Larkinisms
in [Design]
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From: John Larkin on 22 Jul 2010 09:36 On Thu, 22 Jul 2010 01:05:34 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:u7ff4619g56kvnjhclcm7v9anmru46fa3h(a)4ax.com... >> So get a power transformer with one primary and two secondaries. >> Connect the primary to the AC line. Measure the voltage on secondary 1 >> and switch a load on/off secondary 2. If flux density increases with >> load, the voltage on the sec1 winding will go UP as sec2 is loaded. > >Ironically, switching power supplies do this. But they have feedback, so that's simply inversion of the feedback network. > >Tim A lot of switchers, even simple bucks, have mildly negative output impedances. I've never figured out why. John
From: John Larkin on 22 Jul 2010 09:40 On Thu, 22 Jul 2010 07:17:09 -0600, m II <c(a)in.the.hat> wrote: >Ian Iveson wrote: > >> ***Good, although the anthropomorphism may not be helpful. > >Interesting word. You're assuming that the possession of, or being in, a >state of 'stupid' is a purely human trait. I know of a poster here who >uses over seventy aliases, with every single one aspiring to be as thick >as a brick. > >Ergo, if a step upwards, to the level of a brick, advances the quality >of the state of stupid, we can not claim the condition to be a purely >human one and the 'anthropomorphic' label is being misapplied. > >On a side note, does Mister Larkin really deserve this *obviously* >misguided MacCarthyistic attack on his beliefs? I think not. Let's not >surrender so easily to the darker side of human nature. > The real question is whether transformers deserved the slanderous description of their behavior. John
From: Nunya on 22 Jul 2010 09:58 On Jul 21, 8:47 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 21 Jul 2010 23:49:36 +0100, "Ian Iveson" > > > > <IanIveson.h...(a)blueyonder.co.uk> wrote: > >John Larkin wrote: > > > >>> What, George? Is it supposed to be untrue? Explain > >how. > > >>>> Tim > > >>>The magnetic flux density, B, is proportional to i for all > >>>inductor/transformer configurations. > > >>>B = uH, u is approximately constant. > > >>>H = N*i/(2*pi*r) for a toroid. > > >>>At 0 current the magnetic flux density is 0. Simple as > >>>that. > > >> What "I" are you talking about? A transformer has two > >> windings. And > >> each winding has resistance. > > >> Take a typical power transformer. Connect the primary to > >> the AC line > >> and leave the secondary open. Measure or compute the core > >> flux > >> density. Now short the secondary. At least before it > >> catches fire, > >> shorted-secondary flux density will be about half of the > >> unloaded > >> value. > > >> Loading the secondary of a transformer reduces flux > >> density... just > >> look at the directions of the primary and secondary > >> winding currents. > >> It's surprising how many people get this wrong, and think > >> that a > >> transformer "saturates" if you short its output. > > >Yes. The flux arising from secondary current is in > >opposition to that arising from primary current. The two > >currents are related, so the situation is more complicated > >than George's Theory appreciates. > > An easy way to measure the flux density in a transformer core is with > a probe coil, namely any old 3rd winding on the same core. > > So get a power transformer with one primary and two secondaries. > Connect the primary to the AC line. Measure the voltage on secondary 1 > and switch a load on/off secondary 2. If flux density increases with > load, the voltage on the sec1 winding will go UP as sec2 is loaded. > > Of course, it doesn't anything that stupid. It goes down. > > At least AlwaysWrong has a new friend. > > John induced voltage on the open winding is not a function of flux density, it is a function of the amplitude of the excitation winding.
From: Nunya on 22 Jul 2010 10:03 On Jul 22, 4:41 am, "Ian Iveson" <IanIveson.h...(a)blueyonder.co.uk> wrote: > Tim Williams wrote: > > Yes. The flux arising from secondary current is in > > opposition to that arising from primary current. > > Not fluxes. They share identical fluxes, less leakage > inductance. > > ***The two sum to one, obviously, because there is only one > core. > > It's the equal flux that forces equal voltages and > *opposite* currents to flow: EMF = -dPhi/dt. > > ***Opposite? How are the currents opposed? Only in the sense > that the flux arising from one is in opposition to the flux > arising from the other. > > Ian This is commonly known as a feedback winding. Commonly used in switchers to control the drive of the transformer. That, in concert with a voltage feedback reference, allows one to regulate the output properly and efficiently, and with great load change response rates.
From: John Larkin on 22 Jul 2010 10:23
On Thu, 22 Jul 2010 06:58:04 -0700 (PDT), Nunya <jack_shephard(a)cox.net> wrote: >On Jul 21, 8:47�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 21 Jul 2010 23:49:36 +0100, "Ian Iveson" >> >> >> >> <IanIveson.h...(a)blueyonder.co.uk> wrote: >> >John Larkin wrote: >> >> > >>> What, George? �Is it supposed to be untrue? �Explain >> >how. >> >> >>>> Tim >> >> >>>The magnetic flux density, B, is proportional to i for all >> >>>inductor/transformer configurations. >> >> >>>B = uH, u is approximately constant. >> >> >>>H = N*i/(2*pi*r) for a toroid. >> >> >>>At 0 current the magnetic flux density is 0. Simple as >> >>>that. >> >> >> What "I" are you talking about? A transformer has two >> >> windings. And >> >> each winding has resistance. >> >> >> Take a typical power transformer. Connect the primary to >> >> the AC line >> >> and leave the secondary open. Measure or compute the core >> >> flux >> >> density. Now short the secondary. At least before it >> >> catches fire, >> >> shorted-secondary flux density will be about half of the >> >> unloaded >> >> value. >> >> >> Loading the secondary of a transformer reduces flux >> >> density... just >> >> look at the directions of the primary and secondary >> >> winding currents. >> >> It's surprising how many people get this wrong, and think >> >> that a >> >> transformer "saturates" if you short its output. >> >> >Yes. The flux arising from secondary current is in >> >opposition to that arising from primary current. The two >> >currents are related, so the situation is more complicated >> >than George's Theory appreciates. >> >> An easy way to measure the flux density in a transformer core is with >> a probe coil, namely any old 3rd winding on the same core. >> >> So get a power transformer with one primary and two secondaries. >> Connect the primary to the AC line. Measure the voltage on secondary 1 >> and switch a load on/off secondary 2. If flux density increases with >> load, the voltage on the sec1 winding will go UP as sec2 is loaded. >> >> Of course, it doesn't anything that stupid. It goes down. >> >> At least AlwaysWrong has a new friend. >> >> John > >induced voltage on the open winding is not a function of flux density, >it is a function of the amplitude of the excitation winding. The voltage induced is proportional to N * dP/dT, where P is total flux that the winding encloses, namely the center leg of the core. Since the number of turns and the frequency and the core dimensions don't change, the induced voltage tracks flux density. You are right if loading sec2 has zero affect on the voltage observed at sec1, ie, all transformers have perfect cross-regulation. Try it. John |