Dual polarity supply -- no center tap
in [Design]
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From: Phil Hobbs on 30 Sep 2009 14:19 mkr5000 wrote: > You mean just like a simple half wave with another rectifier in > parallel but reversed? > > I never tried that -- didn't even think of it ! -- (or have even seen > it in many schematics > for that matter). (Of course, I wasn't looking for it, so maybe that's > the reason). > > I'll be darn -- except for the ripple compared to a bridge, it will do > the job just as well? > > Is it a good idea to match the rectifiers as far as forward and > reverse resistance? > > Using an over rated diode in a half wave is always a good idea too, > isn't it? > > Thanks. > The RMS diode current in a capacitor-input power supply (i.e. no inductor before the filter cap) is much higher than the average current because a whole cycle's worth of charge has to be delivered in a narrow pulse near the peak of the AC cycle. So yes, you want to use a diode rated for considerably more than the steady DC current. How much more depends on the size and ESR of the filter cap. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Vladimir Vassilevsky on 30 Sep 2009 14:31 Jan Panteltje wrote: > On a sunny day (Wed, 30 Sep 2009 10:26:16 -0700 (PDT)) it happened mkr5000 > <mikerbgr(a)gmail.com> wrote in > <6710fdbd-7150-4c96-a548-6f0477a43177(a)g19g2000yqo.googlegroups.com>: > > >>I'll be darn -- don't know why I haven't considered that before and >>for >>a power amp you don't want a regulated supply. >> >>Is the added ripple a problem? > > > No problem, that chip has good supply rejection, typical 70 dB. BTW, 70dB is not much at all for audio purposes. There is a problem. The consumption of the audio amp is not symmetrical. There is going to be significant DC current in the transformer, and the rail voltage unbalance as well. I would unrecommend using that kind of voltage doubler for audio power amp. Use the traditional diode bridge with center tap, or the single supply bridged amp topology. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Spehro Pefhany on 30 Sep 2009 14:39 On Wed, 30 Sep 2009 09:50:44 -0700 (PDT), mkr5000 <mikerbgr(a)gmail.com> wrote: >You mean just like a simple half wave with another rectifier in >parallel but reversed? > >I never tried that -- didn't even think of it ! -- (or have even seen >it in many schematics >for that matter). (Of course, I wasn't looking for it, so maybe that's >the reason). > >I'll be darn -- except for the ripple compared to a bridge, it will do >the job just as well? To a first approximation, just double up on the capacitance values to keep the ripple voltage the same. The nice thing about this application is that the currents from the + and - supplies will be about the same, so you get full use out of the xfmr core, unlike typical half-wave applications. >Is it a good idea to match the rectifiers as far as forward and >reverse resistance? No need. >Using an over rated diode in a half wave is always a good idea too, >isn't it? > >Thanks. If it fries, it wasn't rated high enough. Turn-on surge can be pretty brutal.
From: George Herold on 30 Sep 2009 14:47 On Sep 30, 2:13 pm, Rich Grise <richgr...(a)example.net> wrote: > On Wed, 30 Sep 2009 09:46:41 -0700, George Herold wrote: > > On Sep 30, 12:03 pm, Phil Hobbs > > <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >> mkr5000 wrote: > >> > I've never used a "voltage inverter" -- I see that Maxim makes a chip > >> > that will convert +5v to -5v and I would imagine > >> > they can be used in a dual polarity supply. > > >> > However, I want to build an audio power amp supply for an LM3886 -- > >> > wonder if there's a chip or circuit that will > >> > do the same thing at higher voltages (+ and - 30v -- even higher > >> > maybe). > > >> > Always used a center tap transformer for this but was wondering about > >> > the creative use of a single secondary > >> > transformer. > > >> > Or should I stick to the conventional dual winding approach? (I do > >> > hate potential problems). > > >> How about a full-wave voltage doubler? Ground one end of the secondary, > >> connect A of one diode and K of the other to the other end. Makes two > >> half-wave rectifiers of opposite polarity, total voltage ~2.5 to 2.8 x > >> VRMS. > > > Phil are you sure you get the doubling effect AND two polarities? > > > you mean a circuit like this, > >http://www.play-hookey.com/ac_theory/ps_v_multipliers.html > > > But most likely I just misunderstood. > > After looking at that circuit, do you still misunderstand? The _total_ > voltage is 2.8 * Vpk, but that's split between the two supplies. And > it's only two half-wave rectifiers, back-to-back, so your ripple per > current will be way more than a full-wave. (I'd have to look up the > numbers, but I'll leave that as an exercise for the student. ;-) ) > > Cheers! > Rich- Hide quoted text - > > - Show quoted text - Yup I understand now. I'd just never seen this configuration for a power supply before. George H.
From: Jan Panteltje on 30 Sep 2009 15:40
On a sunny day (Wed, 30 Sep 2009 13:31:23 -0500) it happened Vladimir Vassilevsky <nospam(a)nowhere.com> wrote in <JfednRXB2YF0Pl7XnZ2dnUVZ_g-dnZ2d(a)giganews.com>: > > >Jan Panteltje wrote: > >> On a sunny day (Wed, 30 Sep 2009 10:26:16 -0700 (PDT)) it happened mkr5000 >> <mikerbgr(a)gmail.com> wrote in >> <6710fdbd-7150-4c96-a548-6f0477a43177(a)g19g2000yqo.googlegroups.com>: >> >> >>>I'll be darn -- don't know why I haven't considered that before and >>>for >>>a power amp you don't want a regulated supply. >>> >>>Is the added ripple a problem? >> >> >> No problem, that chip has good supply rejection, typical 70 dB. > >BTW, 70dB is not much at all for audio purposes. > >There is a problem. The consumption of the audio amp is not symmetrical. >There is going to be significant DC current in the transformer, and the >rail voltage unbalance as well. I would unrecommend using that kind of >voltage doubler for audio power amp. Use the traditional diode bridge >with center tap, or the single supply bridged amp topology. Yea, well, its been working OK since year 2000, and it is actually 75 dB supply rejection (looked it up). I have even used it as driver for piezos at 40 kHz. And run it full power sine at many times. It is my universal amp, use it whenever I need some amp, else it is on the bass speaker. That 'significant DC' is only a little little bit hey:-) But I will not argue with any audiophiles about stuff like that. Somebody send me an email 'your amp has big hum'. I dunno if they ever build it, but it has no hum. All I could say is: Well theoretically maybe, better check your PCB though. Mine has no hum (that I can hear). But you can do the math right, 4700 uF, recharged every 40 mS, you know the load, the current, so the ripple, 75 dB attenuated of that, and that is the output ripple. And that ripple is only that big when driving full just below clipping. No way you can hear that in the presence of such a strong signal, as everything else in the room is then already vibrating... as I mounted the bass speaker against the floor, at 20 Hz even the chairs jump up and down at full output. Thank you for your contribution :-) |