Return-Path: Sender: (Marvin Kaye) To: lml Date: Sat, 11 May 2002 22:56:54 -0400 Message-ID: X-Original-Return-Path: Received: from pop3.olsusa.com ([63.150.212.2] verified) by logan.com (CommuniGate Pro SMTP 4.0b1) with ESMTP id 1235292 for lml@lancaironline.net; Sat, 11 May 2002 21:16:45 -0400 Received: from mail.indian-creek.net ([209.176.40.9]) by pop3.olsusa.com (Post.Office MTA v3.5.3 release 223 ID# 0-71866U8000L800S0V35) with ESMTP id com for ; Sat, 11 May 2002 21:12:04 -0400 Received: from VAIO ([209.176.40.34]) by mail.indian-creek.net with SMTP (IOA-IPAD 3.0) id 3113400; Sat, 11 May 2002 20:16:41 -0500 X-Original-Message-ID: <007c01c1f952$a6ff7ac0$2228b0d1@VAIO> From: "Jim Cameron" X-Original-To: "Lancair Mail List" Subject: Diodes across coils X-Original-Date: Sat, 11 May 2002 20:16:31 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0079_01C1F928.BD5A14D0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2600.0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2600.0000 This is a multi-part message in MIME format. ------=_NextPart_000_0079_01C1F928.BD5A14D0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable The purpose of diodes across inductive coils like a starter relay is = fairly simple. When power is abruptly removed from an inductive coil, = the collapsing magnetic field generates a brief, but intense current = "spike" with polarity opposite that of the previously applied power = (voltage). If you have a diode reverse-wired across the terminals of = the coil, it will not conduct during normal operation, but will conduct = the reverse current of the inductive spike. This effectively shorts the = spike out across the coils and largely prevents it being propagated into = your electrical system. Almost any silicon diode will perform this function. Common types = like the 1N4002, 1N4003, or 1N4004 are fine, and can be bought for maybe = a nickel apiece. The negative end, which is usually indicated by a = solid band around the diode body, goes to the positive pole of the = relay; the other end goes to the ground terminal. Current ratings are = probably not very important, since they're for continuous current, and = inductive spikes last for milliseconds or less. Reverse (breakdown) = voltage is another diode specification you can mostly ignore because at = least in silicon diodes, you're not likely to find anything with = breakdown voltage much less than 100V, which is more than plenty. Jim Cameron, N143ES ------=_NextPart_000_0079_01C1F928.BD5A14D0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
    The purpose of = diodes across=20 inductive coils like a starter relay is fairly simple.  When power = is=20 abruptly removed from an inductive coil, the collapsing magnetic field = generates=20 a brief, but intense current "spike" with polarity opposite that of the=20 previously applied power (voltage).  If you have a diode = reverse-wired=20 across the terminals of the coil, it will not conduct during normal = operation,=20 but will conduct the reverse current of the inductive spike.  This=20 effectively shorts the spike out across the coils and largely prevents = it being=20 propagated into your electrical system.
    Almost any silicon = diode will=20 perform this function.  Common types like the 1N4002, 1N4003, or = 1N4004 are=20 fine, and can be bought for maybe a nickel apiece.  The negative = end, which=20 is usually indicated by a solid band around the diode body, goes to the = positive=20 pole of the relay; the other end goes to the ground terminal.  = Current=20 ratings are probably not very important, since they're for continuous = current,=20 and inductive spikes last for milliseconds or less.  Reverse = (breakdown)=20 voltage is another diode specification you can mostly ignore because at = least in=20 silicon diodes, you're not likely to find anything with breakdown = voltage much=20 less than 100V, which is more than plenty.
 
Jim Cameron, = N143ES
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