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Twisted Wires vs. Parallel

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Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Sun Dec 28, 2008 1:14 pm Post subject: Twisted Wires vs. Parallel

We often see instructions to twist the positive and negative wires together  to reduce noise problems.  Instructions from B&C for their voltage  regulator even specified the number of twists per inch.  

  

 But I'm wondering about the physics.  From studying the 'Connection,  it seems that paralleling the positive and negative wires tightly together would  have the same effect as twisting them together.  Is twisting the wires  together recommended because it is a convenient way to keep the wires in close  contact?  Or is there something about the nature of the electromagnetic  fields that makes twisting more effective?

  

 Thanks,

 Dennis

 Legacy, 220 hours, mostly twisted positive and negative  wires 

   [quote][b]

brucebell74(at)sbcglobal.
Guest

Posted: Sun Dec 28, 2008 2:12 pm Post subject: Twisted Wires vs. Parallel

Hi All,

 Years ago I replaced the windshield on my A35  Bonanza and found the wires to the Mag compass light twisted. This was done by  the beech factory. Must be something to it!

 Regards, Bruce Bell  RV-4, N23BB,   B&C Alternator and starter. All wiring to Aeroelectric Specs. No  smoke!

 DO NOT ARCHIVE!

 [quote]   ---

CamLight

Joined: 19 Dec 2008
Posts: 11

Posted: Sun Dec 28, 2008 2:41 pm Post subject: Re: Twisted Wires vs. Parallel

Twisting reduces the wiring's susceptibility to interference due to the way it can cancel out certain external signals by reducing the wire's loop area (which reduces its degree of coupling to the interference). It also reduces crosstalk between nearby pairs of wire, but you need different twist rates for each pair to minimize coupling between them.



What I'm not sure of is the difference in effectiveness between pairs carrying single-ended signals (signal and ground) vs. differential signals (signal+ and signal-. used in telecomm and computer equipment).

_________________
John M.
Owner
CamLight Systems

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Dec 28, 2008 4:06 pm Post subject: Twisted Wires vs. Parallel

 	  Quote: 	
		    We often see instructions to twist the positive and negative wires together to reduce noise problems.  Instructions from B&C for their voltage regulator even specified the number of twists per inch.  

   

  But I'm wondering about the physics.  From studying the 'Connection, it seems that paralleling the positive and negative wires tightly together would have the same effect as twisting them together.  Is twisting the wires together recommended because it is a convenient way to keep the wires in close contact?  Or is there something about the nature of the electromagnetic fields that makes twisting more effective? 	
 



    A single conductor in relatively "free" space has some

    behaviors of an antenna. I.e, it can radiate as well

    as gather some portion of any electro-magnetic energy

    in the vicinity.  When electrons move relative to some

    local point one can measure both an electric (electro-static)

    force as well as a magnetic (electro-magnetic) force

    as a result of that motion.



    For example, a beam of electrons shot from the rear

    of a cathode ray tube toward a screen can be deflected

    or pointed because we can exploit EITHER the electro-static

    or electro-magnetic properties of a moving stream of electrons.

    It matters not whether those electrons move through a

    conductor or through a vacuum. In a CRT television display



  http://aeroelectric.com/Pictures/Misc/Electro-Magnetic_Deflection.jpg 



    electro-magnets on the neck of the tube were used to

    control direction of the electron beam to produce spots

    of light over the surface of the screen. In an oscilloscope,

    electro-static deflection . . . .



  http://aeroelectric.com/Pictures/Misc/Electro-Static_Deflection.jpg 



    . . . was preferred for its much higher frequency response.

    

    Not only can electron streams be magnetically and

    statically influenced as described above, they exert

    their own, similar influences. 



    Suppose we start with a circular loop of wire say 10'

    in diameter. This is an excellent example of a loop

    antenna with an ability to efficiently radiate or

    intercept energy at some frequency.



    Now, let's  squash the loop in one dimension. As

    it moves toward an elliptical shape, the AREA of the

    loop goes down and the radiation/reception efficiency

    goes down. The obvious end point happens when the

    area approaches zero and the circle becomes a parallel

    pair of wires.



    It's intuitively obvious that electrons flowing in one

    wire are exactly balanced by opposite direction flow

    in the opposite wire thus cancelling their individual

    electro-magnetic and electro-static effects . . . mostly.



    At higher frequencies it can be shown that parallel

    lines do not exactly cancel mutual magnetic effects

    because of the non-zero distance between their centers.

    Here it becomes valuable to twist the two conductors

    such that what ever leakage exists perpendicular

    to the conductors tends to be washed out by the reverse

    polarity of effects longitudinally displaced 1/2 twist

    away.



    Another very effective technique is to use shielded

    wire. Use the center conductor for the outbound electron

    flow and the shield for the inbound flow. Here the two

    fields ARE centered on each other and cancel each other

    exactly.



    In the case of the compass light wire, the goal is to

    null the magnetic field around a wire that MUST be

    positioned in close proximity to the system most

    vulnerable to interference from variable and stray

    magnetic fields.



    Some manufacturers twisted their outbound and return

    wires. Others ran the wire up a small i.d. copper

    tube to make inbound/outbound paths. Similarly,

    shielded wire would be equally effective in this

    regard. 



    Twisting the B&C Alternator wires was done at my

    suggestion back when plastic and glass canard

    pushers were king. The effects of poorly managed

    inbound/outbound conductor pairs had some profound

    effects on compass as well as audio systems. But

    again, this practice IS NOT generally recommended

    for ALL inbound/outbound pairs in an airplane.

    

    Like my oft repeated admonition for "follow the

    manufacturer's instructions" for shielding practices,

    the same advice applies to things like twisting for

    PM alternator installations in plastic airplanes

    and compass lights. Twisting makes a wire bundle

    more bulky and more $time$ consuming to fabricate

    and install.



    Bob . . .     [quote][b]

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Dec 28, 2008 4:08 pm Post subject: Twisted Wires vs. Parallel

At 04:41 PM 12/28/2008, you wrote:

 	  Quote: 	
		  



Twisting reduces the wiring's susceptibility to interference due to 

the way it can cancel out certain external signals by reducing the 

wire's loop area (which reduces its degree of coupling to the 

interference). It also reduces crosstalk between nearby pairs of 

wire, but you need different twist rates for each pair to minimize 

coupling between them.



What I'm not sure of is the difference in effectiveness between 

pairs carrying single-ended signals (signal and ground) vs. 

differential signals (signal+ and signal-. used in telecomm and 

computer equipment).


	


   Paralleled or twisted conductors get you benefit

   only when they carry equal magnitude and opposite

   polarity signals. This is why RS232 single-ended

   data was quickly replaced by RS422 twisted pairs

   under shields for the greatest noise immunity.

        Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

simon(at)synchdes.com
Guest

Posted: Sun Dec 28, 2008 7:19 pm Post subject: Twisted Wires vs. Parallel

Dennis,  

   

There is no difference electrically between two wires that are kept at the same distance in parallel and two wires that are twisted together to stay parallel.  The reason they are twisted together is that it is the simplest way to keep them in parallel while the cable (a combination of wires, shields, insulation, etc.) bends and distorts.  

   

When an RS-422 transmitter generates a signal, it has two circuits that drive the twisted wire pair.  One circuit sources current, while the other circuit sinks current.  When a differential twist wire pair carries two currents from the transmitter to the receiver, they travel from the source driver to the load (a termination resistor next to the receiver), then back to the current sink.  Along each wire, the current generates an electromagnetic field defined by the right hand rule, where the direction of the field is per the direction of the hand’s fingers when the thumb is pointing toward the direction of current flow.  Since the current in one wire is traveling opposite the current in the other wire, the fields oppose each other and ideally cancel each other out.  They also ideally cause no noise to be radiated.  

   

Perhaps the greatest benefit of RS-422 and differential signaling is immunity from external noise in the receiver.  RS-422 receivers are built to reject “common-mode noise, which is noise that is of the same amplitude on both the + and – terminals of the receiver.  This is where the twisted wire pair comes in.  It connects to the receiver’s + and – terminals and brings in both the real signal and common-mode noise.  While the twisted-wire pair is shielded, some noise will leak in, but the electromagnetic and electrostatic fields will cut across wires at the same amplitude, because the wires are so close together.  Thus, the noise’s amplitude will appear at the receiver’s + and – terminals equally and will be rejected by the receiver, since it is designed to reject common-mode noise.  As an example, the AM26LV32 is a commonly used differential line 3.3V receiver.  It has a common mode input voltage range from -0.3 to 5.5V and will reject any voltage that appears on both terminals in this range.  The voltages on its terminals must have a delta of greater than 0.2V for it to change its outputs.  You probably have to be involved in the measurement of real world signals and problems to appreciate just how good the AM26LV32 works to reject common-mode noise.  One of the things that I haven’t mentioned is that the termination resistor not only serves to terminate the twisted wire pair’s characteristic impedance but also provides a low impedance to prevent electrostatic and electromagnetic fields from developing high voltages in twisted wire pairs.  

   

So to reiterate, there is nothing magical about twisted wire pairs other than they keep the wires in close proximity to each other and that’s it.  Everything else I said above is just fluff!     

   

Simon Ramirez  

Copyright 2008

 

   

   

  

       

From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Dennis Johnson

 Sent: Sunday, December 28, 2008 12:29 PM

 To: aeroelectric-list(at)matronics.com

 Subject: Twisted Wires vs. Parallel  

  

  

     

We often see instructions to twist the positive and negative wires together to reduce noise problems.  Instructions from B&C for their voltage regulator even specified the number of twists per inch.    

    

   

    

But I'm wondering about the physics.  From studying the 'Connection, it seems that paralleling the positive and negative wires tightly together would have the same effect as twisting them together.  Is twisting the wires together recommended because it is a convenient way to keep the wires in close contact?  Or is there something about the nature of the electromagnetic fields that makes twisting more effective?  

    

   

    

Thanks,  

    

Dennis  

    

Legacy, 220 hours, mostly twisted positive and negative wires   

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       [quote][b]

CamLight

Joined: 19 Dec 2008
Posts: 11

Posted: Sun Dec 28, 2008 11:33 pm Post subject: Re: Twisted Wires vs. Parallel

 	  simon(at)synchdes.com wrote: 	
		  There is no difference electrically between two wires that are kept at the same distance in parallel and two wires that are twisted together to stay parallel.  The reason they are twisted together is that it is the simplest way to keep them in parallel while the cable (a combination of wires, shields, insulation, etc.) bends and distorts. 	


But don't pairs in untwisted pair cabling suffer from more crosstalk and susceptibility to interference than in twisted-pair cabling?  And, IIRC, can't you do a longer run with twisted-pair than with untwisted-pair cable?

_________________
John M.
Owner
CamLight Systems

raymondj(at)frontiernet.n
Guest

Posted: Mon Dec 29, 2008 12:11 am Post subject: Twisted Wires vs. Parallel

I think this paragraph from Bob N. earlier this week bears on this 

discussion.



================================================

   At higher frequencies it can be shown that parallel

   lines do not exactly cancel mutual magnetic effects

   because of the non-zero distance between their centers.

   Here it becomes valuable to twist the two conductors

   such that what ever leakage exists perpendicular

   to the conductors tends to be washed out by the reverse

   polarity of effects longitudinally displaced 1/2 twist

   away.

=================================================



Raymond Julian

Kettle River, MN



"Hope for the best,

but prepare for the worst."



---

tiethoff

Joined: 02 Sep 2008
Posts: 23

Posted: Mon Dec 29, 2008 3:41 am Post subject: Twisted Wires vs. Parallel

When wiring a tube amplifier it is common practice to twist the filament 

lines to the tubes. Reason is that they are AC fed with a relative high 

amperages (6,3 volt and 2 or 3 amp). The rest of the wiring is or signal or 

DC. When the filament lines are NOT twisted they introduce humm (50 or 60 

Hz) in the system. Since in an aircraftsystem there is no AC feed (except 

the output of the alternator field tot the regulator) the only usance is to 

keep the lines together.



--------------------------------------------------

From: "raymondj" <raymondj(at)frontiernet.net>

Sent: Monday, December 29, 2008 9:09 AM

To: <aeroelectric-list(at)matronics.com>

Subject: Re: Twisted Wires vs. Parallel



[quote] 

 <raymondj(at)frontiernet.net>



 I think this paragraph from Bob N. earlier this week bears on this 

 discussion.



 ================================================

   At higher frequencies it can be shown that parallel

   lines do not exactly cancel mutual magnetic effects

   because of the non-zero distance between their centers.

   Here it becomes valuable to twist the two conductors

   such that what ever leakage exists perpendicular

   to the conductors tends to be washed out by the reverse

   polarity of effects longitudinally displaced 1/2 twist

   away.

 =================================================



 Raymond Julian

 Kettle River, MN



 "Hope for the best,

 but prepare for the worst."



 ---

sglynn(at)us.ibm.com
Guest

Posted: Mon Dec 29, 2008 9:33 am Post subject: Twisted Wires vs. Parallel

Please how do I remove this email from receiving informative notes like the ones below?

 thanks

 

 

 Steven Lynn

 Global Business Services

 IBM Americas

 Cell: 425-280-4404

 Fax: 360-851-2211

 sglynn(at)us.ibm.com

 [img]cid:1__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img]"raymondj" <raymondj(at)frontiernet.net>

 

 

       "raymondj" <raymondj(at)frontiernet.net> 

 Sent by: owner-aeroelectric-list-server(at)matronics.com 

12/29/2008 12:09 AM  Please respond to

 aeroelectric-list(at)matronics.com

  
 
 
    [img]cid:3__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img]

 To

[img]cid:3__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img]

 <aeroelectric-list(at)matronics.com>  [img]cid:3__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img]

 cc

[img]cid:3__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img]

   [img]cid:3__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img]

 Subject

[img]cid:3__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img]

 Re: Twisted Wires vs. Parallel    [img]cid:3__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img][img]cid:3__=07BBFFBDDFF3B99D8f9e8a93df938(at)us.ibm.com[/img]    

 --> AeroElectric-List message posted by: "raymondj" <raymondj(at)frontiernet.net>

 

 I think this paragraph from Bob N. earlier this week bears on this 

 discussion.

 

 ================================================

    At higher frequencies it can be shown that parallel

    lines do not exactly cancel mutual magnetic effects

    because of the non-zero distance between their centers.

    Here it becomes valuable to twist the two conductors

    such that what ever leakage exists perpendicular

    to the conductors tends to be washed out by the reverse

    polarity of effects longitudinally displaced 1/2 twist

    away.

 =================================================

 

 Raymond Julian

 Kettle River, MN

 

 "Hope for the best,

 but prepare for the worst."

 

 ---

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simon(at)synchdes.com
Guest

Posted: Mon Dec 29, 2008 6:04 pm Post subject: Twisted Wires vs. Parallel

Yes and yes, John. Untwisted pair cables suffer from more crosstalk and are

more susceptible to interference because during their run, they tend to be

farther apart in places than their twisted pair counterparts.  Per my

earlier post, when an electromagnetic or electrostatic field crosses their

path and induces a current or voltage into them, it will be a different in

each wire because they are at different relative distances from the source

wire, which is probably running parallel but not necessarily.  Then the

common-mode receiver will not be able to reject this noise as effectively as

if the noise at its two terminals were more "equal."



Also, if a differential transmitter is generating + and - currents through

the two non-twisted wires, they will not cancel each other out as

effectively as their twisted wire counterparts, because they will be farther

apart in places, and their electromagnetic fields will not be mutually

inductively coupled into each other as well along the wires' runs.



Furthermore, it is easier to maintain characteristic impedances more

accurately when the wires are twisted together and their distances apart are

more accurately maintained. By providing a termination resistor at the

receiving end of the twisted wire pair, reflections are more carefully

controlled, if not cancelled.  For this reason and for the reasons cited

above, longer runs are possible with twisted wire pairs than with untwisted

wire pairs.



BTW, the same theory that applies to keeping the distances equal between

wires in twisted wire pairs also is used in printed circuit boards to

propagate signals along traces.  There the traces are geometrically

controlled so that their widths and distances from each other and from other

traces, ground and power planes are maintained, as well as the separation

material, to do a much better job than wires in cables, especially in

Gigabit applications.



Simon Ramirez

Copyright 2008



--

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