I can't find any way to plug these numbers in that will give me a 7v drop. You must have a more pessimistic formula than most ! And then you are missing the part about using both open pairs in the Cat5e.
I found this purpose specific calculator which takes the typical use of paired 24 ga into account:
Try it out, plugging 2 (default) into the pair quantity.
Also, as noted, the Linksys V4 can easily handle over 50% voltage drop (according to Jeff's tests) , so I'm just going to inject 12v from the device's wall wart. Even Linksys claims that their 5v PoE splitters will work with these.
I was not discussing both Cat5e pairs. At 0.0842 ohms per meter, that 120 foot wire is about 6.25 ohms.
I did not see other numbers about a 50% voltage variation and therefore don't know why that number exists. Measurement alone is insufficient and may not take into account other parameters that would only be known with circuit analysis and discovered later by failure. Also taken into account are voltage variations in the AC to DC power supply which is probably nothing more than a wall wart with poor regulation. Voltage numbers must also take into account that regulation.
If provided were numbers (such as from the AP internal electrolytic capacitors and heat dissipation calculations for its internal regulator), then and only then would I agree to a 50% overvoltage conclusion. Testing alone is never sufficient to really know and how failures happen later. Knowing only from a test is also called speculation. Since the single chip power supply costs dimes, it is a far more reliable solution.
Of course, do you want that must reliable? A difference between what I suggested. That long wire between the regulator and AP means additional complications may exist. POE devices compensate for those anomalies. Your AP typically would not unless you compensate for regulation problems created by that long wire. Just another reason why a single chip regulator is a more reliable solution.
Your choice. I am into having things work everytime the first time. Since the 'single chip regulator' solution costs so much less than even Cat5 wire and because it avoids so many 'might be' problems (others not discussed here), that single chip regulator is what I would do.
Again, I assumed only one twisted pair carrying power. I did not see any numbers that suggest the AP can handle voltage variations that large. I did not see numbers based in anything but measurements (therefore only speculation). I did not see any suggestions to avoid other problems created when the regulator is that far separated from the load. And I included margins to avoid other not discussed (potential) problems. Those would account for the differences.
Meanwhile, your intent is clearly superior to an earlier idea of running 120 VAC in the same conduit.
What part is FUD? My suggestions that sharing conduit with power lines is a safety hazard? That's codified in the NEC and certainly isn't in doubt.
Incidentally, online free NEC version:
(account creation required).
Even if it is all FUD (fear, uncertainty, doubt), I think I've made my point. If you follow the NEC, you have a reasonable chance of surviving the installation. You also have a reasonable chance of surviving a fire inspection. If want to be creative, you take your chances. In this case, the only obvious benefit seems to be convenience, which in my never humble opinion, does not justify the risk (whether perceived, real, or FUD).
Incidentally, my drug numbed brain just recalled another experience in conduit sharing. I was working on a pair of mountaintop radio sites. Between the two sites was a single 2.5" PVC buried conduit about 2000 meters long. At one point, the power to one of the buildings failed in a major storm. PG&E was going busy and it looked like it would take perhaps a week for the power to re-appear. The locals didn't like the sound of generators running and would either steal or sabotage the generators. So, the building owner decided to violate his own rules and shove power between building through the conduit. The radio load was only about 300 watts, and there were plenty of extra copper in the bundle. I did some rough calcs, and determined that it would play, as long as nobody increased the load.
It was done, and it worked well for 3 days. One of the service companies showed up to do something with their radios. They turned on all the lights (despite having the breakers turned off, with a huge sign indicating what was happening). They also plugged in some test equipment, and apparently a power tool charger. The load more than doubled. The voltage dropped to about 85VAC and radios started to fail. The buried wire also probably got hot but fortunately nothing caught fire. Disaster was averted when the security service phoned the owner and then the service company asking why nobody bothered to inform them that someone had entered the building and that the alarm system was complaining of cyclic power failures. Things returned to normal when the tech turned off his equipment, turned off the lights, and left.
Nobody was hurt, nothing was damaged, and no insurance company was involved. However, it doesn't take much paranoid imagination to see how this kludge could have turned into a disaster. For example, if the tech had been more ambitious, decided to find out why the voltage was so low, gone troubleshooting, and found the signal wires carrying the AC power. Remember, this is the same tech(s) that ignored the email, phone call to his office, and two very large signs explaining what was happening.
Sometimes, a little fear, uncertainty, and/or doubt might be a good thing to keep people from doing something stupid.
On Mon, 05 Jan 2009 17:20:04 -0800, Jeff Liebermann wrote in :
I guess you expect _all_ AC work (extension cords too?) to be done by electricians? Except they are some of the worst safety and code violators I've seen. When I insisted my new 220 spa wiring be run in hard conduit, I had the first two electricians tell me it was a silly waste of money -- they'd just staple wires to the bottom of the deck.
Do you know of any (even one) actual case of fire insurance being invalidated this way?
As for NFPA having a clue, as I would hope you know, there's quite a bit of stuff in code that doesn't make a great deal of sense -- don't get me started on smoke detectors and on AC drains.
AC power is a safety hazard. Likewise water pressure. Walking across the street. Etc. Etc. Certainly not in doubt.
Then I can only assume you haven't done much work in old buildings, where preaching goodness on remodeling issues can come across as more let-them-eat-cake than practical advice.
Apocryphal story, and how is it relevant to this thread?
fraid i forgot what the original plan was to do, but if it doesn't screw up your plan, I was working at a high school in a rural area, that had power to an xternal classroom (standalone temp building), in an underground conduit, and to get the network/internet to that outbuilding, we used powerline networking (actually a powerline bridge), and plugged one one unit inside the school, and one in the classroom with a wrt, so we had both wired and wireless...did the same from my house to the shop/outbuilding (that had power from the house) about 150 ft away... point is, can you do powerline networking over the exsting ac cable, instead of running another wire?
Hi. It's not like you think, as there is no existing AC line, and I'm already set on Passive PoE, but there is something cool about the idea that if one cannot readily do Power over Ethernet, then an option is to do Ethernet over Power!
Run a cheap exterior extension cord (assuring that the load is kept low- no circ saws or toaster ovens) and put Powerline adapters on each end. Expensive, but still, might be just the solution for temporary installations. Personally, I keep a couple of used XE102s in my problem solving kit.
By the way, this is what I bought for my Passive PoE solution:
I may have to chop off the barrel connectors and just splice in the linksys one, we'll see. But it's the cheapest solution and should work fine over 120 feet of Cat5e.
Thanks. Obviously you are an electrical engineer - officially or otherwise. I'm just a builder/electrician type turned networker ! Not trying to put you on the defensive, but I think there are times when knowing so much can lead one to overthink a problem. Happens to me in building, and then I sometimes I see what some independent lady did to solve a problem at her ranch and I wish I could unlearn a bit!
Refering to voltage variation tolerance - Linksys appears to use a switching regulator, which you might understand better than me.
I dug this up:
Well, the real inovation was to use a switching regulator in the router instead of the cheaper inefficient linear power burner. It keeps the box MUCH cooler. It also makes it tougher to meet FCC and ISO conducted radiation specification and costs somewhat more. The real benifit is that the router will run on just about any wall wart that can be purchased, which compensates for the increased cost of the circuit. It turns out that the 12VDC wall warts are a bit cheaper, use more efficient transformers, smaller diodes, smaller filter caps, and smaller guage output cable. Looks like an all around winner.
Another nice thing is that it will run on either a 6V or 12V gel cell. No need for DC-DC converters. Add a suitable solar battery charger and no need for PoE. The voltage can drift around the nominal voltage and the radio will never notice. Actually, there is a dumb problem at the upper end. The electrolytic filter cazapitors limit the upper end regulator can handle. The typical switching regulator can usually handle up to 40VDC on the input, but the capacitors are usually rated at 15-18VDC. So, I can't run it on 24VDC (without modification) which is what I like to use for solar power.
Google found some more on the subject:
Oh good. I wasn't quite sure what the WRT54G would do. Looking at the internal photos, my guess(tm) is that it's the same regulator circuit.
...snip... Jeff Liebermann
And then this by Jeff also:
The WRT54G apparently will run on any voltage between 3.7VDC and perhaps 18VDC. Current drain will vary, but is probably below .7A at 5VDC which is well within the operating range of the standard 12VDC 1A wall wart. With such a wide range of acceptable input operating voltages, the line loss in the CAT5 cable can almost be ignored.
For example, my previous calculations showed that for 100ft of CAT5, you will have 3 ohms of resistance and lose 3VDC at 1A (worst case). If we start with our 12VDC wall wart, and lose the 3VDC in the cable, then the WRT54G is now running off of 12-3 =3D 9VDC. The WRT54G can easily run off +9VDC and probably tolerate 3 times as much cable loss.
If you simply run a 2nd CAT5 cable, double up two of the wire pairs, and power the WRT54G from the 12VDC 1A wall wart, it will work. If you can't run a 2nd cable, then borrow the 4 extra wires, break out the wires from the RJ45 connector, attach a 3.5mm power connector, and use it as a simple extension cable for the power supply.
I cannot say what that unique hardware design uses. That citation is also only speculating and is typically of most such devices. Most electronics use the 5 volt version single chip power supply - not switching supplies.. But my suggestion is for almost every such device - just not most.
The reason for those recommendations: all other suggestions are more difficult or overly complex either in equipment, costs, or complications due to unknowns. Suggested is a least expenisve single chip power supply (7812) that eliminates a very long list of complications (most of which are not discussed) AND that means any standard or discounted DC wall wart of 22 volts 1 amps or higher can be used. Electronics so 'complex' that it is one of the first electornic circuits constructed by curious early teenagers.
In reality, same design probably would work even with a 13 volt wall wart (using a different single chip power supply). But again, I had stripped out all complications, extra work, surprises, and overthinking by suggesting the simplest 7812, any 20 something wall wart supply, and one twisted pair. If the AP fails, my suggestion means probably no changes with a replacement AP from most every other manufacturer. Just another example of so many complications (current and future) eliminated. I cannot express how simple is this best suggestion that eliminates all those 'what ifs' you are doing. Eliminates so many complications and surprises because this least expensive solution eliminates a very long list of things I have not even discussed yet.
You really are making this more complex than need be. That 7812 single chip power supply makes a long list of surprises and difficulties irrelevant. By far, a least expensive and simpliest solution is a 1 amp wall wart suppy of anywhere from 20 to 35 VDC, via an ajdacent single chip power supply, connected to the AP.
For service drops (entering a building) NEC 800.44(A)(4) states:
(4) Clearance. Supply service drops of 0-750 volts, running above or parallel to communications service drops, shall have a minimum separation of 300mm (12 in) at any point in the span, including the point of and at their attachment to the building, provided the nongrounded conductors are insulated and that a clearance of not less than 1.0 m (40 in) is maintained between the two services at the pole.
That's not exactly what you were looking for, but should give a fair idea of what the NFPA is looking for. I'll see if I can find the section on signal circuits in walls later. 800.133 thru 800.179 covers CMX type cable (which includes CAT5). However, there have been changes since I last read it, and I wanna be sure I've got it correct first.
Incidentally, the NFPA (National Fire Protection Assoc), which writes and publishes the NEC (National Electrical Code), is primarily concerned with fire safety, not electrocution hazards. There's considerable overlap between fire safety and electrocution safety. The basic assumption is that if you are electrocuted, you are also unsafe and given sufficient electricity, also combustable.
OK. I hear you. Considering I have a Linksys, it's not necessary in this case to hunt down a 7812 here in Mexico and then build a device
- but I will remember the trick for future solutions, and the info is here as reference for others. Apparently these posts bounce around the web disguised as various forums for years. Good thing, I'd say.
firstname.lastname@example.org wrote in news:5ce341db-62c7-4465-9dd8-5d2e2f616746 @o4g2000pra.googlegroups.com:
Why do you keep calling the 7812 voltage regulator a 'Single Chip Power Supply' ?
It's best to also add a filter cap to ground on the regs. input and output.
FWIW, a company (formerly) called DaTel makes a line of switching regulators based on the 78xx line that conform to the TO220 package and can replace any linear 7805/7812 simply by putting it in its place, and good for 500 mA I believe. I think Mouser or Digikey carries them. They are more expensive than a standard linear 78xx reg., but take input to 30 volts, and don't require a heatsink. Thermal management can be a nightmare when trying to reg. 12v down to 5v.
here's another fun one (power to one outbuilding came from a different transformer, so we couldn't use powerline, but we ran coax from the house, to allow cable tv/etc, and netgear happens to sell ethernet over coax......