Forums

Devil's Staircase

Started by John Larkin May 31, 2019
On Sat, 1 Jun 2019 00:42:45 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

>John Larkin wrote: >> >> >> >> 60 mV max. >> >> > >> >For a 4 ohm coil that is only 4mA >> >> My transformer primary will be maybe 20 milliohms DCR. At the max 60 >> mV, that's 3 amps DC. >> > > >** Hmmm - so a 4 ohm impedance in the working range with 20mohms of >resistance. That's a ratio of 200:1. > >The transformer will have a regulation factor of 1%, or better. > >150VA trannies normally have regulation factors of 8 to 10% and there is little difference between E-core and toroidal types other than size and weight. > > > >.... Phil > > > >..... Phil
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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sat, 01 Jun 2019 08:23:01 -0700, John Larkin
<jjlarkin@highlandtechnology.com> 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?&#2013266080; The problem is >>>>> that they don't really tell you what the input of the 3255 looks like, >>>>> IIRC (none do).&#2013266080; 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.
I don't think you can count on that, long term. I think you're going to need something to servo the voltage to zero.
>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.
That was my point. They really don't tell you what's going on. A few years ago, I wanted to DC couple an audio amp but couldn't get them (any of the manufacturers) to fork over the designs. We buy tons of these things but none were interested in showing us the details.
On Saturday, 1 June 2019 03:22:53 UTC+1, k...@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? The problem is > that they don't really tell you what the input of the 3255 looks like, > IIRC (none do). It's intended to be AC coupled.
I was wondering that, RC the transformer drive & feed back with greater gain. Trouble is if the load ever has a rectified component, your amp is toast. There's a reason these kinda things are usually done with valves. NT
On Sat, 1 Jun 2019 10:32:19 -0700 (PDT), tabbypurr@gmail.com wrote:

>On Saturday, 1 June 2019 03:22:53 UTC+1, k...@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? The problem is >> that they don't really tell you what the input of the 3255 looks like, >> IIRC (none do). It's intended to be AC coupled. > >I was wondering that, RC the transformer drive & feed back with greater gain. Trouble is if the load ever has a rectified component, your amp is toast. There's a reason these kinda things are usually done with valves. > > >NT
PM alternators often drive shunt regulators, triacs or mosfets. We have to tolerate that. -- John Larkin Highland Technology, Inc lunatic fringe electronics
John Doe wrote:
> > > > > Isn't most audio power class D now? > > My powered computer speakers JBL LSR305 Studio Monitors use class D > amplifiers. > >
** Hardly proves the point about numbers but here is a link to some interesting technical stuff on that model. http://rdimitrov.twistedsanity.net/blog/show.php?entry=JBL%20LSR305%20Teardown%20and%20Analysis .... Phil
On Sat, 01 Jun 2019 09:27:54 -0700, John Larkin wrote:

> The problem is that the DeLoreans are getting hard to find. > > ebay has an '82 for $40K.
There were not that many made before JdL got taped by the FBI drooling over the prospect of a massive cocaine deal. -- 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 Saturday, 1 June 2019 16:38:01 UTC+1, John Larkin  wrote:
> On Fri, 31 May 2019 22:49:02 -0700 (PDT), whit3rd <whit3rd@gmail.com> > wrote:
> It would take a giant capacitor, easily the biggest thing on the > board. This is a nominal 400 Hz system, but we may as well go down to > 50 Hz to have more market. I can just poke in a 33,000 uF cap per amp, > I guess.
Some folk may want to test with varying f to their 50Hz equipment, so lower may be better.
> >If this is for a three-phase power drive, a small three-phase motor with no > >load is a great filter/flywheel/phase balance component. So, how much ripple > >current is the blocking capacitor going to pass? The voltage should be kept low by the > >parallel resistor, of course, and that means a 6V capacitor is overrated for the job. > >Without blocking, you'd run DC into your three-phase loads (not recommended). > > > >Do your AC loads burn up when given DC? Or, just magnetize and hum loudly? > > We need to be frequency/voltage/phase agile, and we want to be able to > program complex output impedance. The point of this box is that the > customer wants to simulate alternators without spinning actual > alternators, which they certainly could. > > Problem is, this customer likes us so they throw all the hard problems > at us, and we have to hope they'll buy 300, and not 3. I guess I > shouldn't complain about being furnished interesting problems.
so many load problems can occur. There's still a reason people use valves. NT
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
On Saturday, 1 June 2019 19:30:42 UTC+1, John Larkin  wrote:
> On Sat, 1 Jun 2019 10:32:19 -0700 (PDT), tabbypurr wrote: > >On Saturday, 1 June 2019 03:22:53 UTC+1, k...@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? The problem is > >> that they don't really tell you what the input of the 3255 looks like, > >> IIRC (none do). It's intended to be AC coupled. > > > >I was wondering that, RC the transformer drive & feed back with greater gain. Trouble is if the load ever has a rectified component, your amp is toast. There's a reason these kinda things are usually done with valves. > > > > > >NT > > PM alternators often drive shunt regulators, triacs or mosfets. We > have to tolerate that.
I'm wondering how you plan to do that with a toroidal, dc on the 2ndary having the same effect on the core as dc on the primary. NT
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. -- John Larkin Highland Technology, Inc lunatic fringe electronics