On 08/12/17 00:16, John Larkin wrote:> On Sat, 12 Aug 2017 00:01:11 +0000, Chris<xxx.syseng.yyy@gfsys.co.uk> > wrote: > >> On 08/11/17 20:27, John Larkin wrote: >>> >>> I want to build an AC source to test synchros/resolvers/RVDTs. The >>> target is 400 Hz 30 VRMS at maybe 30 VA, but we might want to simulate >>> aircraft "wild power" and drive LVDTs, so maybe 250 Hz to a few KHz. >>> >>> I'm thinking of using half of a TPA3251 audio amp and a step-up >>> transformer. Like this maybe: >>> >>> https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 >>> >>> The transformer DCR is low, so a blocking cap might be prudent to >>> prevent DC current in the primary. I have a cute little 1000 uF 2.5V >>> polymer cap in stock, so I could use a few of those in parallel. The >>> diodes keep the cap from exploding if the h-bridge ever rails. >>> >>> How does that look? Any advice? >>> >>> I've heard horror stories about "the devil's staircase" progressive >>> saturation, which I don't really understand. The transformer is a >>> toroid, low resistance and pretty dramatic saturation behavior. >>> >>> >> >> Keep it simple. For that sort of voltage output, any audio amplifier >> with suitable output swing should get the job done, and distortion >> will be insignificant at that sort of frequency. Either that, or find >> one of the variable output lab ac supplies from Ebay or similar. >> >> Do you want to design it, or use it :-)... >> >> Chris > > Use it and maybe sell it if it works nice. It would complement our > synchro/LVDT boards, which don't output much power. > > There aren't many low-power 400 Hz sources around. 1 KVA is small in > this world. >Have a couple of the older rack mount 3ph 115v 400Hz psus, one 500 and another 750Va. Used to build 400Hz inverters for mil surplus gyros etc for leisure flyers, transformers at the time, but Scott connected to get 3ph from 2 channels. 400Hz is useful to have around the lab anyway, if you do any avionics type work... Chris
class D amp
Started by ●August 11, 2017
Reply by ●August 12, 20172017-08-12
Reply by ●August 12, 20172017-08-12
John Larkin wrote: -----------------------> > > >** The "staircasing" story is 100% bogus in the example under discussion. > > > > > > Why? >** No point discussing "why" when you do not know the facts. There are several ways your scheme can get into trouble with core saturation, only one of which you have addressed. I mentioned low frequencies, so good HPF filter at the input to the class D amp is advisable. Also large inrush surges are generated when signal is first applied and this needs addressing. The third one is if the class D amp has response down to DC *and* when the signal is asymmetrical. I don't mean that it has a DC offset, but the peak values are non identical. If allowed to clip the amp, a large DC offset is produced at the output that will saturate the toroidal core. I guess this scenario is similar to "staircasing". The normal fix is to make sure the class D amp's NFB loop reduces the gain to unity or zero at low frequencies as is the case with the vast majority of audio amps. Another way is to use an offset servo loop. See this article on by my colleague Rod Elliot, whom I alerted to the phenomenon. http://sound.whsites.net/clipping.htm ..... Phil
Reply by ●August 12, 20172017-08-12
On Sat, 12 Aug 2017 14:59:29 -0700 (PDT), Klaus Kragelund <klauskvik@hotmail.com> wrote:>On Saturday, August 12, 2017 at 11:49:31 PM UTC+2, Klaus Kragelund wrote: >> On Saturday, August 12, 2017 at 10:31:12 PM UTC+2, Tim Williams wrote: >> > "Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message >> > news:gJKdnUUGRLAIhBPEnZ2dnUU7-R3NnZ2d@supernews.com... >> > > The caps will force the volt-seconds to average to zero, but not >> > > necessarily within a cycle. There may be some DC on the caps because >> > > once the average current in the core gets off zero, it'll walk in the >> > > direction of increasing magnetization. It does have to stop on account >> > > of the capacitor, but maybe there could be a bit of distortion as a >> > > result. >> > > >> > >> > Ferromagnetism is weird. >> > >> > Once had a 10kW, "high frequency" (low kHz) stripwound toroid in an >> > induction heater. The inverter (just simple square wave output) wasn't >> > quite balanced, so it went into saturation (audible with the loud, harsh >> > noise) pretty quickly.. >> > >> > So we put a coupling capacitor in, and ran it again. DC is gone, but it's >> > still saturating. Huh. Even stranger, it's fine for a few seconds, then >> > "walks" over to saturation. >> > >> > Put in transformer #2. No problem, nice and quiet (well, relatively >> > speaking). >> > >> > Some ferromagnetic materials have saturation and history problems: NiZn >> > ferrite often warns not to saturate the material (though I don't know why; I >> > haven't tested that yet). I've seen the same behavior, the long time >> > constant, "flux walking" saturation, with square permalloy cores. >> > >> > Those transformers were interesting. We quoted a dozen transformer shops >> > across the country. Most "no quote". A few came back with unrealistic >> > designs, like, a 24" wide toroid. Guessing their "designer" was using >> > winding and loss tables for 60Hz. ;-) The place we finally bought from, >> > gave us something about $800 and 10" o.d., with a copper tube in the middle >> > for cooling water, exactly what I was expecting. Couldn't find any thermal >> > problems with them. >> > >> > As far as I know, those units are still in service at LANL. Some sort of >> > material processing system. Something about a rare, toxic metal -- probably >> > nothing interesting. :^) >> > >> Staircase saturation can be difficult to manage with full confidence >> >> Until recently I only looked at the primary side control, but you also need to consider unbalance on the secondary. Typically on the primary the staircase is prevented by a series capacitor, but that only works for voltage mode control systems >> >> For the secondary, one could have say a halfbridge converter with primary series capacitor, but secondary could be what is called a current doubler type: >> >> http://www.encyclopedia-magnetica.com/doku.php/current_doubler >> >> The problem with that type is that if the diodes are not equal (and they never are), then different volt seconds are presented to positive and negative cycles leading to staircasing. In the halfbridge, the staircasing can be mitigated by series resistance which offers negative feedback. But staircasing exists for sure >> >> Cheers >> >> Klaus > >Oh, forgot to link that to Larkins shcematics, is secondary unbalance possible?The loads will be low-Q inductors, like synchro or LVDT windings. I don't expect any diodes or anything.> >Bytheway John, nice idea, good use of the IC if it works > >Cheers > >Klaus-- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●August 12, 20172017-08-12
On Sat, 12 Aug 2017 16:10:56 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Friday, August 11, 2017 at 1:27:25 PM UTC-7, John Larkin wrote: >> I want to build an AC source to test synchros/resolvers/RVDTs. The >> target is 400 Hz 30 VRMS at maybe 30 VA... >> I'm thinking of using half of a TPA3251 audio amp and a step-up >> transformer. Like this maybe: >> >> https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 > >Annoyingly, you can't feed low-impedance input to those digital amps, >you gotta DC-block with capacitors. So, that pic has some >issues (won't bias the inputs right). That also means that if you have >a toroid doing voltage-boosting, and care about DC, you can't correct >the DC level at those input terminals.I didn't show the input caps in the sketch, but the chip needs them. Given the input caps, the half-bridge output DC offset is 60 mV max. It's impressive how much important stuff they can leave out of a 42 page data sheet.> >You might, however, run a second winding on the toroid with a modulated load, that >would let you balance the induction with a resistor-to-small-voltage load that opposes >the (DC-offset) current.The caps in the transformer primary are supposed to kill any DC. The diodes make it more interesting. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●August 12, 20172017-08-12
On Sat, 12 Aug 2017 18:47:42 -0700, John Larkin <jjlarkin@highlandtechnology.com> wrote:>On Sat, 12 Aug 2017 16:10:56 -0700 (PDT), whit3rd <whit3rd@gmail.com> >wrote: > >>On Friday, August 11, 2017 at 1:27:25 PM UTC-7, John Larkin wrote: >>> I want to build an AC source to test synchros/resolvers/RVDTs. The >>> target is 400 Hz 30 VRMS at maybe 30 VA... >>> I'm thinking of using half of a TPA3251 audio amp and a step-up >>> transformer. Like this maybe: >>> >>> https://www.dropbox.com/s/eikiu79jv4cjxqp/P548_WB_1.JPG?raw=1 >> >>Annoyingly, you can't feed low-impedance input to those digital amps, >>you gotta DC-block with capacitors. So, that pic has some >>issues (won't bias the inputs right). That also means that if you have >>a toroid doing voltage-boosting, and care about DC, you can't correct >>the DC level at those input terminals. > >I didn't show the input caps in the sketch, but the chip needs them. >Given the input caps, the half-bridge output DC offset is 60 mV max. > >It's impressive how much important stuff they can leave out of a 42 >page data sheet. > >> >>You might, however, run a second winding on the toroid with a modulated load, that >>would let you balance the induction with a resistor-to-small-voltage load that opposes >>the (DC-offset) current. > >The caps in the transformer primary are supposed to kill any DC. The >diodes make it more interesting.I've successfully kept Ethernet transformers happy by watching the current*time balance in the center-tap. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website. Thinking outside the box...producing elegant & economic solutions.
Reply by ●August 13, 20172017-08-13
On 13/08/2017 03:30, Jim Thompson wrote:> > I've successfully kept Ethernet transformers happy by watching the > current*time balance in the center-tap. > > ...Jim Thompson >I like the sound of that. You mean JL could compare the primary center-tap against supply mid-point and feedback to eliminate asymmetry problems? piglet --- This email has been checked for viruses by AVG. http://www.avg.com
Reply by ●August 13, 20172017-08-13
piglet wrote: ------------------> Jim Thompson wrote: > > > > I've successfully kept Ethernet transformers happy by watching the > > current*time balance in the center-tap. > > > > ... > > > > I like the sound of that. >** You like the sound of your own bullshit.> You mean JL could compare the primary center-tap against supply > mid-point and feedback to eliminate asymmetry problems? >** JL has no centre-tap or need for one. Dickheads like you have no idea what the topic is about, at all. No does it appear anyone else here, but me. Cos it is a *power* audio electronics, *interface* issue with a wound core toroidal tranny. If you have not dealt with and mastered that scenario, at the design level, you have no fucking clue. At all. .... Phil
Reply by ●August 13, 20172017-08-13
On 13/08/2017 13:43, Phil Allison wrote:> piglet wrote: > > ------------------ > >> Jim Thompson wrote: >>> >>> I've successfully kept Ethernet transformers happy by watching the >>> current*time balance in the center-tap. >>> >>> ... >>> >> >> I like the sound of that. >> > > ** You like the sound of your own bullshit. > > >> You mean JL could compare the primary center-tap against supply >> mid-point and feedback to eliminate asymmetry problems? >> > > > ** JL has no centre-tap or need for one. > > Dickheads like you have no idea what the topic is about, at all. > > No does it appear anyone else here, but me. > > Cos it is a *power* audio electronics, *interface* issue with a wound core toroidal tranny. > > If you have not dealt with and mastered that scenario, at the design level, you have no fucking clue. At all. > > > > > .... Phil > > > > > > --- > This email has been checked for viruses by AVG. > http://www.avg.com >Ouch! Phil you are one of the very few really knowledgeable contributors here but I was only trying to explore JT's CT idea because some people had doubted if a capacitor alone would solve the saturation worries JL asked about and in an earlier post you mentioned an offset servo loop as one option. piglet
Reply by ●August 13, 20172017-08-13
On Sun, 13 Aug 2017 15:08:07 +0100, Piglet wrote:> Ouch! Phil you are one of the very few really knowledgeable contributors > hereLOL!:-D You haven't been here very long then, Piglet. Allison is about the most ignorant and bigotted arsehole who ever posted here. When he fucks up (which is almost constantly) he attacks anyone who attempts to correct him. KF him. You will find life much more pleasant without the ravings of this conspicuously autistic retard to content with. -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.
Reply by ●August 13, 20172017-08-13
On Sun, 13 Aug 2017 12:35:27 +0100, Piglet <erichpwagner@hotmail.com> wrote:>On 13/08/2017 03:30, Jim Thompson wrote: >> >> I've successfully kept Ethernet transformers happy by watching the >> current*time balance in the center-tap. >> >> ...Jim Thompson >> > >I like the sound of that. > >You mean JL could compare the primary center-tap against supply >mid-point and feedback to eliminate asymmetry problems? > >piglet >Not quite, but close. Rather than observing volt-seconds, which is slow, watch current to/from center-tap for asymmetry, cycle-by-cycle. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website. Thinking outside the box...producing elegant & economic solutions.
