Folks,
Have to send 12V/500mA across a connection. So far there is only an RJ45. Looking at datasheets I saw current ratings of 1.2A and 1.5A which sounds like a lot.
Is this kosher? I know PoE sends up to 600mA across but I've always felt uneasy about it when they went up from the (older?) 350mA.
-- Regards, Joerg http://www.analogconsultants.com/
it's not the wire you need to worry about it's the cheap crimp style connectors.
A company I deal with has 2 products that do 5V/500mA on a RJ45. The products have different types of RJ45s. One is a Dual stacked with a EMI shell. The other one has an internal vert RJ45. No problems with either one. I have tested the stacked RJ45 with 5V/1A out of a bench supply for most of a day, just as kind of a torture test. No noticeable issues.
(that was only 1 of the connectors on the dual)
-- Chisolm Republic of Texas
I think I remember PoE as something like ~400-500mA per pair, but that's gotta be including everything from connectors, wire size and how many cables that are usually packed together in a large installation
-Lasse
Looking at datasheets I saw current ratings of 1.2A and 1.5A which sounds like a lot.
Amphenol make a range of connectors rated at 1.5A and I've successfully used them in NRTL listed POE powered products drawing >800mA
Charlie
Don't forget distance. PoE is one of the few times I've had to worry about "maximum power transfer". BTW, 400-500mA sounds right but my memory is a bit fuzzy on that, too.
RJ45 isn't the best connector. In my experience you need to reseat the connector after a few years for it to make good contact. IMHO sending a lot of current through it may make things worse. BTW PoE always uses two parallel wires to carry the supply to the device whether the power is phantom fed or through unused pairs.
-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
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sure distance matters when you need to figure out how much power you have left at the of the cable, but it doesn't affect current rating
-Lasse
Thanks, Nico. Also thanks to all others. One of the things concerning me is the fact that datasheets derate to zero milliamps at slightly above
70C. Got that info this morning and it was quite the surprise. This is from the connector we are using:The unit will sit in full sun occasionally and can become as hot as the inside of a car under that condition.
-- Regards, Joerg http://www.analogconsultants.com/
Note that the maximum recommended current (for power conduction) per wire for #24AWG CAT5e is 577ma.
I found some references to a TIA recommendation for 360ma per pin maximum, but never could find the original document.
One of the cleanup jobs I dealt with had 12V at about 600ma going through an RJ45 as in PoE but not using 802.3af power control. (Actually, it was an RJ50 connector, which is the 10 pin version of the RJ45). The problem was that after about 20 insertion and removal cycles, the power connectors were becoming intermittent. Examination with my microscope shows pits, craters, holes, missing plating, and evidence of arcing. The problem was that the connector was being inserted and removed with power applied. The load looked like a BFC (big fat capacitor) which produced current spikes well above the recommended 360ma. A peak current limiting resistor in series with the power connections was the best I could do to reduce the problem to acceptable levels.
While working on the problem, I got diverted into measuring how much gold was being plated onto the RJ45 pins. The better connectors have
50 micro inches. The junk connectors I bought on eBay barely had 10 micro inches with exposed nickel plating in places. I also had issues with surface roughness, which reduced the contact area and seemed to cause some intermittents without arcing:(This site is worth reading if you're using RJ45 connectors).
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
You can get 22AWG TP for that. Usually only two-pair but that would suffice here.
I found 1.2A as a low value at 25C, so far. But the surprise was a rather fast derating to almost zero at 70C.
The worst is a BFC on the source side plus a BFC on the other side and the only inrush limiter is a short cable and an RJ45 connection ... bzzzt ... *PHUT*
But we'll avoid that here.
Thanks! I have bookmarked it. In this case we'd be ok because there would be essentially only two insertions. One at production test and another at site installation. Both non-powered.
-- Regards, Joerg http://www.analogconsultants.com/
We regularly see plastic boxes supposedly rated for 85 degC badly deformed after being in a car in Australia.
-- Regards, Adrian Jansen adrianjansen at internode dot on dot net Note reply address is invalid, convert address above to machine form.
Careful. That's #22AWG solid, not stranded. Stranded CAT5 is #24AWG which is what you'll probably see on "portable" equipment.
That's odd. The only thing I can think of is that either the plastic connector or the insulation becomes sufficiently soft to cold flow. That reduces the pressure on the insulation displacement connection, possibly resulting in a smaller contact area between the stake and the wire. (This is a guess, this is only a guess).
That's exactly what I had. I don't know how big the cazapitor was inside the power supply as it would have required explosives to disassemble. However, the device had about 10,000uF across the PS input pins: 12V at 0.6A = 20 ohms t = RC = 20 * 0.01F = 200msec which was enough to keep the device alive during minor power glitches. The designer wanted to add more energy storage but ran out PCB real estate. A supercap would have been better, but at the time, they were rather pricy.
Hopefully, it's running on battery, solar, or something not susceptible to power outages and glitches. If not, run a test to see how long your device will fly without power. You may want to do some inrush current limiting.
Assumption, the mother of all screwups. The first indication of a problem showed up in final test, where one power supply and I/O cable combination was used to connect to the automagic test equipment. After about an hour of testing, the ATE was rejecting nearly every device due to intermittent and "noisy" connections. I traced it to the RJ45 connectors and stupidly supplied the test line with a box of 25 replacement cables. That lasted about a week. I knew I was in trouble when I got a casual email requesting more cables.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
True, but the 577mA of 24AWG suffice in this case.
We just did a test run. Stays under 60C. Whew ...
10,000uF across RJ45 pins? Wow! That is brazen.
We'll have a generator with uC triggered starts as needed to replenish a large battery. So always juice. But the battery will not directly feed the system, there's a current limiter.
That's probably the same feeling people get when a request comes in via collect call, "Dad, I'd need another $500, soon" :-)
-- Regards, Joerg http://www.analogconsultants.com/
What will it do when it's out in the real world? 60C - 25C ambient is a 35C temp rise. That's not too awful, but with a "commercial" temperature range of 0 - 70C, that's 105C on the case. That should boil the tea on your product case.
Yep. (Disclaimer: It wasn't my design). The designer originally had a much smaller cazapitor. However, the initial customers experienced problems with noise on the power line. Rather than fix it in the power supply (there were several thousand already in stock), he applied the fix in the device by adding a huge cazapitor. Nobody noticed a problem until final test started rejecting devices. Better power supplies were eventually purchased, but the BFC and my thermistor fix remained.
Incidentally, it wasn't my responsibility to fix the RJ45 problem. I was doing the FCC type acceptance dry run(s), and was asked to "take a look" at the problem. That somehow morphed into inheriting a production engineering function. It was a safe thing to do for the company as I was working on a fixed price contract and would not get paid extra for fixing their connector problem.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
I've seen 85 degC capacitors (I assume it was the cap) quit in similar circumstances When I parked I had to unplug the blinker unit and put it under the seat so that it would stay cool. I never got round to replacing the cap with a 105 degree part.
-- ?? 100% natural --- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net
The connector rating and the spec for a given data streaming schema including power are two different things.
Of course the connectors will have a higher rating than any circuit one would design using them.
The wire is the big factor. The spec should be below the gauge standard as well.
So, never use small wire RJ-45 for a long run. Get the 24 or 22 Ga stuff.
These connectors connect at the very end of the wire, usually using a pair of barbs into the wire/strand bundle. That will likely carry far more than they are actually even rated at. They usually end up practically gas tight as well. Certainly if one pots or molds onto it.
Our cat 6 stuff gets an RJ-45 shaped shroud over it, connected to the drain line/shield during fabrication. Makes for a better eye.
Eye eye.
All the Gold utilized has changed all that.
Hundreds of billions of normal air connections made every day (all around the world), and years later still fine. And there is nobody wandering around flexing all the lines either. Gold does not oxidize at any rate you would understand. Obviously. Platinum, practically never.
Your experience must have been with tinned 'stingers' on CB radios.
Cat 5 and 6 say for the last 15 years, has been just fine as far as contact issues go.
And half an amp is trivial.
This could simply mean that a change in colour or surface finish of the casing is required. Different colours and materials have different albedo qualities.
White is three times better than grey, order of magnitude+ better than black.
RL
Not sure if the difference is that stark in real life. I own a white car and a couple of weeks ago during really hot summer days I had a black Volkswagen rental. I didn't feel that much of a difference when coming back to the parked car after severak hours.
-- Regards, Joerg http://www.analogconsultants.com/
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