Wiring for custom 40watt rgbw led fixtures

Discussion in 'Computers, Cabling & Other Miscellaneous Hardware' started by robt, May 21, 2014.

  1. robt

    robt Apprentice Elf

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    I have a led engin 40W rgbw led that i'm building a fixture around, and i've been puzzling about how to do some of the wiring. Eventually i'll have several of these fixtures, each with one led, and i was hoping to daisy chain power and data, preferrable on one cable. What is the best wiring to use?

    The specs for the LED, 700ma per channel, 12VDC max on each, except red is 8v max. The fixtures will be dmx controlled, and powered by 12v. The idea is to have two connectors on each fixture, and in and an out. The IN connector carries power and dmx data to the 4ch constant current controller, and that sends power to the leds in response to dmx. The OUT connector is there so i can connect it to another fixture, so with one single cable, i get power and data to it without having messes of cables everywhere.

    The main problem i see, dmx needs to be twisted pair, that means cat5 (i want to avoid standard dmx cable). Cat5 is usually 24 gauge, which won't carry enough current.

    What i want to do is have the power going to the IN connector sent to that fixtures led controller and also wired to the OUT connector, that way, each fixture is wired to the power in parallel.

    I thought that i might be able to use TWO twisted pairs on the cat5 for power instead of one (one pair for positive, one pair for negative). Do you see any problems with doing that to get around the current limits of a single 24gauge wire? I'm not sure what gauge equivelant that would make it so that i can tell how much current can be use that way though.

    Here is where my electronics experience lacks... If i have power wired this way (power in wired in parallel to power OUT), will the entire path for power be carrying the full load of all of the fixtures that are connected together? In other words, two fixtures will have eight LEDs (two red, two green, two blue and two white). Each needs 600ma (the LEDs are rated to 700mA, but the controllers will be sending 600mA constant current). eight times 600mA is 4.8 amps. Do all of the power leads need to carry 4.8 amps when they're all hooked this way?

    A similar question, the controller is hardwired so it has four "-" outs and one "+". Since the + will be hooked to the + of all four channels on the LED, is it carrying 4*600mA, while the "-" ones are only carrying 600mA each?

    Also, if i'm reading right, people generally omit the ground connection when sending DMX over cat5, is that right? Are signal problems common when doing it this way?

    The last question, the controller i am using has dmx in, but no dmx out. The manufacturer says it can be used for both. This seems more and more common these days. But i keep seeing in places that you should never ever Y a DMX signal. But if i hook the DMX on the LED controller both to the DMX coming in and to the DMX of the next controller, isn't that exactly what i'm doing?

    Thanks for any help!

    RobT
     
  2. AAH

    AAH I love blinky lights :) Community Project Designer

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    Hopefully this will answer all your questions but they'll be in no particular order.
    Pretty well all dmx boards have the dmx in and out as a commoned set of connections with the +, - and gnd being interchangeable across both connectors. There are a few exceptions with things like dmx controlled lasers where you can have a master and a slave.
    Having the above configurations isn't really a problem for the RS-485/dmx spec. Where the problem occurs is when there are long "Y"s as you get reflections in the signals.
    If you want to use cat5 cable for both dmx and power then you can probably do 2 things. The 1st is to change to cat 6 cable as it is much heavier. The 2nd thing would be to supply a higher voltage down the cat6 cable and have stepdown regulation via a switchmode regulator to 12V at the fixture/dimmer. The higher the voltage you start with the lower the current will be in the cat6 cable. 24V power supplies are common and 24V to 12V stepdown regulators are too. Something like http://www.ebay.com.au/itm/UN3F-DC-DC-Converter-Step-Down-Buck-12V-24V-to-5V-10A-50W-Car-Power-Adapter-/271251783088?pt=AU_GPS_Accessories&var=&hash=item3f27ddbdb0 could be used or you could go with something more exotic. Having the stepdown switchmode regulator means that you have an actual 12V right at the board and also that the cable has less current to get the same output power. For 24 to 12V you'll only have about 55% of the current (allowing for 90% efficiency). For 36->12 you're down to about 37% of the 12V current.
    The common carries 3 times the current that the R, G an B wires carry.
     
  3. OP
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    robt

    robt Apprentice Elf

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    Thanks. I take it ground isn't needed? And i just wire dmx coming in, to dmx on the first controller, and from those pads to the second controller, etc?

    For the power, current adds in a parallel circuit, so wired the way i was saying, the wires carrying the power would be taking the total current draw of a fixture (600mA *4), multiplied by the number of fixtures hooked together, does that sound right? i.e. if i have one fixture, the wires in the cat 5 carry 2.4 amps, but if i have two fixtures, the same wires would be carrying 4.8 amps? This seems right, but cat5 (or cat6) won't come close to working if so.

    Thanks again,

    RobT
     
  4. AAH

    AAH I love blinky lights :) Community Project Designer

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    Ground isn't really required as it's a differential signal but it's usually good practice to have it there as a shield.

    Cat 5 cable is sorta suitable up to 2 amps per strand. If you are using 2 strands per voltage rail then you could squeeze out 4 amps per cable.

    Cat 6 is good up to about 2.5A per strand. If you use 3 strands for +ve, 3 for -ve and trash having a shield/gnd for the dmx then you could potentially be getting 7.5A per cable. IF you then use switchmode voltage regulation and put 24V down the cable then you could effectively get about 162W of 12V power out of the cable.

    I would assume that even though the dimmer is 4 channel you would only be using 3 channels for the RGB and as such you would only be needing 1.8A@12V (21.6W). Theoretically if you really wanted to push the friendship you could probably daisy chain up to 6 deep with this configuration with cat6 cable (4 with cat5). Without using stepdown regulation you would be really lucky if you got 3 and 2 and it would depend a fair bit on cable length which is still a factor whichever way you go. ideally you'd use 48V power over ethernet type voltages but getting regulators could be harder.


    Doh. just noticed that you said RGBW. Don't know how you'd sequence these. You could either be sequencing R+G+B for white or just use the white which affects the current. The same prinicples apply regardless
     
  5. OP
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    robt

    robt Apprentice Elf

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    Ok, that clarifies it for me, except the part about 2 amps per strand, maybe i am looking at the wrong thing- cat5 is normally 24 gauge i think? And from the gauge chart at http://www.powerstream.com/Wire_Size.htm 24 gauge is rated for only 0.577a for power transmission. I hope i'm wrong though, i like your number better.. Ahh, maybe the gauges are different between US and AUS? Thanks for explaining the other parts, some of it's hard to find straight yet reliable answers to.
     
  6. davrus

    davrus Silent Elf

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    AAH - can you explain more about Ethernet POE ? That is 48V, over just one pair of wires, if my memory is correct. I have a few POE injectors, are you saying that they would be a suitable power supply for power over Cat5 (together with DC-DC converters, of which I also have several.)


    I intend doing my DMX connections with Cat5, and hadn't thought of using POE.
     
  7. OP
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    robt

    robt Apprentice Elf

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  8. OP
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    robt

    robt Apprentice Elf

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    The table at

    http://www.powerstream.com/Wire_Size.htm

    shows how many amps i can put through a given gauge. It's got two columns, one for chassis wiring, one for power transmission, i was thinking the power transmission column would be the one to use.

    But then i see at:

    http://www.mjobee.com/projects&news/NEC%20Art%20310.15.pdf

    It indicates completely different numbers for a given gauge. How do i know which table to go by? I've been looking into that for a couple of days, one site seems to imply that i'm reading the right chart (1st link), another site seems to ignore those numbers completely and use something closer (but not exactly) what's in the second link. So far i haven't been able to find any info on why the discrepancy, but i'm sure i must be seeing something wrong.. At some point once i get further along, i'll need to have an engineer take a look for safety, but not an option quite yet.. Safety's number 1 for this project, but i need to know i'm looking at the right info so i can move forward and get to that point.
     
  9. AAH

    AAH I love blinky lights :) Community Project Designer

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    I don't know a whole lot about it other than the concept. A number of the J1Sys controllers including the P2 are configurable for Power Over Ethernet (POE). David_AVD has also had POE configured for a bunch of his devices too. Somewhere around on either the forum or the wiki is some details on what wiring convention is used for POE.

    As for cable ratings I grabbed the 2 amps and 2.5 amps from here on the wiki. 0.2mm cable=2A. 0.25mm cable=2.5A based on a simplistic approach. It is the rough ability for wire to carry 10A/mm2 that is the reason I always like (and will always like) to use mm2 for wire size definitions rather than AWG.

     
  10. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    Just be aware that the size of the cable also effects the voltage drop, so the closer you run a cable to its maximum rating then the more voltage drop you will see.
     
  11. battle79

    battle79 Full Time Elf

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    Firstly, two types of POE exist.

    Passive POE - basically just using the unused wires to send the power to the unit. This is what all the controllers are using. This is equivalent to connecting the unused pairs to a power supply at one end and to the power input at the other. (Always refer to it as passive POE.)

    Active POE - This is what is built into a switch to power PTZ cameras and WIFI units. It only sends the power down the line after it receives an acknowledgement from the device at the other end to send power. This is in no way, currently used in our hobby. Ignore it. (Normally just called POE)

    So sending out the power through the controller is really not that magical, just very practical as it keeps the cabling setup simple. Only one RJ45 connector can power and send data to unit.

    Regards,
    Rowan
     
  12. davrus

    davrus Silent Elf

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    Thanks for the info on POE.
     
  13. OP
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    robt

    robt Apprentice Elf

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    Thanks for the info about amps/gauges. I know it's supposed to be simple stuff, i do ok with normal everyday things like driving a car and making the door lock before i leave for work, but some of the basic electronics principles are confusing, have had trouble getting any answers from other forums, i think i must look like a dimwit for asking.

    Thanks for that, eddy, i forget that heat is an issue as far as current running though the wire as well. basic ideas that the elec-tech heads take for granted, but i have to actually think still.

    But to stay on topic, thanks for the link to the wiki on gauge sizes. But this is the area that has me the most. One page says gauge X takes a maximum of Y amps, another page says something ludicrously different, either the whole world has all gone funky, or i'm being an airhead and missing something simple.

    Example, the page you point to:

    http://en.wikipedia.org/wiki/Power_over_Ethernet

    That seems more inline with what i expected. For example, according to that page, 20 gauge wire can carry a max of 5 amps.

    Here is a page that apparently disagrees:

    http://www.powerstream.com/Wire_Size.htm

    According to it, 20 gauge wire can carry a max of only 1.5 amps for power transmission, or 11 amps for chassis wiring. So i am thinking, this spread has just gotten wider apart than the earth and the moon.

    It's not the only page.

    http://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity

    That page says the same 20 gauge will carry 7.5 amps for "CU Max enclosed Amps", or 11 amps for "CU Max free-air Amps", whatever those terms mean.

    I know there are tolerances, and lots of variables like temperature, material, elevation, and the positions of the other planets, but i could have made a complete guess and got something well inside of those ranges.
     
  14. OP
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    robt

    robt Apprentice Elf

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    No response- that's ok though, i'm perfectly capable of electrocuting myself without any help.
     
  15. David_AVD

    David_AVD Good news, everyone!

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    CAT5 uses 24 AWG wires, not 20 AWG as far as I know.

    According the the ACL wire size wiki, 24 AWG is good for less than 2 Amps.

    Some of the differences in current rating are to do with how much the cable can carry in free air as opposed to being part of an enclosed cable bundle.

    Some specifications will also derate the cable for maximum safety margin.

    For my P-DMX wiring distribution, I use one pair of a CAT5 for V+ and another pair for V- (ground) and limit the current via a 2 Amp fuse.

    Hope that helps in some way.
     

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