Why does twisting two wires create capacitance?

Thread Starter

Cretin

Joined Dec 13, 2012
69
I learned about this concept the other day...while the capacitance is minimal, i read an audio repair blog that discussed the effects that twisted wires can have on sound.

So what's the deal? What is the physics behind this? As far as I understand a capacitance is created by a di-electric field, so how does twisting two wires create that?
 

russ_hensel

Joined Jan 11, 2009
825
Twisting does not "do that" it does help lower the inductance. Audio guys sometime know what they are talking about and sometimes not so much.
 

#12

Joined Nov 30, 2010
18,210
Capacitance exits between any two conductors, regardless of the intervening dielectric. Twisting 2 wires together merely assures that their proximity stays the same across time.

and yes, I did not address anything about what some audiophile said.
 
If you twist wires together you increase the capacitance between them and so get more crosstalk.

A lot depends on the length of wire and the impedance of the signals.

I once had great trouble with a ribbon cable going to a keyboard, it had lots of crosstalk.
 

WBahn

Joined Mar 31, 2012
26,272
If you twist wires together you increase the capacitance between them and so get more crosstalk.
You are mixing apples and oranges.

If you start with four insulated wires, the + and- for one signal and the + and - for another signal, and run them parallel to each other in that order and as close as possible (so insulation touching) , then youi have a ribbon cable and a signal on any of them couples inductively and capacitively to all of the others. If you then twist the two signal pairs together you have very littel effect on the capacitive properties between the wires in a pair since theystay pretty much the same distance from each other. You will reduce the capacitance between wires in different pairs, but probably not by more than a factor of two or so. However, you greatly reduce the inductive coupling, particularly if the twist pitch is different between the two pairs, because whatever is induced by one loop of the twist is largely cancelled out by another.

A lot depends on the length of wire and the impedance of the signals.

I once had great trouble with a ribbon cable going to a keyboard, it had lots of crosstalk.
Yet a ribbon cable is not an example of twisted pairs, hence the crosstalk between signals is normally pretty bad. You can get twisted pair ribbon cable, but it is quite a bit more expensive and only does you any good if you can properly assign signals and return paths for those signals to the pairs.
 

MrChips

Joined Oct 2, 2009
22,067
I have made my own twisted pair multi-conductor cables in the past with as many as 25 pairs on a 50 pin connector. Sometimes you gotta do what you gotta do.

Note that high speed Internet cables are all twisted pairs. This debunks the comment about crosstalk.
 

atferrari

Joined Jan 6, 2004
4,110
Few weeks ago, recalling the use of two twisted wires (or very small plates) to compensate capacitances in (RF) tube amplifiers I twisted two wires over a length of 8 cm IIRC.

Measured 8 pF. Progresive pruning showed a linear relationship between capacitance and length.

Such a trick was used in a project (Radio Electronics?) to filter the output of what I believe was an NCO.

Still wondering about its stability against temp and if it is an objectionable solution (albeit not practical).

Thread derailing? Maybe you could say so.
 

#12

Joined Nov 30, 2010
18,210
I have known about the short twisted pair in radios for a long time. It was called a squib when I heard about it. Ask this: If a squib has 2 pf of capacitance, how much can it drift with temperature? Not much, I think.
 

WBahn

Joined Mar 31, 2012
26,272
I have made my own twisted pair multi-conductor cables in the past with as many as 25 pairs on a 50 pin connector. Sometimes you gotta do what you gotta do.

Note that high speed Internet cables are all twisted pairs. This debunks the comment about crosstalk.
When I was doing work at NIST we made twisted pairs out of 36 guage wire that were about 20 feet long and packed about a dozen pairs into a braided sheath. All of that capacitance and "crosstalk" we created didn't keep us from measuring nV scale signals -- quite the opposite.
 
Top