of 1.2 amps. The
current flow in a series circuit is the same throughout the circuit so
the voltage drop across the transmission line would be determined by
multiplying the amps of current times the ohms of resistance which means
1.2 amps X 9.06 ohms = 10.872 volts. This means that if a radio needs
12 volts with 1.2 amps current flow then you would need to supply a
minimum of about 24 volts at the POE injector. This is because if we
start with 24 volts and lose 10.872 across the transmission line this
leaves about 13.128 volts to the radio. If you do not know ohms law then
you need to do some reading on it. It is easier than I have described
here once you play with it a bit and you really do use it in your life
if you do anything at all with electricity. Here is a page I just
Googled and seems to cover most of the basics:
http://www.awrr.com/descorn.html
Cheers,
Scriv
Brian Rohrbacher wrote:
I'm a little confused here. I'm working on a 300 ft run of cat5 and
have a question. The radio is acting sparatic. The power supply has
already been upped from a 18v to a 24v. Both 1 amp. Will it hurt to
put a 24v 2 amp power supply in? If I "over do" on amps or volts,
what blows a radio. One or both? I seem to remember being told that
a radio only takes what amps it needs, so putting a higher amp power
supply in won't hurt, but if you put too many volts in, that will fry
them. Please clarify me on this.
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