Sunday, November 13, 2005


Wire wrapping test

I've been experimenting with the idea of using plain copper wire for conductors, as "bend alloy" has so far been shown to be unobtainable in NZ. The L-sectioned posts I'd blogged earlier don't look easy to fabricate, so I wondered about creating a 'trap' out of wire to hold chip legs and components in. I wrapped wire into 3 figure-of-eight shapes around some posts (the remainder were just to keep the wire out of the way) and pressed a PIC into the gaps, taking care not to actually touch the posts themselves:

Even the single strand on the far left produced a contact resistance of <0.1 Ohms. A heated hypodermic could trail a wire around on a RepRap platform, and terminate the wires by pushing them into a Polymorph surface. Some form of cunning nipper would be needed to cut the wire off, a few mm clear of the end of the needle and fold it off to the side. I think the next stab at the Polymorph surface would then form the wire into a J-shape, creating a primitive barb to anchor the wire in the setting Polymorph. Once the wire is in place, of course, the RepRap can bury it in plastic if necessary.

Do the assembled think it would be worth pursuing this concept? I can find a hypodermic and warm it up a bit to test the Polymorph anchoring. The wrapping would need some FDM'ing of suitable plastic posts of various shapes for straightforward wire-wrapping (junctions) and making sockets with. Suggestions on potential post shapes welcome.

Vik :v)

You should be able to find low melting point metal in the form of some type of soildering metal. Bends alloy is the best but hardly the only one available.

Personally, I like the idea of working with the copper wire. It will require higher tolerances around the connections and specialized designs for forming the figure eights etc. But it is cheap, easy to get anywhere in the world and carries current a thousand times better than bends alloy. And if we ever want this thing to get to the point of winding motors and power supplies bending wire is going to be necessary.

I am a chemist and dont understand how a chip wires in but I picture a connection block using the spring tension of the polymorph to hold a small length of wire against the pin. That would give you the abiltity to wire each pin seperatly. It might reguire addition of a conductive grease but salt water would even do in a pinch.
I'd have thought that a salt would (a) need to be wet to conduct, and (b) have significant corrosion problems with regards to the wire and component legs. Standard procedure is to keep greases and salts out of the way. Conductive grease is only thermally conductive.

If the wire itself does not offer significant tension, there are other options.

I could wrap a post, and then snap another plastic part down on top of the chip to push the posts against the chip legs.

Or I could have the snap-down part exert additional stress on the wire to guarantee contact.

The trick is to get the square edge of the component leg against the wire so that the edge digs into the wire.

Vik :v)
You could insert the chips upside down in the build, and then wire wrap directly to the pins on the chip, istead of trying to form sockets. Standard wire-wrapping tools and materials would then be useable. Admittedly, at higher power levels there could be heat dissipation issues.
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