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Contactor Diodes

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dvanlanen

Joined: 25 Jul 2006
Posts: 122
Location: Madison, WI

Posted: Wed Jun 18, 2008 11:35 am Post subject: Contactor Diodes

I am reading Chapter 11 of the AeroElectric manual, pp. 11-20 and 11-21 regarding battery and starter contactors, and I am having trouble understanding the use of diodes for spike protection.  

In figure 11-20 for a battery contactor, the diode is connected between the coil terminal and the large battery terminal.  When the contactor switch is opened, and the magnetic field of the coil collapses, what prevents an electrical spike from traveling directly out of the coil back through the 22AWG wire to the master switch, bypassing the diode, which is shown as being wired “in parallel” to the coil?    

In figure 11-21 for a starter contactor, the direction of the diode appears to prevent a flow to ground, where there are no electrical components to protect.  Again, what would prevent the spike from traveling back through the 20AWG wire to the starter switch, bypassing the diode?  

I’m sure these are “dumb” questions, but I’m not very quick at picking up some of these things as some folks are.  I would appreciate help in understanding it.  

Thanks,  

Dave Van Lanen  

Do Not Archive     [quote][b]

bob(at)bob-white.com
Guest

Posted: Wed Jun 18, 2008 12:02 pm Post subject: Contactor Diodes

On Wed, 18 Jun 2008 14:29:23 -0500

"Dave VanLanen" <davevanlanen(at)sbcglobal.net> wrote:



 	  Quote: 	
		   I am reading Chapter 11 of the AeroElectric manual, pp. 11-20 and 11-21

 regarding battery and starter contactors, and I am having trouble

 understanding the use of diodes for spike protection.

 

 In figure 11-20 for a battery contactor, the diode is connected between the

 coil terminal and the large battery terminal.  When the contactor switch is

 opened, and the magnetic field of the coil collapses, what prevents an

 electrical spike from traveling directly out of the coil back through the

 22AWG wire to the master switch, bypassing the diode, which is shown as

 being wired "in parallel" to the coil?  

 

 In figure 11-21 for a starter contactor, the direction of the diode appears

 to prevent a flow to ground, where there are no electrical components to

 protect.  Again, what would prevent the spike from traveling back through

 the 20AWG wire to the starter switch, bypassing the diode?

 

 I'm sure these are "dumb" questions, but I'm not very quick at picking up

 some of these things as some folks are.  I would appreciate help in

 understanding it.

 

 Thanks,

 Dave Van Lanen

 

 Do Not Archive

 

Hi Dave,
	




When you interrupt the current flow through a coil, there is a voltage

produced that is proportional to the speed that the current changes.  It

is also the opposite polarity to the voltage that was applied to

generate the current flow in the first place.  What the diode does is

provide a low impedance load for the reverse polarity spike.  Voltage

will be limited to the forward voltage of the diode, and current flow

will decay more slowly.  That's why the coil drop out time is increased

when the spike protection is used.



Bob W.



-- 

N93BD - Rotary Powered BD-4 - http://www.bob-white.com

3.8 Hours Total Time and holding

Cables for your rotary installation - http://roblinstores.com/cables/

nuckolls.bob(at)cox.net
Guest

Posted: Wed Jun 18, 2008 3:37 pm Post subject: Contactor Diodes

At 02:29 PM 6/18/2008 -0500, you wrote:



 	  Quote: 	
		  I am reading Chapter 11 of the AeroElectric manual, pp. 11-20 and 11-21 

regarding battery and starter contactors, and I am having trouble 

understanding the use of diodes for spike protection.



In figure 11-20 for a battery contactor, the diode is connected between 

the coil terminal and the large battery terminal.  When the contactor 

switch is opened, and the magnetic field of the coil collapses, what 

prevents an electrical spike from traveling directly out of the coil back 

through the 22AWG wire to the master switch, bypassing the diode, which is 

shown as being wired in parallel to the coil?



In figure 11-21 for a starter contactor, the direction of the diode 

appears to prevent a flow to ground, where there are no electrical 

components to protect.  Again, what would prevent the spike from traveling 

back through the 20AWG wire to the starter switch, bypassing the diode?



I m sure these are dumb questions, but I m not very quick at picking up 

some of these things as some folks are.  I would appreciate help in 

understanding it.


	


   No problem Dave,



   See:



http://www.aeroelectric.com/articles/spike.pdf



http://www.aeroelectric.com/articles/spikecatcher.pdf



   If these don't answer or satisfactorily explain

   then get back with us here on the List.



   Bob . . .

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

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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

dvanlanen

Joined: 25 Jul 2006
Posts: 122
Location: Madison, WI

Posted: Thu Jun 19, 2008 3:04 pm Post subject: Contactor Diodes

I am still confused, but I think my reading of the second document might help me clarify my questions.  This document states that the spike is negative-going, which I assume means that it is flowing toward ground (?)  If this is true, then it leads me to two related questions:  1) If the spike is travelling toward ground, why would it hurt the switch, which is installed in the positive wire?, and 2) Why would the spike be stopped by the diode when it could more easily travel through the coil wire directly to ground?  This last question may be a lack of understanding of how a diode works - does it “block” the spike from travelling against the arrowhead in the diagram, or does it actually “absorb” the spike?  

Thanks,  

Dave  

  

Date:   Jun 18, 2008   

   

From:   "Robert L. Nuckolls, III" <nuckolls.bob(at)cox.net>    

   

Subject:        Re: Contactor Diodes   

   

I am reading Chapter 11 of the AeroElectric manual, pp. 11-20 and 11-21   

>regarding battery and starter contactors, and I am having trouble   

>understanding the use of diodes for spike protection.  

>  

>In figure 11-20 for a battery contactor, the diode is connected between   

>the coil terminal and the large battery terminal.  When the contactor   

>switch is opened, and the magnetic field of the coil collapses, what   

>prevents an electrical spike from traveling directly out of the coil back   

>through the 22AWG wire to the master switch, bypassing the diode, which is   

>shown as being wired in parallel to the coil?  

>  

>In figure 11-21 for a starter contactor, the direction of the diode   

>appears to prevent a flow to ground, where there are no electrical   

>components to protect.  Again, what would prevent the spike from traveling   

>back through the 20AWG wire to the starter switch, bypassing the diode?  

>  

>I m sure these are dumb questions, but I m not very quick at picking up   

>some of these things as some folks are.  I would appreciate help in   

>understanding it.  

   No problem Dave,  

   See:  

http://www.aeroelectric.com/articles/spike.pdf  

http://www.aeroelectric.com/articles/spikecatcher.pdf  

   If these don't answer or satisfactorily explain  

   then get back with us here on the List.  

   Bob . . .     [quote][b]

nuckolls.bob(at)cox.net
Guest

Posted: Thu Jun 19, 2008 5:42 pm Post subject: Contactor Diodes

At 05:58 PM 6/19/2008 -0500, you wrote:



 	  Quote: 	
		  I am still confused, but I think my reading of the second document might 

help me clarify my questions.  This document states that the spike is 

negative-going, which I assume means that it is flowing toward ground 

(?)  If this is true, then it leads me to two related questions:  1) If 

the spike is travelling toward ground, why would it hurt the switch, which 

is installed in the positive wire?


	




    Polarity isn't the risk . . . it's VOLTAGE. The dielectric

    strength of air (insulation quality) is on the order of 700

    volts per mil. Okay, how far apart are the contacts of a switch

    when they first open. Microinches? Nanoinches? Naw . . . damned

    SMALL inches. How many volts does it take to jump across

    damned small inches? damned small volts.



    So, this says that any time you break the contacts of any circuit

    there is a tiny little fire that strikes up in the gap. Now,

    depending on the energy stored (no much but enough to certainly

    feel if you put your finger on it), how fast the rate of rise

    for votlage (pretty quick, see graphs), spreading velocity of

    contacts (the faster they open the more likely they are to break

    the arc) and their mass (the heavier, the more heat they take

    out of the arc and promote quenching), there is some erosion

    of contact surface.



    We largely ignored it during the first 4 decades of aviation

    mostly out of lack of technical imperative combined with

    some ignorance. The cost of replacing a switch from time to

    time was quite small in the overall cost of owning an airplane.



    But about 10-12 years ago, the first of the two-stage

    starter solenoids started showing up on both OBAM and

    TC aircraft. The energy stored on the coils for these

    devices took a big jump and we saw a rash of contact

    failures in the legacy OFF-L-R-BOTH-START style keyswitches.



    This prompted the AD against ACS and similar switches

    for replacement of starter solenoid control contacts

    in the switch and ADDING the spike catcher diode. Problem

    was, as described in the article, the diode was originally

    installed across the switch in a manner that did not

    perform as needed. It was ultimately corrected and

    re-published to put the diode across the contactor coil.



 	  Quote: 	
		  , and 2) Why would the spike be stopped by the diode when it could more easily

  travel through the coil wire directly to ground?


	


    Don't understand this . . . since the induced voltage

    caused by coil collapse is negative going at switch

    when the switch opens, and the diode (a check valve for

    electron flow) is positioned to conduct for any voltage

    that attempts to go more negative than ground, the

    spike current presented at the switch end is indeed

    shunted to ground and returns to the coil through

    ground. Virtually ALL the energy stored in the coil's

    magnetic field is dissipated in THE RESISTANCE OF

    THE COIL over tens of milliseconds.



    Review the write up on Kettering ignition system

    in chapter on OV relays. Kettering's points/coil/

    distributor exploited this effect to generate a

    multi-kilovolt spike from a 6v car battery. Here

    he WANTED the spike to live long and prosper. In

    our case, the same kind of spike has deleterious

    effects on the contacts of the controlling switch.

    Kettering used 'condenser' to mitigate deleterious

    effects on points.

 	  Quote: 	
		     This last question may be a lack of understanding of how a diode works 

 - does it block the spike from travelling against the arrowhead in the 

 diagram, or does it actually absorb the spike?


	


    It's a check valve for electrons. Electrons flow

    through the device only in direction opposite the

    arrowhead. See description in Chapter 1.



    Oh yeah, sorry about that first article. Noticed

    later it didn't have the information I was thinking

    about. I'm not sure where the right one got off too.

    I've been reorganizing the website indexes. I'll find

    it evenually.



    Bob . . .

dvanlanen

Joined: 25 Jul 2006
Posts: 122
Location: Madison, WI

Posted: Sun Jun 22, 2008 5:22 pm Post subject: Contactor Diodes

Bob,  

I think I mis-stated question 2.  What I meant to say was, “why would the spike be stopped by the diode when it could more easily travel through the coil wire directly to the opening switch?”     

In reading your reply carefully, I believe I might be misunderstanding the actual “flow” of energy in this instance.  When you say that the diode is positioned to conduct for any voltage that attempts to go more negative than ground, are you saying that the spike will be conducted through the diode (in the direction of the arrow) and back to the coil, and that this cycle will continue until the resistance of the coil dissipates it?  If so, what makes the diode a “preferred” path over the path through the wire to the opening switch?  Is this just the nature of a diode?  

I apologize if I am taking up too much of this venue’s resouces in trying to understand this concept.  If this is an issue, please let me know and I will try to gain an understanding by finding someone locally to discuss in more detail.  

Thanks,  

Dave 

 

  

and 2) Why would the spike be stopped by the diode when it could more easily  

>  travel through the coil wire directly to ground?  

    Don't understand this . . . since the induced voltage  

    caused by coil collapse is negative going at switch  

    when the switch opens, and the diode (a check valve for  

    electron flow) is positioned to conduct for any voltage  

    that attempts to go more negative than ground, the  

    spike current presented at the switch end is indeed  

    shunted to ground and returns to the coil through  

    ground. Virtually ALL the energy stored in the coil's  

    magnetic field is dissipated in THE RESISTANCE OF  

    THE COIL over tens of milliseconds.  

    Review the write up on Kettering ignition system  

    in chapter on OV relays. Kettering's points/coil/  

    distributor exploited this effect to generate a  

    multi-kilovolt spike from a 6v car battery. Here  

    he WANTED the spike to live long and prosper. In  

    our case, the same kind of spike has deleterious  

    effects on the contacts of the controlling switch.  

    Kettering used 'condenser' to mitigate deleterious  

    effects on points.     [quote][b]

nuckolls.bob(at)cox.net
Guest

Posted: Mon Jun 23, 2008 6:02 am Post subject: Contactor Diodes

At 08:18 PM 6/22/2008 -0500, you wrote:



 	  Quote: 	
		  Bob,



I think I mis-stated question 2.  What I meant to say was, why would the 

spike be stopped by the diode when it could more easily travel through the 

coil wire directly to the opening switch?



In reading your reply carefully, I believe I might be misunderstanding the 

actual flow of energy in this instance.  When you say that the diode is 

positioned to conduct for any voltage that attempts to go more negative 

than ground, are you saying that the spike will be conducted through the 

diode (in the direction of the arrow) and back to the coil, and that this 

cycle will continue until the resistance of the coil dissipates it?  If 

so, what makes the diode a preferred path over the path through the wire 

to the opening switch?  Is this just the nature of a diode?


	


   Reactive devices (capacitors and inductors) store energy.

   Capacitors in terms of the voltage to which they are charged

   and the value of their capacitance. The quantity of energy

   stored is



    J(watt-seconds)=(CE^2)/2



   Inductors do the same thing with current where energy

   stored is



    J=(LI^2)/2



   Capacitors can give up their energy in the form of

   high current pulses (like shorting a charged capacitor)

   while inductors are able to produce high voltages (when

   their magnetic fields are allowed to collapse without

   restraint).



   In the case of energy stored on an inductor like a

   starter or battery contactor coil, the breaking of the energizing

   circuit allows rapid collapse of the magnetic field within

   the inductor . . . and if totally unrestrained, can generate

   short spikes of voltage 300 volts or more.



   Obviously, this voltage will easily jump the gap in the

   relatively slow spreading contacts of the controlling

   switch. So as demonstrated on the bench and subsequent

   scope traces, a tiny fire is kindled between the spreading

   contacts and most of the store energy is dissipated in

   the stoking of that fire . . . with consequences to the

   surface of the contacts that reduce switch life.



   The goal is to re-route that energy. There's a host of

   methodologies, all perform as advertised and as dictated

   by the physics of their respective capabilities. A simple

   diode around the coil provides a shunt path for that

   energy that not only prevents the voltage from building

   at all, but causes it to dissipate almost totally within

   the resistance of the subject contactor coil. See:



http://www.aeroelectric.com/Pictures/Curves/S704-1_Drop-Out_Delay_without_Diode.jpg



   In views C and D, I show the path of electron flow for

   an energized contactor and a de-energized contactor. Note

   that the diode has no conduction in the energized mode (C)

   but provides an alternate path for electron flow in a tight

   loop (D) when the circuit is broken.



   This does cause the magnetic field to decay more slowly

   than if allowed to collapse unrestrained. A few weeks ago

   I published a trace that shows how the contactor or relay

   is delayed in reacting with a diode installed



http://www.aeroelectric.com/Pictures/Curves/S704-1_Drop-Out_Delay_with_Diode.jpg



   . . . as opposed to no diode at all:



http://www.aeroelectric.com/Pictures/Curves/S704-1_Drop-Out_Delay_without_Diode.jpg



   Note in the first trace coil voltage is clamped off

   at just below ground (.7v diode drop) while in the second

   trace we see the voltage go WWWAAAaaaayyyy negative. In

   the second trace, had a fire been allowed to form between

   the spreading contacts of a switch, the energy dissipated

   would have removed metal from the contacts. In the first

   trace, arcing is a tiny fraction of what does happen when

   the stored energy is not managed.



   This whole discussion is about improving life of the controlling

   device with one of several techniques . . . with a simple diode

   being the simplest and easiest to acquire.

 	  Quote: 	
		  I apologize if I am taking up too much of this venue s resouces in trying 

to understand this concept.  If this is an issue, please let me know and I 

will try to gain an understanding by finding someone locally to discuss in 

more detail.


	


   This is a classroom, not a country club. New students and

   teachers come and go with a wide range of knowledge

   and skills. This venue's resources are dedicated to

   precisely the needs you have expressed.

   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1932
Location: Riley TWP Michigan

Posted: Mon Jun 23, 2008 4:51 pm Post subject: Contactor Diodes

Dave, 

    When battery power is  removed from a coil by opening a switch, the coil now becomes a power source and  tries to keep the current flowing through itself in same direction that it was  flowing before the switch was opened.   Current that leaves a source wants to return to that same source (but to  the terminal of opposite polarity) by easiest path possible.  Although the diode blocked current flow  from the battery, the polarity of the current generated by the coil is such that  the diode easily conducts it (as long as the voltage is above approximately 1  volt).  The majority of the  coil-generated current will find it much easier to travel through the short path  of the diode to get back to the other end of the coil, rather than through  the air between the opening contacts of the switch.  The above events only last for a  fraction of a second. 

    It might be easier to  understand if one uses a water analogy.   Suppose there is a water pump instead of a battery, pipes instead of  wires, coiled tubing instead of a contactor coil, valves instead of switches,  and a check valve instead of a diode.   If the pump were running and valves were suddenly closed on each side of  the coil of tubing, the momentum of the water flowing through the coil of tubing  would want to keep that water flowing in the same direction.  The check valve would give the water an  easy path to keep water flowing in the same direction from the outlet of the  tubing back to the inlet of the tubing, much easier than trying to go through a  closed valve.  In a short time, the energy would be spent and the water  would stop flowing. 

    I hope that helps. 

Joe Gores

   [quote][b]

_________________
Joe Gores

bobf(at)feldtman.com
Guest

Posted: Mon Jun 23, 2008 6:21 pm Post subject: Contactor Diodes

you must be a teacher! this is excellent



bobf



On 6/23/08, Joe <fran5sew(at)banyanol.com (fran5sew(at)banyanol.com)> wrote:[quote]         

Dave, 

    When battery power is  removed from a coil by opening a switch, the coil now becomes a power source and  tries to keep the current flowing through itself in same direction that it was  flowing before the switch was opened.   Current that leaves a source wants to return to that same source (but to  the terminal of opposite polarity) by easiest path possible.  Although the diode blocked current flow  from the battery, the polarity of the current generated by the coil is such that  the diode easily conducts it (as long as the voltage is above approximately 1  volt).  The majority of the  coil-generated current will find it much easier to travel through the short path  of the diode to get back to the other end of the coil, rather than through  the air between the opening contacts of the switch.  The above events only last for a  fraction of a second. 

    It might be easier to  understand if one uses a water analogy.   Suppose there is a water pump instead of a battery, pipes instead of  wires, coiled tubing instead of a contactor coil, valves instead of switches,  and a check valve instead of a diode.   If the pump were running and valves were suddenly closed on each side of  the coil of tubing, the momentum of the water flowing through the coil of tubing  would want to keep that water flowing in the same direction.  The check valve would give the water an  easy path to keep water flowing in the same direction from the outlet of the  tubing back to the inlet of the tubing, much easier than trying to go through a  closed valve.  In a short time, the energy would be spent and the water  would stop flowing. 

    I hope that helps. 

Joe Gores

    	  Quote: 	
		  

	


[b]

Joemotis(at)aol.com
Guest

Posted: Mon Jun 23, 2008 8:29 pm Post subject: Contactor Diodes

You win the internet!

  

 Great example.

  

 Thanks

 joe

  

  no    archivos hombre!!



Gas prices getting you down? Search AOL Autos for fuel-efficient used cars.

  [quote][b]

dvanlanen

Joined: 25 Jul 2006
Posts: 122
Location: Madison, WI

Posted: Tue Jun 24, 2008 10:17 am Post subject: Contactor Diodes

Thanks to Bob and Joe for the great explanations.  I now understand clearly.  On to the next subject….  

Dave  

Do Not Archive     [quote][b]

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