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Wire


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Wire is the stuff which allows the electrons to move around your robot. It's the stuff which, if you try to squeeze too many electrons down it too fast, it will heat up and produce smoke. It's the stuff which will break if you bend it too much.

It's a misunderstood little thing.

Wire for Robotics

In my robots, there are really only three types of wire I concern myself with.

1) "heavy" flex

Leftmost in the main photo, this reel is happy with 17 amps. I would use this for connection between a heavy duty motor driver (FETs or relays) and the motors. I might also use it for battery connections. This is the cable I would consider for your 5-20 Amp motors.

Being "flex" (flexible) it can be jiggled and vibrated quite happily without too much risk of breaking. The cable has multiple cores of copper which is what allows it to be flexible.

You can get it at car parts stores in small quantities which is great because it comes in several current ratings and I rarely need much of it.

I wouldn't recommend soldering it to a PCB. Yes, of course, you can, but if it's flexing, you will almost certainly separate the PCB track from the board. Also, cables this thick needs lots of heat to allow the solder to flow. I would recommend screwdown terminals. If you're making your own PCBs, remember that there's no point in having a heavy cable fed from a PCB with only thin tracks. The tracks will act as a fuse.

Other sources are electrical mains cables, for example. If you can salvage one of these rated for your desired curent, they are flexialbe good quality cable. The cable off an electric iron will have a good 15-20A rating. Electric light cables should be good for 5-8A.

2) "light" flex

Pictured above in the middle is the cable I use most. I attach motors and sensors with it, make connecting cables and sometimes use it for inter-PCB jumpers. It's perfect for servos and all low current motors.

This is a reel of alarm cable. Again, it's flex and it usually comes in 4, 6, or 8 wires per bundle. It's not desperately expensive. I'vebeen using this reel for about 5 years and as you can see, I still have loads left!!

Other sources of similar (although slightly lighter) cable would be flexible category 5 twisted pair calbe. Beware, cat 5 cable comes in flexible (patch cable) for and inflexible versions (for burying in conduits and walls).

You can solder it easily to PCBs, it's great for small motors (up to maybe 4A max draw) and it's superb for making up specialised cables.

3) bell wire

I'm not sure if "bell wire" is teh right name for this stuff. I've always called it that historically. The roll pictured above on the right is actually category 5 twisted pair networking cable. This is the less flexible stuff aluded to in (2) above. It's normally buried in a wall, or whatever, so it's not supposed to be flexible. It's cheaper to produce this way.

I wouldn't recommend it for anything which needs to bend. I wouldn't attach motors, for example, with it. I use it for PCB links and for making jumpers for breadboards.

WireCores.jpg

Above, an illustration of (left to right) light duty multicore flex, single core bell wire and heavy duty multicore flex.

Other types

There are hundreds of thousands of types of wire each with many purposes.

  • One type you are probably using without knowing it is enamelled copper. Inside your electric motors, there are miles of this stuff. It's a single strand, with a very thin enamel coating as an insulator, allowing it to be wound into motor coils with very little space between each "turn".
  • If you're sending a signal into an amplifier (I suppose you might in robotics), you would typically use a screened cable. That is to say that there is one (or more) cables running inside another. The external "other" is known as the screen or sheath. The purpose of the this is that the screen would be grounded and acts as a Faraday cage in that it does not allow ambient electrical noise onto the inner core. Consequently, this interference does not become amplified into your signal so you have a much more clean signal. One example of this would be where a microphone signal is carried to an audio amplifier.

Care of

You'll note that my cables are stored on rolls or in loose coils. Cables don't like being flexed or folded. Even flexible cables don't like being stressed more than is necessary. Consequently, I will shoot anyone who I see wrapping the lead for their MP3 player around the device and stuffing it into their pocket.

I'm slightly obsessive-compulsive about this. If I need to store a cable, I coil it, then uncoil it in the opposite direction. Do not coil cables up and then just pull at them to unwrap them: this will cause them to twist and strain. For longer cables, or cables which need stowed but are used frequently, I would lightly fold them 2 or three times and tie loose knot so that they never twist up.

Don't underestimate the danger of badly stowed cables. Not ony can your MP3 player sound crackly, but if you repeatedly twist and fold the cable of an mains electrical device, the cores can break and over time, you may have all the current going through a progressively smaller and smaller number of cores until one day you get heat and smoke. Heat ans smoke from mains electrics is very very bad.

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Wires are generally much more tolerant to abuse than you think. Solid core wires are the only ones you should be concerned about, most multi-stranded wires handle stress just fine. Wrapping up headphone wires isn't going to eventually break them.

Wires are generally tolerant to quite a lot of abuse, but perhaps not so much as most people might think.

Wrapping, twisting, folding and bending of wires, even multi core wires IS eventually going to break them. Granted, this will take a lot longer with flex than it does with solid core wires, but it is a CERTAINTY. Care of the wires and the selection of appropriate wires will increase the length of time before failure.

Headphone cable is no different. Bend it, twist it, fold it repeatedly and it will break. The thing about headphones when folk wrap them around their players is they are always bend and fold at exactly the same place! Repeatedly wrapping them the same way exacerbates the problem.

Here, the plastic insulation plays a large part, too. Some less expensive headphones have a harder plastic insulation which is less flexible than the more expensive rubberised ones. The plastic actually increases the strain on the copper.

My only concern with headphone wrapping is weakening the connection to the headphone jack, that's much easier to break compared to the rest of the wire.
...because it is always bent and folded in exactly the same place.
no, because the connection is weakest at that point.

Can you give more detail?

I perceive that the connection is probably the strongest, most durable part of a cable, becoming a weak point because the interface between the flexible copper and the relatively inflexible solder provides a single point at which the cores continually bend, fold, twist and stress. I suppose this might be augmented by crystallisation of the copper by heating and as the solder starts to flow.

It's a somewhat moot point as, regardles the source of the break, all this can be alleviated by coiling rather than wrapping or folding your cable. 

I'd be interested in a reasoned thread on why earphones break.

I say that because the bend at the jack is usually the largest, compared to the rest of the wire. The actual wire wrapped around the MP3 player/whatever usually undergoes much less stress than the wire at the jack.