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Author Topic: Connectors for stranded wire + PC mount screw terminals  (Read 1474 times)
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Hello all,

I recently completed a project (art installation) built around sensors fed into an RS485 serial network.

The protoboards all use the following screw terminals:
http://www.mouser.com/ProductDetail/TE-Connectivity/282834-3/?qs=A%252bip%252bNCYi6M69kAjqV38yA%3d%3d

I'm using some 22 AWG stranded wire (1 twisted pair + ground) to make the connections, but they are a real pain (at least to me) to get into the terminals securely; the wire runs the risk of fraying and the holes are small... So what connectors could I use for this setup?

Will ferrules fit securely into the screw terminal mentioned above, or will they fit oddly once crimped?
What about open-barrel crimp connectors? (assuming a rounded end, not the squarish ones designed for pin headers)

Maybe it would be easier overall to get solid core wire like this:
http://www.homesecuritystore.com/p-1194-win-w4-250-250ft-224-solid-station-wire.aspx

I really appreciate any input, and links if you have any. Thanks!


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There are many things you can do... But I would immobilize the wires right at the point where they connect to the screw terminals with tie wraps or plastic wire clamps and make SURE that the wire ends are tightly twisted before clamping them in the screw terminal blocks. Crimping Ferrules are a good choice as long as you have the "Proper" crimper... Pliers won't really do the job and then you are back to loose ends. Soldering and solid wires are OUT for the same reason, Both are solid and although it takes longer in a higher vibration environment both methods will fail and break. There is also a "preferred" wire, one with many small wires. Many of the jumpers purchased form Asia work well with repeated use... for 2 reasons, 1. The wire may have 15 or 20 strandds and 2. there is a strain relief at the shell or molding on the end of the wire. Many small strands will work for RS485 Very well and are more difficult to fray and break because of their size and number smoe might break, those are more likely to be ones nicked in stripping and in the end... (no pun intended) prevention of this condition depends strongly on tools and technique... In My Direct Experience.

Doc
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This type of screw terminal is designed for wire (stranded wire, unless I'm badly mistaken), not any sort of ferrule.  If you want to crimp things onto the wire, why not use the connecters designed for square pin headers?

Can you redesign the PCB to use a slightly larger (e.g. a 3.8mm version of the terminal you reference)?

-j
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Thanks for the replies so far.

kg4wsv, my perfboards already have the screw terminals soldered in, so for the time being I can't switch to pin headers (good suggestion for the future though). Are you sure a 22 gauge ferrule will not fit?

Also, how do terminals with greater pitch improve the situation? Are you saying the hole size changes as well? At any rate, I'm using perfboard with 0.1" spacing.

Doc, do you have a link to that "preferred" wire you are talking about? Bear in mind my setup involves about 100 feet of cable altogether.
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something like this is commonly used in industry to make wiring easier and cleaner -

http://www.automationdirect.com/adc/Shopping/Catalog/Wiring_Solutions/Wire_End_Connectors/Insulated_Ferrules/Standard_Color_Single_Wire/BM-00501

They are crimped on to the end of the stranded wire and then inserted into the terminal block. I even had a small collection of jumpers with these on the end. Made it real easy to hook something up temporarily.

They come in a large variety of sizes.

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kf2qd,

Hey, great price on those! 500 for 7.50? The Mouser ones are 22 cents a piece for 500.

Are you confirming that ferrules will work on the screw terminals I mentioned?
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No sadly I would have done it differently. In production you bend everything to fit together, choices are and and changed as conditions require rather than deciding to address the connectors after the board is finished. If you expect the wires to move in use then provide for the movement by using one or more of the methods I mentioned. You can buy wire with as many conductors as required If you have a situation that requires that kind of wire then you might re-think what you are doing or order a "custom" cable (expensive for a "One of" type of device or product). The ferrules are very nice but in  the long run rather expensive as you must have some kind of crimper to use them or worse tin the wires in production. Soldering or "Tinning" the wire ends is ok IF you have a situation where the wires don't move (Wire clamps) but effectively makes solid wire and worse IF you have "Nicked" a wire or two in stripping the wire you have started the breaking process. I am totally spoiled as I have used strippers that are nick free all my life (They strip Teflon as well as Kevlar easily too) but they are expensive as they are used to strip wires intended for aircraft production... I don't think that the ferrules are the final answer as they only provide an "Interface" rather than a direct solution. If you can immobilize the wires, use the ferrules... they look real nice but they add little to the integrity of the job.

Doc
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Doc,

Well yes, this was very much a learn-as-you-go type of project; in future projects I will certainly do things differently.

In the meantime, I ordered a relatively cheap ferrule crimper and a bunch of ferrules. Perhaps not ideal given the screw terminals, but no doubt better than what I have now. I'll look into immobilizing the wires. I'm using a 3-wire, 22 AWG cable.

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I would just tin the wire, keeping strands together, preventing them from fraying and gives a nice solid connection to the terminal.

A lot of my professional audio gear has these types of screw terminals and I tinned all of my stranded copper wires before screwing them down.  It's quick, easy, and effective.

Just a little solder on the end and you're good to go!
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I don't think the last poster has the whole thread in mind or perhaps he disagree's with my statement... as follows and quoted from my last post:

Quote
Soldering or "Tinning" the wire ends is ok IF you have a situation where the wires don't move (Wire clamps) but effectively makes solid wire and worse IF you have "Nicked" a wire or two in stripping the wire you have started the breaking process
.

Which part of what I said do you disagree with? I'm curious.

Doc
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I would just tin the wire, keeping strands together, preventing them from fraying and gives a nice solid connection to the terminal.

It also provides a failure point - the wire will break at the end of the solder joint.

If you aren't handling much current (e.g. this is data, or low power) the ferrules are OK. The screw terminals will clamp down on a bundle of stranded wire and make a good solid connection, in sort of the same way that a crimp connector clamps down on stranded wire.

-j
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It's not that I necessarily disagree with you Doc, but it's served me well, even in situations where there might be a little wiggle in the wires.  In regards to nicking a strand, I would say that it depends on the size of each strand.  If there aren't many strands, it might be a little more imperative to make sure you don't knick one, whereas if there are many, nicking one or two I don't see as being a big deal.

Perhaps it's just because my experience is all in audio, using high quality cabling, but I've never been terribly worried about taking a strand or two with the insulation when stripping, because I know that the rest of the strands are in good shape.

Maybe I've just been lucky, but I have about 40 XLR connectors that obviously have tinned ends, as well as many terminal blocks similar to what the OP described in a road case that's traveled all over the country via airline for 5 years.  I've never had an instance of a wire breaking at the end of the solder joint.  I think we're all aware of the type of luggage "handling" the airline ramp agents are capable of.

In my experience, it's worked well.  Maybe it's not the absolute proper way to go about it, but I've never had a problem doing it my way.

I've also noticed that when the end is tinned, putting it in the terminal block seems to mold the solder to fit the shape of the socket and gives it more holding power.

Just my two cents.
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Quote
If you aren't handling much current (e.g. this is data, or low power) the ferrules are OK. The screw terminals will clamp down on a bundle of stranded wire and make a good solid connection, in sort of the same way that a crimp connector clamps down on stranded wire.

-j


In building Industrial control panels we used ferules on EVERY wire in the cabinet. With the combination of the crimp on ferule and the screw terminal there was never any problem with making a good connection. Larger wires still needed to be retightened, but that is standard practice with copper wire, as it needs a retorque after installation. Failure to retorque connections on copper is the biggest reason for terminal failure in high current applications.
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