Re: Current draw through ignition switch

["Dean W. Cains" <dwc@domain.elided>]

You guys seem to have gotten way off track with your discussion of current 
draw through an ignition switch.  Isn't the current draw through the 
ignition switch limited to the current needed to actuate just the starter 
solenoid (and the cold-start solenoid on a Spica car)?  Isn't the point of 
the starter solenoid to act as a relay, taking the brunt of the current 
draw away from the comparatively tiny ignition switch?  No need to be 
concerned with back-EMF, locked or rotating armatures, internal battery 
resistance, etc.

Regards,

Dean W. Cains
Lutz, FL
'74 Spider Veloce (with both a starter relay and solenoid)

At 09:10 PM 3/31/2002, you wrote:
>Date: Sun, 31 Mar 2002 10:32:03 -0800
>From: "Paul Irvine" <pi@domain.elided>
>Subject: Re: Current draw through ignition switch
>
>Wayne Derossett wrote:
>- ----------
>Motor winding DC resistance is only part of the current draw.  Obviously, a
>loaded motor draws more current than an unloaded motor.  This is because
>most
>electro-mechanical devices exhibit reverse EMF
>- ----------
>
>and John Fielding wrote:
>- ----------
>the current drawn by the starter is not simply the volts applied and the
>armarture DC resistance!
>The rotating armarture in the magnetic field induces a "back-EMF" in the
>armarture windings which negates part of the applied voltage, so reducing
>the effective applied voltage from the battery.
>- ----------
>
>This is true but for the purpose of rating a pushbutton starter switch that
>you want to have long and reliable service from you need to figure the
>maximum instantaneous current draw that occurs at the moment you first push
>the switch. That is before the rotor starts turning and the back EMF comes
>into play to reduce the current.
>
>Paul Irvine - Antioch CA
>Project Alfa - http://home.pcmagic.net/pi

--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided