Power over ethernet

Way too complicated. What I've done several times when I couldn't get AC power to the Linksys, I ran a "poor man's PoE" with splicing the power leads in to and out of the CAT5 cable at both ends.

Takes only five minutes and a few beenies (white telephones connectors), but regular crimp connectors would work.

Yup, I ran an Actiontec router like that in my conservatory for some years.

I did it with some insulating tape.... its only 5v.

Only the WRT54G v1 and *SOME* v1.1 versions used 5VDC power. I never could get one to work through a mess of CAT5 because the router required exactly 5.0000VDC to work correctly and the relatively high current through the copper produced excessive voltage drops. Later WRT54G versions were supplied with 12V 1A wall warts, and included a wide range regulator that works from about 4.0VDC to 18VDC. Those worked very well with the method you detailed. I've done much the same thing, but without the electrical tape. I use shrink tube.

Ever wonder why real PoE uses 48VDC? It's to reduce the current through the CAT5 cable and RJ45 connectors. At a constant power level, running something at 5VDC requires approximately 10 times the current of running it at 48VDC. Voltage drop and heating are related to current, not applied voltage. Also, the RJ45 connectors are rated at 1A max per pin. The WRT54G will not exceed the 1A max rating, but higher current devices might do so.

Yep you can do that but you can also damage a device that way if you are not careful. I was trying to come up with something a little more idiot proof. An example, recently I showed my friend how I added an external battery pack to my camera. A few days later he is complaining to me that he destroyed his camera like it is my fault he wired the thing up backwards.

Jimmie

Jimmie

PoE seems to be used a lot in HVAC and Alarm systems now that a lot of the controls and sensors are LAN based. I havent seen PoE used extensiely in any other way. Jeff, as always your accurate and thoughtful comments are always appreciated.

I had offered my way of hooking it up because I was thinking of mounting a WRT54 in the cupola on my garage. I had seen methods on the internet that suggested the router be opened up and diodes be placed inside the router and connections to the power supply be made inside the router. I had also coome across the simple methods suggested by others here in this thread. The first I considered way too much trouble and the second way too dependent on me wiring it up and not goofing something up. Its posible the router may already have some reverse polarity protection, even if this is the case I dont want to have to troubleshoot the reason the router isnt working. By using a small bridge rectifier at the router end it allows me to connect the power supply at the other end with either polarity reducing the reason my setup could fail. The extra few minutes I spent building my adapter is insurance against an extra trip into my attic.

Jimmie Jimmie

I doubt it - worst case you'll have too low a voltage at the head end. You /could/ damage your CAT5 mind you, if you had too much current.

I make a point of never showing actual friends how clever I am with stuff like that, unless I already know they're technically savvy.

Hmm. Consumer electronics goods are pretty immune to people putting the batteries in backwards.

Nope! I did a physics degree...

My technique is to cut the wall-wart cable in two, pull the unused two pairs out of the cat5, and splice the two power lines into the pairs. This avoids any current flow through the RJ45s. I've no idea what the rating of cat5 is but a pair (still twisted) seemed ok to carry an amp. Cable didn't get warm anyway....

I thought physicists ended all their correspondence with "More research is necessary".

That will work, but the less disgusting looking method is to use an RJ45 wall "muffin" (connector block) for interconnect.

It's not cable heating that limits operation over CAT5 or what inspired the 48VDC standard for PoE. It's the voltage drop that the CAT5 cable presents. A single #24 awg wire in the CAT5 bundle has a resistance of about 9.4 ohms per 100 meters (the official maximum for

802.11af PoE). If your WRT54G v1.0 required about 1.5A of current, and you used 4 wires to carry the current, 100 meters of CAT5, the delivered voltage to the WRT54G would be: Loop resistance = 9.4 ohms E = I * R = 1.5A * 9.4 ohms = 14.1 V which is more than the applied 5VDC. That means more power is dissipated in the wire than in the WRT54G. That won't work at all.

If I shorten the cable to 20 meters, I get: Loop resistance = 20/100 * 9.4 = 1.9 ohms E = I * R = 1.5 * 1.9 = 2.9 volts Voltage delivered = 5.0VDC - 2.9VDC = 2.1VDC I don't think the WRT54G v1.0 will run on 2.1VDC. However, if you replace the 5VDC wall wart, with a 7.1VDC wall wart to compensate for the drop, you will get about 5VDC at the radio, and it will work.

For ones that use batteries, perhaps. But for something intended only for use with a power adapter it's still potentially a risk.

Jeff Liebermann wrote: >

I did - in layman's terms "the cable didn't get warm, anyway..." surely indicates further research necessary? :-) See my sig for further info.....

Well, both can be factors. But you're right - if you want to get 12V out of the far end of a 100mtr cable, you'll need to stuff in way more than

12V at the near end.

Luckily I wasn't using a WRT54G... :-)

I was using an actiontec wireless home gateway. My run was about (quick sum) 100ft. I'm assuming that the vendor-provided wallwart chucked out more voltage than the router really needed.

Quite possibly - I read it that the "external battery pack" was being attached to a battery-powered camera. I can't imagine a mains-powered camera would be much use....

Well, yes. It's a stretch but certainly within acceptable limits for soliciting new projects.

Now you tell me.

Good assumption, but with two possible problems. Some devices (i.e. Buffalo wireless routers) use 3.3VDC and 5.0VDC wall warts with internal regulators. There's no regulator inside the box and the chips run directly from the wall wart power. If you apply to high a voltage, it smokes. Too low, and it won't play. The easy test is to measure the wall wart voltage, with no load, using a DVM. If it's exactly 3.3 or 5.0VDC, forget about trying your method. If the voltage is somewhat higher (up to perhaps twice as high), the regulator is inside the box and you can probably run it through the CAT5 extension cord. Extra credit if you test it with a variac (adjustable 117VAC xformer).

Actually, I said it much earlier in the thread. I suspect youre muddling me with the OP.

Absolutely. However I did measure the voltage at the head end to see if it was sensible. I forget what it was, this /was/ a couple of years back.

If I found a wallwart sold with a Extra credit if you test it with a variac

Round here, the voltage is a pretty steady 240v.