From owner-freebsd-questions@FreeBSD.ORG Sat Nov 29 23:13:20 2014 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 4E14E491 for ; Sat, 29 Nov 2014 23:13:20 +0000 (UTC) Received: from agora.rdrop.com (agora.rdrop.com [IPv6:2607:f678:1010::34]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 07CBF208 for ; Sat, 29 Nov 2014 23:13:20 +0000 (UTC) Received: from agora.rdrop.com (66@localhost [127.0.0.1]) by agora.rdrop.com (8.13.1/8.12.7) with ESMTP id sATNDBoK076418 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NOT); Sat, 29 Nov 2014 15:13:11 -0800 (PST) (envelope-from perryh@pluto.rain.com) Received: (from uucp@localhost) by agora.rdrop.com (8.13.1/8.14.2/Submit) with UUCP id sATNDBvZ076417; Sat, 29 Nov 2014 15:13:11 -0800 (PST) (envelope-from perryh@pluto.rain.com) Received: from fbsd81 by pluto.rain.com (4.1/SMI-4.1-pluto-M2060407) id AA24255; Sat, 29 Nov 14 15:12:41 PST Date: Sat, 29 Nov 2014 15:12:41 -0800 From: perryh@pluto.rain.com (Perry Hutchison) To: ralf.mardorf@rocketmail.com Subject: Re: OT: UPS for FreeBSD Message-Id: <547a52e9.tCBMi6xWobou5Fcd%perryh@pluto.rain.com> References: <54777AB1.9010800@bluerosetech.com> <54779629.302@bluerosetech.com> <5478BD4F.7020306@yahoo.com> <5478BEE6.30308@bluerosetech.com> <5478CC08.9090307@yahoo.com> <20141128204722.561f948e@archlinux> <5478F16A.80605@yahoo.com> <54791d3a.w/pI0kak03d+3nKC%perryh@pluto.rain.com> <20141129113405.3d1bd1d6@X220.alogt.com> <54798883.saa13h6lE6rPwZCf%perryh@pluto.rain.com> <20141129113018.17759e2a@archlinux> In-Reply-To: <20141129113018.17759e2a@archlinux> User-Agent: nail 11.25 7/29/05 Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: erichsfreebsdlist@alogt.com, kudzu@tenebras.com, freebsd-questions@freebsd.org X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 29 Nov 2014 23:13:20 -0000 Ralf Mardorf wrote: > On Sat, 29 Nov 2014 00:49:07 -0800 > perryh@pluto.rain.com (Perry Hutchison) wrote: > > multitapped > > 120VAC ==> step-down ==> 6VAC ==> full-wave ==> ~15VDC ==> battery1 > > transformer rectifier > > | > > +---> 3VAC ==> full-wave ==> ~7.5VDC ==> battery2 > > rectifier > > Now I'm confused. Being con-fused was in a different branch of the thread :) > The ratio should be around 1.41, the square root of 2 We may be thinking of different "full wave" rectifier designs. Presuming a reasonable amount of capacitance on the output, a half wave (single diode) rectifier should produce a DC output roughly equal to the peak value of the AC input, which is indeed ~1.4 * the RMS AC voltage. A two-diode full-wave rectifier using a center tap will also produce DC of ~1.4 * the RMS voltage of the entire winding (and with the advantage of less ripple). However, a full wave bridge (4 diodes) should produce a DC output roughly equal to the peak-to-peak value of the AC input, or ~2.8 * the RMS voltage. > "-" 1.5V assumed it's a diode bridge rectifier. Yes, at these voltages the diode drop really should not be ignored: 6 VAC(RMS) * 2.8 - 1.5 = 15.3 VDC leaving 3.3 VDC for regulator drop to produce 12 VDC regulated. That may be a bit thin; perhaps we should use the same winding ratio as in an old-time filament transformer: 6.3 VAC => 16 VDC or so. Alternatively, 12.6 VAC-CT (center-tapped) would produce approximately the same result with only two diodes. OTOH 3 VAC(RMS) * 2.8 - 1.5 = 6.9 VDC leaving only 1.9 V for regulator drop to produce 5 VDC regulated. That "3VAC" winding should be more like 3.8 VAC (or 7.6 VAC-CT), producing 9 VDC.