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:

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.

WISPA Wireless List: wireless@wispa.org


Archives: http://lists.wispa.org/pipermail/wireless/

Reply via email to