Using a DB25 cable as "wire"

Discussion in 'The Projects Forum' started by leonhart88, Feb 10, 2011.

  1. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Hey Guys,

    I have a project where I have 16 wires in total controlling 8 valves. I'd like to extend the wire in order to have the valves and electronics separated. Maybe 3 or 6 feet away. Since there are 16 wires, I was thinking about using a DB25 cable to extend them. I don't really want to use normal wire as it's not shielded and I don't have the budget to buy multi-conductor shielded cable. Has anyone done this before? It sounds like it should work as long as the DB25 cables wires straight through (pin 1 goes to pin 1, etc.). I'm currently looking at these two cables:

    http://www.infinitecables.com/pop/mn_mac1.htm
    http://www.infinitecables.com/pop/sr-110.htm

    The DB25 sounds like it would work, but not sure about the DB15.

    Thanks!
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    Well how much current are you going to run through each wire? A couple of milliamps should not be a problem. Heavy currents probably would not work. How can you not have the money for raw cable. Premade DB25 cables gotta be more expensive.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Pair of cat 5 cables would be much cheaper, current limit around 500mA/wire for the distances you are looking at.

    That current is if you are using RJ45 Jack/plug on either end.
     
  4. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Sorry, should have mentioned that earlier. Each valve is 12VDC and uses 0.33A of current. Would this be too much for a DB25 cable to handle?

    Its not that I can't afford wire, I already have lots of that. I am just worried about having a bunch of non-shielded wire that will run 3-6 feet long (I have valves as well as a stepper motor that I will be extending). The shielded cables with multi-conductors in them are expensive (and only come in rolls).
     
  5. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    thatoneguy:

    Yes, I did look at cat 5 cables as well. I also have 12 wires for a pump module that I didn't mention before. So in total I would need 4 cat 5 cables. If I used DB15 and DB25 cables, I would only require 2.

    Are cat 5 cables superior?
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    You can get foil shielded cat 5 cables. It is cheaper in the way of the connectors that are molded on the ends. An RJ45 end is about 10 cents and is easy to install, while a DB-25 runs a couple dollars or more plus labor.
     
  7. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    thatoneguy:

    OK thanks. I will search on Digikey for them. Is there any difference in the pinouts of cat6, cat5e, etc.? I know I don't want a crossover cable...

    Just out of curiousity, is the current rating for DB connectors roughly the same as the cat 5s?

    Thanks.
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    I don't see the need for shielding to run pumps and motors. Are there strong electric fields you're trying to deal with? What is it you think shielding will do for you?
     
  9. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Papabravo:

    I probably don't need shielding for the valves, since it's just power and not a signal. However, I have a stepper motor driving a syringe pump with an encoder on it. The encoder and motor would be connected to electronics through the long wire. I'm worried that will cause problems with this. Maybe it is unnecessary worrying? I don't have too much experience with this type of stuff...I just know that long wires will pick up noise that can affect the signals being transmitted.

    Also, the thing I am building is being placed near a cyclotron so I am trying to be as safe as possible.
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    For the low level signals, you may want to use a Differential Driver and receiver, using one pair for each signal.
     
  11. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    thatoneguy:

    Unfortunately, I am not designing any electronics. I have been asked to simply use some OEM parts, put them together, and program them. I have been told that the stepper motor and encoder in our OEM pump requires 22 gauge wires. This means a DB25 (~28 AWG) is out of the question...and a cat 5 might not be good enough (~24 AWG). I don't know the exact current rating for the pump, but I am asking them now.

    It looks like maybe I could just solder 22 gauge wire onto two DB25 headers and make my own "cable", without worrying about shielding for now. I will continue to look into it.

    Thanks for the suggestions/help.
     
  12. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    After looking around a bit, I noticed that copper shielding foil is quite cheap.

    If I wanted to just add shielding onto my own wires, would I wrap this around my wires and solder a drain wire onto it? Then connect this drain wire to my PCB ground?

    I see several types of foil...which ones are preferred? Could you even use typical house hold aluminum foil? Although I'm not sure if those would hold up to soldering...
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
  14. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    There are two coupling mechanisms that you have to worry about. Electric field disturbances couple to your wire harness capacitively. This is where shielding can be effective. It has to surround the conductor everywhere especially where the harness enters and exits a box. Integrity of the shield in and around connectors is a huge and difficult problem which is often overlooked. I don't see how "tape" can be of much use, but creativity is not necessarily my strong suit.

    The other kind of coupling is magnetic and here what makes a difference is the area of a "loop" consisting of a conductor and a ground return. When you twist a signal wire with a ground return you minimize the area of the individual loops and knock this kind of interference along with it. High speed data routinely goes over UTP (Unshielded Twisted Pair).

    Unless you have reason to believe that your equipment will operate in the presence of strong electric fields I would not worry about it. On the other hand if you discover that your information was in error believe that shielded cables will be the least of your worries.

    Using differential drivers and receivers eliminates the problem by making the noise "common mode". That is it affects both wires in a differential pair in the dame fashion. A differential receiver is immune to large amounts of common mode noise.
     
  15. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Papabravo:

    Thank you for the explanation. I don't think there will be strong electric fields. The device is being placed inside a metal chamber which also contains lead blocks, so I think most of the EMI would be cancelled out by the thick metal. The only thing inside that I think would be generating electric fields would be the stepper motor and solenoid valves themselves. However, the wire might be run out of this chamber, so I'm not sure if there are strong fields in the other room.

    Thanks for the help guys. I will simply start with some non-shielded 22 gauge wire and then see if I will need anything fancier. I always tend to get worried about shielding when the wires start to get long.
     
    Last edited: Feb 11, 2011
  16. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    One way to give a fair immunity to noise is to twist the wiring pairs together. If you have a VSR drill, this is quite simple.
     
  17. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    No difference between Cat5, Cat5e or CAT6 except for frequency capability 5e had better insulation, 6 had even better but it was also 6 twists per inch. Few know it but you can run 1Ghz ethernet through 5e up to around 350'.

    They're all #24 AWG so can easily carry 3.5A over a short distance or a bit over 0.5A if you aren't trying to run across the world,: http://www.powerstream.com/Wire_Size.htm

    Being twisted pairs I wouldn't worry about interference. Heck of a lot less expensive and easier to work with too.
     
    Last edited: Feb 11, 2011
Loading...