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Double Shielded Coaxial Cable for a Vehicle Tachometer

Discussion in 'General Electronics Discussion' started by dzlvs8, Sep 16, 2011.

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  1. dzlvs8

    dzlvs8

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    Sep 16, 2011
    Hello, I work for a large automotive company and we have some photocells and tachometer boxes that read out a tach signal. There is a photo cell that reads the frequency of a piece of reflective tape on something spinning and that signal is sent via a double shielded coaxial cable to the tach box.

    These cables are terrible. And I am looking for a suitable replacement cable. The cables short of very quickly after a few uses and they are very very expensive. It costs $5 for a foot of cable and $50 for the connector that goes on each end. I would like to find an alternative cable and connector that can be rewired to the box. But I want to be very careful so that I don't compromise the signal that I receive.

    Does anyone know what I need to look for in a cable and connector, based on what I have described???

    The cable is double shielded with copper "mesh" wire used for both sets of shielding.

    Again, I will repeat. I am looking for new connectors and wire. I am willing to rewire the tachometer boxes and photo cells so that they will accept a new wire.

    Thanks in advance for any help.
     
  2. poor mystic

    poor mystic

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    Apr 8, 2011
    Hi :)
    Please describe the electrical load on the cable, including such details as peak voltage, peak current, and waveshape,so that we can see why the existing arrangement seems so flimsy. (Do you mean they short out? Why?)
    Then, we might be able to come up with somethng.
     
  3. daddles

    daddles

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    Jun 10, 2011
    You don't give any engineering details, so it's hard to say. What's the signal level, frequency, and length of cable run? Personally, I would aim for using RG-58 cable with BNC connectors. This might mean you need a preamp at the tach output.

    However, if this is for typical engines, it would be hard to believe that the frequencies involved would be more than (at most) audio frequencies. So it sounds like it should be a fairly undemanding application -- unless you haven't mentioned all the details.

    If you're at a large company, you probably have access to a number of good instrumentation engineers who could help you with the details.
     
  4. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Consider TRIAX cable/connectors.
    (Unless that's what you're using now, ...you weren't very clear).
    I'm guessing your problem is induced vehicle electrical noise onto your cable.
    Be careful exactly WHERE you ground one shield of the triax, to cut your noise.
     
  5. dzlvs8

    dzlvs8

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    Sep 16, 2011
    All I know is that it is a low power signal running through the cable and the frequency is between 300-8000rpm. The cables are only about 10 ft long. I measured the voltage on the cable and it was 5v dc. For the life of me I couldnt get a get resistance measurement. I know thats probably the important one but the damn thing would register different everytime. All I can tell you is that the center wire in the coaxial cable is very very small. This is supposably a very undemanding system. Thats all I really know. I of course have a measurement system to verify that the signal is good when I make the new setup. But I would like to at least be in the ball park when making this thing.

    FYI - Of course there are people in my company that know the info that I am looking for but, my company is huge. Its not like I can just go to a deparment and ask a random person I never met to spend there time working on this. Not that they wouldn't be willing but they're boss and my boss wouldn't probably be happy about it.

    So, the way the wire is set up, the inner lining of shielding carries the DC- voltage on the cable.

    I must also stress the important thing about selecting a new cable is that the end needs to be durable. What we have now comes loose inside when the cable is pulled on even in the slightest bit. The shielding in the wire basically sandwiches in between metal washers of sorts and a nut tightens the washers together to supposably clamp the wires in place. But it doesnt work well. The connector on the end is a BNC. These BNC connectors have multiple parts inside them and are solderless connections. They are $50 per connector and the wire is $7 per foot! So the cables are expensive AND crappy.


    Any help would be great. Something that works with BNC would be great. Since thats whats on the ends of the photo cell and tachometer box.
     
  6. dzlvs8

    dzlvs8

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    Sep 16, 2011
    THE PROBLEM IS that the cable is crappy and breaks. Thats all. They are crappy and expensive. Two things that dont go together well. Thats all. It provides a good signal.

    Triax is not an option. Its expensive, too complex, and there is not enough room to connect three bnc's to our tach box.
     
  7. daddles

    daddles

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    Jun 10, 2011
    I haven't heard anything that says you couldn't use some standard RG-58 cables with BNC connectors. You can get them quite cheap here or a bit more expensive from a place like Pomona. I've got some HP coax cables that I've been using for 30 years, so some good ones should last a long time with reasonable care. You may of course have to modify things if you don't have regular female BNC connectors on the equipment, but that's straightforward to do. If you need custom cable lengths, you can also buy the cable and connectors and make up your own -- it's not hard to do.
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    you havent told us what stresses the cable is being subject to that its constantly failing
    it would not be normal for most cables to fail so regularly unless your installation was particularly bad and providing no protection to the cables

    you sure they are BNC connectors ? At $50 each you must be getting ripped off, even for the best quality ones, virtually mil spec, I have never spent more
    than $20 each, standard ones vary from $3 to $10 each

    Dave
     
    Last edited: Sep 17, 2011
  9. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Brother, the more you talk about it, the more I'm thinking your existing cable IS already triax. Take a close look at the 'BNC' connector. It's probably not a two conductor conventional coax cable you've got.
    You're misunderstanding the word 'triax', a triax cable and connector look outwardly
    like a conventional coax BNC. But there are multiple leads in the cable and connector.
    Closely examine the connector. Are there more than one connector inside of it?
    Google it when you see what's inside the BNC connector, and see if it's triax or not.
    That would explain the high cost you're talking about, and what you consider the
    fragility of the hook-up.
    I don't know who's making your cables. Your own company or a vendor supplying them
    to you. You sound like you're low-level on the food chain there.
    Study the problem, write a report about what you find, and look good to your bosses.
    Your problem isn't the wire or the connector. It's the lousy assembly of the cable that's
    the problem.
    Tell your bosses to dump your existing supply vendor for the cable, and hire somebody
    who can make the cables correctly (the mating of connector to cable).
    You've got low bid going on the cables. Somebody at your company is going to have
    to realize that low bid doesn't cut it, if it means 'no-working'.
    Good luck.
    It ain't the cable or connector, it's the assembly that's screwing you.
     
  10. daddles

    daddles

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    Jun 10, 2011
    Based on the stated cost, shrtrnd could be right that the connectors are triax. If the OP is right that the signal is on the order of 5 volts and less than 1000/6 Hz, I'd say triax is overkill for such a system.
     
  11. poor mystic

    poor mystic

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    Apr 8, 2011
    I don't know what would be wrong with a shielded, twisted pair. Surely no high frequencies are involved!
     
  12. shrtrnd

    shrtrnd

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    Jan 15, 2010
    daddles & poor mystic make sense.
    But this is a new design for a new vehicle. They may be running some kind of higher
    frequency RF for the signal to some central computer or even a wireless reporting instrument.
    We're just guessing from the description of the problem here.
    I used to work instrumentation that sent two signals from the instrument to a computer,
    The manufacturer accomplished this with a single triax cabling system.
    I didn't think the original post was asking what the best system ought to be.
    I thought he was trying to solve a problem with his existing system.
    I might be barking up the wrong tree, but it'll give the guy an idea to check on.
     
  13. dzlvs8

    dzlvs8

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    Sep 16, 2011
    WOW. Slow down and listen please. The cable connectors are terrible. Then are not strong enough and the cables become disconnected in the connectors because there is no strain relief on the cable. The only thing stopping the cable from comming out of the connector is the direct connection of the wires to the connector. For the third time THAT is the problem. Nothing to do with the frequencies or anything else. The cable works fine when its not broke. The damn thing is breaking.

    I am looking for a connector that wont fail so much. Or maybe even just a good strain relief system that is removable so the cable can be serviced.

    I did misunderstand what you were saying about a triax cable. Obviously when I say I have a three connector BNC on the end of the cable, a triax cable is exactly what I have. But the type we are using is absolutely terrible. Sometimes tachos fall off the car or whatever and the cable gets pulled out. It really doesn't take even that much to ruin them.

    In regards to the RG-58 cable, that cable looks great. ....well at least the connector looks different on the end. But I can't figure out if the cable is a triax cable with two "sheild wires" in it. There is a nice strain relief and the connector looks like the internals may hook connect to the wire and sheilding in a different/better way. I would be willing to try something like that even though it doesnt look serviceable. When I say connector, I am not talking about the BNC end (thats the same as what I have...said that already). I am talking about the internal connections to the wires on the inside of the connector.
     
  14. dzlvs8

    dzlvs8

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    Sep 16, 2011
    Here are some pictures of the crappy connector. Like I said before, it is a three wire BNC. The wire is held to the connector by the sheilding being squished between two pieces of metal. It sucks. I need a different, cheaper connector that is hopefully more durable. But it should just at least be cheaper than what we use now. about $250 per cable!!!!! Maybe even a heavy duty strain relief boot for this cable end would be a god solution. Any ideas?

    [​IMG]

    [​IMG]

    [​IMG]
     
  15. shrtrnd

    shrtrnd

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    Jan 15, 2010
    That looks like an Amphenol Triax connector to me. The part number etched at the
    end of the connector will give you the part number.
    I think we told you everything we could.
    Triax is used specifically for low loss applications. I'm assuming your engineers selected
    specifically for that reason.
    Only the best connector manufacturers do Triax connectors, and none of them are cheap.
    Unless you can convince your people to try coax like others recommended on this
    site, you are probably screwed.
    All I can tell you is. It's your cable to connector assembly that's causing your problem. If you're stuck with the assembly in your picture, hire King Kong to tighten
    the connector when you reassemble it. You have absolutely no retention for the center
    connector, that's the nature of this beast. You have to torque down the sheild braid
    to get your connection (Unless you buy one that solders or crimps).
    I searched Winchester(Kings), Pomona, Amphenol, Trompeter, Fischer, Nemal.
    You've got no 'strain relief' for this type of connector from the manufacturer.
    If your people get themselves into enough trouble from this, they can consider having
    a custom-built cable made to meet your specs.
    Good luck. Triax isn't usually used in rough and tumble environments. You usually
    just install them and let them work.
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, so now that we know the connector and cable are of presumably high quality (which presumably justifies their cost -- but possibly not their use in this application) we need to know why they're breaking.

    Are the cables in the way of something and get caught on things? It sounds to me like the solution may be protecting the cable, not changing it.
     
  17. shrtrnd

    shrtrnd

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    Jan 15, 2010
    It sounded like the cable is being yanked out if it's connector.
    I don't see how a strain relief is going to fix that.
    Armored cable, soldered to the BNC barrels at both ends?
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Or cable placed so that it can't be yanked.

    It sounds to me that the cable is failing because something is being done to it that it is not designed for. So stop it happening and your problems will go away.

    I realise it sounds simple,,,
     
  19. davenn

    davenn Moderator

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    Sep 5, 2009
    the pics you are showing are standard coax connectors a centre core and a screen, no sign of a 3rd wire
    they are not crappy connectors !! :) they are used successfully in their 100's of 1000's world wide for many many years I personally have used many dozens of them in both amateur and commercial radio installations in 2 different countries. From use in vehicles to mountain top repeater installations. Its Extremely rare for one to fail.

    As Steve said you are far better off determining why the cable and connector is being stressed so badly and solve that problem. :)
    The usual trick is to put a loop in the cable so as to give strain relief. You should never have a tight cable between 2 locations, should always be at least a bit of slack

    cheers
    Dave
     
    Last edited: Sep 20, 2011
  20. dzlvs8

    dzlvs8

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    Sep 16, 2011
    Hmmm Thanks for the help. But wow, some of these are some weird responses. The people using these cables are normal people. Therefore we know what not to do with them but we cant prevent accidents. Obviously we've tried to prevent these cables from being damaged. ????

    I dont know why someone is doubting me but it is a three wire xonnector. It sounds like you fuys are saying its a triax. Ok. Like I said before. I would like to find a cable like this (a triax, I guess) thst has different connectors inside it that connct the wire s to the end connector. Someone mentioned a triax with soldered connectors. That would be excellent. Like I said before, with the way that these cables attach, it sucks and tightening down the cable connectors is the only possible improvemnet we can make with these connectors. We already have hired king kong to tighten down the ends. The pin still pulls out whe the tach falls or whatever we do to destroy them. Like Ive said, Im just looking for a different cable connector (internal) that will be a little more rugged or a strain relief for this type of connector.


    It sounds like no one knows of any sort of thing?
     
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