Why does twisting two wires create capacitance?

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Cretin

Joined Dec 13, 2012
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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
18,222
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
26,398
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
23,539
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
4,329
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
18,222
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
26,398
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.
 

crutschow

Joined Mar 14, 2008
27,229
Yes, ive heard of them called gimmicks too. Never had need of one, I feel like they were more used in the tube days.
Probably.
But they can still be useful, such as when compensating the feedback resistor for high frequency, high impedance inverting op amp circuits where a few pF of capacitance may be needed.
 

Audioguru again

Joined Oct 21, 2019
3,201
A telephone cable is a twisted pair. But it is balanced with opposite signal phase on each wire. Then crosstalk from other twisted pairs and interference are cancelled. The twisted pair has capacitance between the close-together wires which cuts high audio frequencies.

A telephone line has an impedance of 600 ohms then the capacitance of a long twisted pair as a load muffles the sounds.
 

Tonyr1084

Joined Sep 24, 2015
5,992
Jumping in with what may be accurate - may be wrong, but a Twisted Pair (TP) of wires, one used as a ground or common part of the circuit will act like a shielded cable. No - I didn't say it becomes a shielded cable, just that outside interference is negated when the wires are twisted.

A capacitor is simply two plates with a spacing between them and some sort of insulator. Sounds a lot like a TP of wires; doesn't it? The difference between a cap and a TP is that the capacitance between the plates is deliberate, and it is not in itself a conductor. A TP on the other hand IS a conductor. But it's like having a capacitor across the conductors.

What does a capacitor do? Many things. Basically it charges and discharges at a specific rate. They can be used as filters, decouplers and some other things. A TP wire has one purpose. Well, it can serve as a dual purpose as others have said sometimes the TP is more useful than a simple set of conductors.
 

DickCappels

Joined Aug 21, 2008
7,442
I used to use them in the lab on prototypes when I knew I need a tiny bit of capacitance but probably needed less than 1 pf (50 Mhz video amp comes to mind).

On some home projects I have used them as trimmers -need more capacitance, just twist it some more, lower capacitance: trim the length a little.

I've always called them gimmicks, probably because of something I read in Electronics Illustrated or Popular Electronics.
 

Yaakov

Joined Jan 27, 2019
3,161
I used to use them in the lab on prototypes when I knew I need a tiny bit of capacitance but probably needed less than 1 pf (50 Mhz video amp comes to mind).

On some home projects I have used them as trimmers -need more capacitance, just twist it some more, lower capacitance: trim the length a little.

I've always called them gimmicks, probably because of something I read in Electronics Illustrated or Popular Electronics.
I've always known them as gimmick capacitors and I'd have learned that from either old hams or QST or both.
 

BobaMosfet

Joined Jul 1, 2009
1,776
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?
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3
 

AnalogKid

Joined Aug 1, 2013
9,257
Jumping in with what may be accurate - may be wrong, but a Twisted Pair (TP) of wires, one used as a ground or common part of the circuit will act like a shielded cable. No - I didn't say it becomes a shielded cable, just that outside interference is negated when the wires are twisted.
While you are correct that twisting and shielding both reduce interference pickup (called RS - radiated susceptibility - in MIL-STD-461), the two mechanisms are not at all comparable; "outside interference is negated" for two very different reasons.

The shield in shielded cable forms a Faraday cage around the central conductor, and both absorbs and conducts away the impinging energy. It works best with a single-ended signal when the shield is firmly grounded.

Unshielded twisted pair (UTP) relies on manufacturing precision and algebraic subtraction. Interference is picked up equally by both conductors in the pair, and is subtracted out by a differential receiver. The interference reduction is greatest with a balanced signal where neither conductor is ground.

There are other wiring arrangements and combinations, but them's the basics.

ak
 
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