Coils

[jevans]

What is the effective diameter of a coil (ignore legs) for calculating resistance?
Is it mandrel size
mandrel size + 0.5 wire diameter
mandrel size + Wire diameter
mandrel size + 2x wire diameter


Jim

[GeoffUK]

Dunno, I just punch the numbers I want into steam and it tells me.

[Deleted]

If a coil diameter is quoted as say, 2.5mm, then this is the internal diameter of the coil and the diameter of the mandrel.

[chykensa]

Yep jevans, as @yinyang said, ID (internal diameter) is the one to use!

[phatfil]

oooooooo makes my head hurt thinking this one thru.. when coiled is the inner side of the coiled wire compressed to the same extent that the outer-side is stretched?? if so the length of wire will equal the Number of coils X mandrel diameter, if not its a bit more complicated and will no doubt have some relationship to the malleability of the metal used.. I suspect that it is quite a complicated calculation for a high precision of accuracy, in real terms however the variance i suspect is so small that its virtually irrelevant, but tbh i have no idea

thats a perfect Monday night down the boozer discussion

[jevans]

phatfil
Brilliant answer!


Jim

[jevans]

Nov 18, 2015 12:02:13 GMT chykensa said:

Yep jevans, as @yinyang said, ID (internal diameter) is the one to use!

In that case I can only assume the the ohms/metre figures that the "Mesh Co." give are a load of bollocks!


Jim

[tim]

It's the length of wire that determines the resistance, not the diameter of the coil. The resistance would be the the same whether it was straight or coiled. The diameter is made smaller for more coils (more coils=good), or larger if you have too many to fit the atty (55    ).

[phatfil]

Nov 18, 2015 23:04:33 GMT tim said:

It's the length of wire that determines the resistance, not the diameter of the coil. The resistance would be the the same whether it was straight or coiled. The diameter is made smaller for more coils (more coils=good), or larger if you have too many to fit the atty (55    ).

BUT!!!  "have consumed some beers warning.."

coiling a wire will/may have some effect in changing its diameter and length..  an experiment to prove or disprove this hypothesis would be to coil a known length of wire, then straighten it and remeasure its length and diameter.   

the wire when coiled will undergo 2 opposing forces, 1 compressing the wire on its internal side and 2 stretching the wire on its outside, its a property of malleability, if this wasnt going on the wire would snap when coiling was attempted..    

Or is it??  i dunno but would enjoy discussing over more beers  

next up get a cube drill holes through the centre of each face,,  whats the shape of the hole in the centre???  

next up for discussion is infinity..  anyone read white light by Rudy Rucker??

[tim]

Nov 18, 2015 23:17:19 GMT phatfil said:

Nov 18, 2015 23:04:33 GMT tim said:

It's the length of wire that determines the resistance, not the diameter of the coil. The resistance would be the the same whether it was straight or coiled. The diameter is made smaller for more coils (more coils=good), or larger if you have too many to fit the atty (55    ).

BUT!!!  "have consumed some beers warning.."

coiling a wire will/may have some effect in changing its diameter and length..  an experiment to prove or disprove this hypothesis would be to coil a known length of wire, then straighten it and remeasure its length and diameter.   

the wire when coiled will undergo 2 opposing forces, 1 compressing the wire on its internal side and 2 stretching the wire on its outside, its a property of malleability, if this wasnt going on the wire would snap when coiling was attempted..    

Or is it??  i dunno but would enjoy discussing over more beers  

next up get a cube drill holes through the centre of each face,,  whats the shape of the hole in the centre???  

next up for discussion is infinity..  anyone read white light by Rudy Rucker??

Indeed, the wire will become distorted. But the change in resistance is what you see from a calc site (like Steam-engine) and in general is insignificant. 0.03 ohm change to a 1.2ohm coil is not noticeable.

I would love to discuss the drilled cube with you one day over a few beers though..... I reckon, for starters, it's a cube, without hard edges.

[thatguy]

coil resistance is a tricky beast to calculate. You have to factor in variations in impurity and diameter of the wire, how tightly coiled the wire is, coil spacing and even the temp of the wire. Most coil calculations will not use the internal diameter of the coil, but the median diameter, half way between the Internal and external diameter) as the measurement of choice, unless the calculations have been adjusted.


In general, don't expect any coiling calculator to be any more accurate than +- 0.05 ohms, or an even wider tolerance for the poorer ones. Please bear in mind that TC coils are even harder to calculate since they are more prone to variations of temp.

Build based on a proven calculator such as steam engine, then test the resistance of each coil, and match them as best you can.

[jevans]

Nov 18, 2015 23:04:33 GMT tim said:

It's the length of wire that determines the resistance, not the diameter of the coil. The resistance would be the the same whether it was straight or coiled.

I'm not stupid Tim!
Resistance = DIAMETER x Pi x No. turns x ohms/meter!!
+ a couple of mm for the legs.


Jim

[jevans]

Cracked it!
Using the O/D (mandrel + 2x wire dia) and 3mm for each leg, the ohms/metre figs work out.


Jim