Building a transformer
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From: meow2222 on 14 Aug 2006 15:46 John Fields wrote: > On 14 Aug 2006 11:17:33 -0700, meow2222(a)care2.com wrote: > >John Fields wrote: > >> On 13 Aug 2006 06:03:46 -0700, meow2222(a)care2.com wrote: > >> >To get +-50v dc you need the same transformer, no reason to have 2 > >> >windings at all. So you just need one 50v winding. > >> I don't think that's true. (View in Courier) > >Youre not the only person to say this, and I find it very puzzling, as > >the means to get +/- 50v is elementary: > > > > +-----|<|------- V- > >MAINS>----+ | > > | +-----|>|------- V+ > > P||S > > R||E > > I||C > > | | > >MAINS>----+ | > > | > > +--------------- 0V > > Well, I can't argue with that, LOL, but by doing it that way you've > just made the smoothing caps twice as big as they have to be since > you've halved the frequency of the rectified AC. Also, you've > increased the current the diodes _and_ the transformer need to > handle. Secondary is twice the current, half the V. Higher diode i, half the number of diodes. Smoothing caps would need to be a little over twice the capacitance irl, as i is flowing out of the caps for more than twice the length of time between topups. When youre building from salvage the choice comes down to what you happen to find. NT
From: John Woodgate on 14 Aug 2006 15:50 In message <21h1e2h0lq8e5hsr4j4fa5frohljgritmg(a)4ax.com>, dated Mon, 14 Aug 2006, John Fields <jfields(a)austininstruments.com> writes >On 14 Aug 2006 11:17:33 -0700, meow2222(a)care2.com wrote: > >>John Fields wrote: >>> On 13 Aug 2006 06:03:46 -0700, meow2222(a)care2.com wrote: >>> >>> >>> >To get +-50v dc you need the same transformer, no reason to have 2 >>> >windings at all. So you just need one 50v winding. >>> >>> --- >>> I don't think that's true. (View in Courier) >> >>Youre not the only person to say this, and I find it very puzzling, as >>the means to get +/- 50v is elementary: >> >> +-----|<|------- V- >>MAINS>----+ | >> | +-----|>|------- V+ >> P||S >> R||E >> I||C >> | | >>MAINS>----+ | >> | >> +--------------- 0V > >--- >Well, I can't argue with that, LOL, but by doing it that way you've >just made the smoothing caps twice as big as they have to be since >you've halved the frequency of the rectified AC. Also, you've >increased the current the diodes _and_ the transformer need to >handle. > > > It's a very convenient way of getting + and - rails from an AC wall-wart for low-current op-amp applications, where the larger filter caps aren't TOO large. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
From: John Popelish on 14 Aug 2006 17:46 meow2222(a)care2.com wrote: > Youre not the only person to say this, and I find it very puzzling, as > the means to get +/- 50v is elementary: > > +-----|<|------- V- > MAINS>----+ | > | +-----|>|------- V+ > P||S > R||E > I||C > | | > MAINS>----+ | > | > +--------------- 0V > In addition to the filter and secondary current comments: If you don't load the two outputs equally, there will be Dc in the transformer core, causing it to saturate at the end of alternating half cycles. It will hum and get hot.
From: jasen on 15 Aug 2006 03:42 On 2006-08-14, John Woodgate <jmw(a)jmwa.demon.co.uk> wrote: > In message <ebpb61$4s3$2(a)gonzo.homenet>, dated Mon, 14 Aug 2006, jasen ><jasen(a)free.net.nz> writes > >>You can call it two half-wave rectifiers but it's the same circuit as a >>voltage doubler, and it puts a common mode ripple on the power rails > > What do you mean by 'common-mode ripple'? both go negatve or positive at the same time. to an op-amp connected accross the rails it looks like the earth is moving, with a full-wave rectifier and centre tap the ripple ripple is symmetric and at twice the frequency, which makes it easier to filter out. Bye. Jasen
From: John Woodgate on 15 Aug 2006 06:09
In message <ebrts8$gv0$1(a)gonzo.homenet>, dated Tue, 15 Aug 2006, jasen <jasen(a)free.net.nz> writes >On 2006-08-14, John Woodgate <jmw(a)jmwa.demon.co.uk> wrote: >> In message <ebpb61$4s3$2(a)gonzo.homenet>, dated Mon, 14 Aug 2006, jasen >><jasen(a)free.net.nz> writes >> >>>You can call it two half-wave rectifiers but it's the same circuit as a >>>voltage doubler, and it puts a common mode ripple on the power rails >> >> What do you mean by 'common-mode ripple'? > >both go negatve or positive at the same time. Thanks; that's what I thought. If you draw out what happens (and look with a scope, of course), you find that the two ripple waveforms are unsymmetrical sawtooth waveforms, displaced in time. So there is some common-mode and some differential mode ripple. I would use this technique only for low-current applications, where low ripple can be achieved with reasonable filter capacitor values, or for feeding regulators. > >to an op-amp connected accross the rails it looks like the earth is >moving, It seems to me that the op-amp doesn't 'see' the rails until its input or output voltage gets close to them. If it did, PSRR wouldn't be so high. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK |