Return-Path: Received: from mail.cruzio.com ([208.226.92.37]) by truman.olsusa.com (Post.Office MTA v3.1.2 release (PO203-101c) ID# 0-44819U2500L250S0) with ESMTP id AAA11329 for ; Sun, 27 Sep 1998 03:30:07 -0400 Received: from testelectronics.mail (sa-207-251-8-110.cruzio.com [207.251.8.110]) by mail.cruzio.com with SMTP id AAA02993 for ; Sun, 27 Sep 1998 00:30:05 -0700 (PDT) Received: from laptop [192.168.1.4] by testelectronics.mail [192.168.1.1] with SMTP (MDaemon.v2.5.rB.b2.32-R) for ; Sun, 27 Sep 98 00:30:08 -0700 Received: by localhost with Microsoft MAPI; Sun, 27 Sep 1998 00:27:52 -0700 Message-ID: <01BDE9AD.A9AC5760.ed@testelectronics.com> From: Ed Armstrong Reply-To: "ed@testelectronics.com" To: "Lancair [lancair.list@olsusa.com] (E-mail)" Cc: "'tomcat@gigapixel.com'" Subject: MOV Flyback Circuit Diode Date: Sun, 27 Sep 1998 00:27:50 -0700 Organization: Test Electronics X-Mailing-List: lancair.list@olsusa.com Mime-Version: 1.0 <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> >> Tom made me look at this circuit again. I realized that the series diode is not going to isolate noise all that much. The primary noise will be a negative 14V spike. At very high frequencies, when the switch is turned off, stray capacitance around the wire will tend to hold the wire at +14V for a very short time. The diode will be forward biased by the -14 volts from the flyback circuit, so the major noise from the flyback circuit still gets through to the wire. The diode will isolate any secondary ringing noise, but that is very minor and probably not very significant. MOV's do have a little capacitance, not enough to overload the switch contacts, but enough to dampen the high frequency noise a little. As a result, the -14V spike that the MOV allows is not going to have a very fast rise time (a high frequency component), so, I don't think the MOV circuit is going to need any extra components to help filter out the -14V spike. In other words, drop the series diode idea, it is not going to help significantly. Every extra component increases the probability of failure. Diodes usually fail shorted, but they can on occasion fail open. Keep it simple, the little noise this series diode will get rid of is not worth the added complexity. We are stuck with either a low noise parallel diode, or a more noisy but safer MOV. There is no way that I can think of to really help the MOV, but it will not be that much more noisy than the diode. Tom suggests putting all three, the diode MOV and Solenoid in parallel. The diode would take the full flyback current, and limit the noise to the low 0.7 Volts. The MOV would limit secondary ringing voltages to 14V. Secondary ringing voltages would quickly short out the diode if they exceeded 50Volts. This is a safer way to use the lower noise diode circuit. I think I will use Tom's idea on my high current inductive loads. Thanks Tom, Regards, Ed.