Re: battery polarity

Posted by Gary Schafer on February 13, 2006 at 17:06:35 from (71.57.149.211):

In Reply to: Re: battery polarity posted by John T on February 13, 2006 at 13:39:23:

Hi John,

Yes it is interesting! In the operation of making a spark the ringing that occurs is just a by product and not a necessary ingredient for the spark plug to fire. Only a voltage transition in one direction at a rapid rate causes the plug to fire.

When the points open an arc occurs between the contacts as they open. An arc once started presents a low resistance across the points as long as the arc is there and sustains the arc. Actually a small plasma trail is formed in the arc which has a lower resistance than air. When switching, DC arcs are hard to put out without changing the polarity of the voltage or widening the gap of the switch (points).

Switching AC is much easier as it is constantly changing polarity and the change in polarity, voltage swinging down through zero volts, extinguishes the arc. That is the reason that you will see different ratings on a switch that may have both AC and DC current ratings. The AC rating will be much higher than the DC rating.

With a DC arc metal gets transferred from one side to the other of the contacts when the contacts open. I believe what happens to transfer metal back to the other side of the points is when the charged capacitor gets shorted by the points closing and releasing its energy which causes the points to arc again. This arc has a polarity opposite of when the points open so metal transfers to the opposite contact.

If the capacitor is of the proper size the same amount of material will transfer in one direction with points opening as the amount of material transferring in the opposite direction when the points close. The net result is no pitting or buildup of material on either side.

But since nothing is perfect there is always erosion of the point surfaces. During the arc some molten material does not transfer but gets boiled or thrown of the contact surface and lost.

If one looks at the points after some time in use usually one side or the other will have a spike (built up material) and the other side will have a matching pit from which the spike material came from. If new points and capacitor are installed and inspected some time again later it may be found that the pit may be on the opposite side of the points that it was on the last time. The capacitor may have gone from too small to too large.

With contacts switching an AC voltage the contacts last much longer as the random switching of the AC voltage results in sometimes one contact being positive and at other times the other contact being positive so the metal transfer kind of equals out and you usually only see small pits on each contact from lost metal that has gotten thrown clear during the arc times. Otherwise the metal is transferred back and forth between contacts depending on the polarity of the voltage at switch time.

There of course methods to limit the amount of arc in AC switching too, which is done on higher power circuits.
There other methods to limit arcs in DC switches also in addition to just the capacitor.

Regards
Gary

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