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Devil's Staircase

Started by John Larkin May 31, 2019
søndag den 2. juni 2019 kl. 16.06.47 UTC+2 skrev John Larkin:
> On Sat, 1 Jun 2019 19:16:39 -0700 (PDT), Lasse Langwadt Christensen > <langwadt@fonz.dk> wrote: > > >s&oslash;ndag den 2. juni 2019 kl. 04.05.06 UTC+2 skrev John Larkin: > >> On Sat, 1 Jun 2019 18:50:48 -0700 (PDT), tabbypurr@gmail.com wrote: > >> > >> >On Saturday, 1 June 2019 17:36:48 UTC+1, John Larkin wrote: > >> > > >> >> We're selling a lab-grade voltage source, so we want the output to be > >> >> stiff. I estimated the 20 mohms primary DCR by scaling from a smaller > >> >> toroid that we have. The AC output impedance is a separate issue from > >> >> the DC primary current hazard. > >> >> > >> >> We will be sensing output current to control the complex output > >> >> impedance, so we can in theory tune the box Zout to zero by canceling > >> >> the transformer impedance. That approaches building an oscillator, but > >> >> we might take out some of the native impedance. > >> >> > >> >> But alternators are very inductive, and our main goal is to simulate > >> >> alternators. > >> > > >> >How do you intend to simulate the voltage spike when they suddenly reduce load? > >> > > >> > > >> >NT > >> > >> A bit of our simulated inductance will be a real inductor. It can do > >> the fast stuff, and our DSP loop can take over behind that. > >> > >> It's similar to making a super-wideband inductor by making a series > >> string of little and big inductors. People do that in bias tees and > >> such. > >> > >> The real-life load on the alternator will probably be shunt regulator, > >> bridge rectifier, capacitor, so really fast spikes probably wouldn't > >> matter much. > >> > >> We're trying to get them to lend us an alternator. We could spin that > >> somehow and see how it behaves. > >> > > > >https://youtu.be/D4DqyElNFAs ;) > > I'm thinking we could just chuck it in one of our milling machines and > spin it up. That guy did an awful lot of machining. And talking.
sure, when you don't have to a deal with part from a 60's fighter jet and keep it in a state that it can go back on the airplane you have a bit more leeway
On Sun, 02 Jun 2019 07:06:39 -0700, John Larkin wrote:

> I'm thinking we could just chuck it in one of our milling machines and > spin it up. That guy did an awful lot of machining. And talking.
And that cat was very distracting. -- 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.
On Sun, 02 Jun 2019 07:13:58 -0700, John Larkin wrote:

> The class D amps are inherently low impedance out. We's drive each amp > from an FPGA and a DAC, to synthesize our sine waves.
Synthesising sine waves is a crime against nature. I urge you to reconsider. -- 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.
s&oslash;ndag den 2. juni 2019 kl. 17.13.00 UTC+2 skrev Cursitor Doom:
> On Sun, 02 Jun 2019 07:06:39 -0700, John Larkin wrote: > > > I'm thinking we could just chuck it in one of our milling machines and > > spin it up. That guy did an awful lot of machining. And talking. > > And that cat was very distracting.
nice with a shop kitty, https://youtu.be/1mZhOlr5v-s?t=5m20s
On Sun, 02 Jun 2019 08:32:53 -0700, Lasse Langwadt Christensen wrote:

> nice with a shop kitty, https://youtu.be/1mZhOlr5v-s?t=5m20s
Haha! Good reply. :) Actually, that Old Tony is one of the best metalworkers out there on YT and I really enjoy watching his films and those of Tubelcaine, Myfordboy and so forth as well. Interesting when he was cutting that left-hand thread he didn't pre-cut any relief on the RHS of it. I wouldn't have got away with that, but his machining skill level is orders of magnitude better than mine. And so is his video editing ability. The bastard. -- 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.
On Sun, 2 Jun 2019 15:14:02 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Sun, 02 Jun 2019 07:13:58 -0700, John Larkin wrote: > >> The class D amps are inherently low impedance out. We's drive each amp >> from an FPGA and a DAC, to synthesize our sine waves. > >Synthesising sine waves is a crime against nature. I urge you to >reconsider.
It's legal in most states if it's over 16 bits. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sun, 02 Jun 2019 12:09:08 -0700, John Larkin wrote:

> It's legal in most states if it's over 16 bits.
It's not a matter of legality; more one of morality. But if you can live with yourself, then fine. -- 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.
On 6/1/19 11:23 AM, John Larkin wrote:
> On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote: > >> On 6/1/19 12:23 AM, bitrex wrote: >>> On 5/31/19 10:55 PM, John Larkin wrote: >>>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >>>> >>>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>>>> <jjlarkin@highland_snip_technology.com> wrote: >>>>> >>>>>> >>>>>> >>>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>>>> TPA3255 maybe. It's good for 600 watts mono! >>>>>> >>>>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>>>> and the class D part will surely have some DC offset. The TI spec is >>>>>> 60 mV max output offset, which could be a problem into a good >>>>>> transformer. Speakers don't mind a little DC, but transformers do. DC >>>>>> can cause stairstepped increase in circulating current, the Devil's >>>>>> Staircase, until they saturate. >>>>>> >>>>>> So I'm thinking I'll add a series blocking cap so I can ignore any DC >>>>>> problems. It will have to be big, 10s of millifarads at least. Biggest >>>>>> thing on the board. Maybe use a low voltage electrolytic with >>>>>> antiparallel power diodes, or a shorted bridge, to protect it from >>>>>> accidental forward or backwards over-voltage. >>>>>> >>>>>> Lytics will be big, and supercaps don't seem to like ripple current. I >>>>>> think. >>>>>> >>>>>> Any other ideas about driving a transformer from an audio amp? >>>>> >>>>> Feed back a (heavily) filtered signal to the input?&nbsp; The problem is >>>>> that they don't really tell you what the input of the 3255 looks like, >>>>> IIRC (none do).&nbsp; It's intended to be AC coupled. >>>> >>>> TI does spec 60 mV max DC offset at the output, which is pretty good, >>>> but even that could push a lot of DC into my transformer. >>>> >>>> There is probably some way to tweak the input with a little DC. >>>> Feedback loop or even a trimpot. >>>> >>>> >>> >>> You can drive the transformer bridged but using current-sense feedback >>> instead of voltage feedback; put a small sense resistor in line with >>> each amp output going to each end of the transformer and take off the DC >>> feedback to the opposite amp from the junction. That way the amps should >>> act as their own servo to keep DC out of the transformer. >>> >>> Lower power example like this for driving audio isolation transformer >>> for XLR cable: >>> >>> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> >>> >>> >> >> In the case of the TPA3255 I think you would put a low-offset op amp in >> front of the inputs and AC couple into that and do something similar but >> the 3255 just acts as a power buffer, I don't immediately see anything >> in the datasheet that says you can't intentionally apply a small DC >> feedback generated offset to its single input per channel as an error >> signal. > > It's not stated how the normally AC coupled inputs affect the DC > offset, but we have the eval board and I could have a scut bunny set > it up and try it. > > One trimpot and 30 seconds of tech time, to turn it, is sure > appealing. If it turns out we don't need it, we can leave it off the > board. > > The input resistance of the TPA3255 is 10K, and they want 10 uF input > coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to > me that the input caps are also used as lowpass filters for the DC > feedback loop, which then suggests we could push the input pins gently > to change the output offset. > > Datasheets tend to hide the good stuff. > > >
Does LTC make any class D amp modules included with LTSpice? I'm curious now about how using a DC servo loop into a first stage opamp driving the D-amp as a power buffer, instead of a cap would work out in practice. The problem with that XLR transformer driver circuit as drawn in a modification is that while there are two feedback loops, one local to the op-amps and one around the opposite side current sense resistor, it's assumed that the phase of the AC signals on both inputs of the op amps will be similar. but the phase shift produced by the class D output filter complicates things, where to put the sense resistor. If after the output filter need to compensate the phase shift somehow and if before need to filter the switching frequency down to DC. It may not be nearly as easily workable in the class D bridged power-buffer topology as with linear amplifiers driving the transformer, bridged.
On Sun, 2 Jun 2019 21:20:39 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Sun, 02 Jun 2019 12:09:08 -0700, John Larkin wrote: > >> It's legal in most states if it's over 16 bits. > >It's not a matter of legality; more one of morality. But if you can live >with yourself, then fine.
It's just a phase I'm going through. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 6/2/19 6:04 PM, bitrex wrote:
> On 6/1/19 11:23 AM, John Larkin wrote: >> On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote: >> >>> On 6/1/19 12:23 AM, bitrex wrote: >>>> On 5/31/19 10:55 PM, John Larkin wrote: >>>>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >>>>> >>>>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>>>>> <jjlarkin@highland_snip_technology.com> wrote: >>>>>> >>>>>>> >>>>>>> >>>>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>>>>> TPA3255 maybe. It's good for 600 watts mono! >>>>>>> >>>>>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>>>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>>>>> and the class D part will surely have some DC offset. The TI spec is >>>>>>> 60 mV max output offset, which could be a problem into a good >>>>>>> transformer. Speakers don't mind a little DC, but transformers >>>>>>> do. DC >>>>>>> can cause stairstepped increase in circulating current, the Devil's >>>>>>> Staircase, until they saturate. >>>>>>> >>>>>>> So I'm thinking I'll add a series blocking cap so I can ignore >>>>>>> any DC >>>>>>> problems. It will have to be big, 10s of millifarads at least. >>>>>>> Biggest >>>>>>> thing on the board. Maybe use a low voltage electrolytic with >>>>>>> antiparallel power diodes, or a shorted bridge, to protect it from >>>>>>> accidental forward or backwards over-voltage. >>>>>>> >>>>>>> Lytics will be big, and supercaps don't seem to like ripple >>>>>>> current. I >>>>>>> think. >>>>>>> >>>>>>> Any other ideas about driving a transformer from an audio amp? >>>>>> >>>>>> Feed back a (heavily) filtered signal to the input?&nbsp; The problem is >>>>>> that they don't really tell you what the input of the 3255 looks >>>>>> like, >>>>>> IIRC (none do).&nbsp; It's intended to be AC coupled. >>>>> >>>>> TI does spec 60 mV max DC offset at the output, which is pretty good, >>>>> but even that could push a lot of DC into my transformer. >>>>> >>>>> There is probably some way to tweak the input with a little DC. >>>>> Feedback loop or even a trimpot. >>>>> >>>>> >>>> >>>> You can drive the transformer bridged but using current-sense feedback >>>> instead of voltage feedback; put a small sense resistor in line with >>>> each amp output going to each end of the transformer and take off >>>> the DC >>>> feedback to the opposite amp from the junction. That way the amps >>>> should >>>> act as their own servo to keep DC out of the transformer. >>>> >>>> Lower power example like this for driving audio isolation transformer >>>> for XLR cable: >>>> >>>> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> >>>> >>>> >>>> >>> >>> In the case of the TPA3255 I think you would put a low-offset op amp in >>> front of the inputs and AC couple into that and do something similar but >>> the 3255 just acts as a power buffer, I don't immediately see anything >>> in the datasheet that says you can't intentionally apply a small DC >>> feedback generated offset to its single input per channel as an error >>> signal. >> >> It's not stated how the normally AC coupled inputs affect the DC >> offset, but we have the eval board and I could have a scut bunny set >> it up and try it. >> >> One trimpot and 30 seconds of tech time, to turn it, is sure >> appealing. If it turns out we don't need it, we can leave it off the >> board. >> >> The input resistance of the TPA3255 is 10K, and they want 10 uF input >> coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to >> me that the input caps are also used as lowpass filters for the DC >> feedback loop, which then suggests we could push the input pins gently >> to change the output offset. >> >> Datasheets tend to hide the good stuff. >> >> >> > > Does LTC make any class D amp modules included with LTSpice? I'm curious > now about how using a DC servo loop into a first stage opamp driving the > D-amp as a power buffer, instead of a cap would work out in practice. > > The problem with that XLR transformer driver circuit as drawn in a > modification is that while there are two feedback loops, one local to > the op-amps and one around the opposite side current sense resistor, > it's assumed that the phase of the AC signals on both inputs of the op > amps will be similar. > > but the phase shift produced by the class D output filter complicates > things, where to put the sense resistor. If after the output filter need > to compensate the phase shift somehow and if before need to filter the > switching frequency down to DC. It may not be nearly as easily workable > in the class D bridged power-buffer topology as with linear amplifiers > driving the transformer, bridged.
Putting it after the LC also means have to take the (possibly not well- defined) R of the L into account as a component of the current sense impedance.