Devil's Staircase

On Fri, 31 May 2019 17:33:47 -0700, John Larkin
<jjlarkin@highland_snip_technology.com> wrote:

On Fri, 31 May 2019 17:15:14 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

lřrdag den 1. juni 2019 kl. 01.55.43 UTC+2 skrev John Larkin:
On Fri, 31 May 2019 23:38:58 -0000 (UTC), Cursitor Doom
curd@notformail.com> wrote:

On Fri, 31 May 2019 15:36:39 -0700, John Larkin 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!

Why class D? Doesn't seem like an obvious first choice for audio unless
it's for a PA system or something of that sort.

We plan to do three channels, 150 watts each, in a 2U rackmount
chassis.

It will simulate a 3-phase PM alternator hung on the gearbox of a jet
engine. We'll be able to program frequency, voltage, and complex
output impedance.


why do you need a transformer?

For isolation, to make 3-phase and other configurations, and to get
various output voltages.

Some of the downstream voltage regulators are weird. People like to
short out PM alternators for regulation. Our box will have to tolerate
that.


and why not some COTS amplifier, you can a 4 channel several 100watt amplifier in 2U for a few hundred $

We'll probably do our own class-D modules. We could use one of the TI
eval boards, but they are big and have a zillion jumpers and goofy
connectors.

We can get a 48V, 600 watt MeanWell power supply for $75!

We were thinking about putting the TPA3255 on the bottom of the board
and heat sinking it to the bottom of the box, which avoids a fan. It
has a huge, grounded power pad on the top of the chip.

Isn't most audio power class D now?

Most, except in automotive (where the TPA3255 is intended to play).
Automotive is still mostly class-AB, or some "high efficiency"
variation of AB (i.e. 'SB', 'KB', or 'TB'). It's changing though.

 

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.

 

[John Larkin]

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? 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.

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.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Fri, 31 May 2019 16:41:05 -0700 (PDT), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

You have 15mV offset on the output

See page 8 of the datasheet:

http://www.ti.com/lit/ds/slasea8a/slasea8a.pdf

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.

You have no problem, get rid of the caps

Cheers

Klaus

Why does TI use such goofy file names?


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[bitrex]

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? 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.

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>

 

[bitrex]

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?  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.

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.

 

[Jan Panteltje]

On a sunny day (Fri, 31 May 2019 15:36:39 -0700) it happened John Larkin
<jjlarkin@highland_snip_technology.com> wrote in
<q4a3fe9nlmflqfmadlb1dbfrtg7slvr7f0@4ax.com>:

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.

As somebody else already pointed out, maybe a DC control loop could zero the current.
Not so easy, low pass, some opamps, but not impossible.

The idea of supercaps is nice, maybe I should order some to see how those
perform in a similar application (driving cryo cooler here).
The ebay class D unit did not like the step up transformer... and died.
The normal class B audio amps with output coupling caps work fine for that.

 

[whit3rd]

On Friday, May 31, 2019 at 7:56:00 PM UTC-7, 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
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.

I'd just capacitor-couple the output, with a medium-size resistor parallel to the
capacitor. The digital amps I've seen want to self-bias, do NOT like
injected DC.

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?

 

[Phil Allison]

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

 

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.

Is it necessary to use a toroid, why not ordinary EI transformer with
possibly air gap ?

Or how about a high inductance but low resistance external solenoid
across toroid primary ? Apparently the frequency is in the 400 Hz
ballpark, so even less solenoid inductance should suffice, compared to
50/60 Hz.

 

[Cursitor Doom]

On Fri, 31 May 2019 17:33:47 -0700, John Larkin wrote:

> Isn't most audio power class D now?

I should jolly well hope not!

 

[Cursitor Doom]

On Fri, 31 May 2019 16:00:33 -0700, bloggs.fredbloggs.fred wrote:

There's nothing special about audio:

https://www.pes-publications.ee.ethz.ch/uploads/
tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf

Ah, so that's how flux capacitors arose. I've always wondered. ;-)




--
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 Friday, May 31, 2019 at 7:46:40 PM UTC-4, John Larkin wrote:

On Fri, 31 May 2019 16:00:33 -0700 (PDT),
bloggs.fredbloggs.fred@gmail.com wrote:

On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin 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?

There's nothing special about audio:

https://www.pes-publications.ee.ethz.ch/uploads/tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf


A capacitor looks easier.

No need. Lots of work has already been done to handle this hazard.

https://www.academia.edu/23741127/A_simple_control_algorithm_to_avoid_flux_density_bias_in_isolated_full-bridge_topologies

"The flux density bias of the transformer in such topologies is solved by using a fully digital strategy without any auxiliary circuit, thus, core losses are decreased and the total conversion efficiency is improved. Circuit complexity and cost are reduced, enhancing at the same time the reliability of the system. The possibility of magnetic saturation due to steady-state or transients is strongly reduced, being an important performance."

There are plenty of other papers on exactly this topic. And there's a small niche market of add-on protectors.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Joerg]

On 2019-05-31 22:08, Jan Panteltje wrote:

On a sunny day (Fri, 31 May 2019 15:36:39 -0700) it happened John Larkin
jjlarkin@highland_snip_technology.com> wrote in
q4a3fe9nlmflqfmadlb1dbfrtg7slvr7f0@4ax.com>:



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.

As somebody else already pointed out, maybe a DC control loop could zero the current.

That's what I'd do. No output cap, much smaller, much cheaper.

Not so easy, low pass, some opamps, but not impossible.

It is quite easy if you use that same loop for everything, DC as well as
drive signal. That's how I do it with class D stuff. Mine have also
always driven weird loads, not speakers. Use another opamp up front that
has very low offset.

It would be good to have that blessed by TI, to make sure the analog
loop filter in the IC doesn't become upset.

The idea of supercaps is nice, maybe I should order some to see how those
perform in a similar application (driving cryo cooler here).
The ebay class D unit did not like the step up transformer... and died.

Be careful not to get into a resonant mode at some frequency. That can
hammer them, they see an output AC short, become instable like an opamp
with a capacitive load and potentially croak.

The normal class B audio amps with output coupling caps work fine for that.

Not always. You were probably lucky. I had the overcurrent trip come on
a regular big old stereo amp when I drove a load capacitively for
aerospace testing.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[John Doe]

John Larkin <jjlarkin@highland_snip_technology.com> wrote:

> Isn't most audio power class D now?

My powered computer speakers JBL LSR305 Studio Monitors use class D
amplifiers.

 

[John Larkin]

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?  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.

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.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Sat, 01 Jun 2019 10:59:58 +0300, upsidedown@downunder.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.

Is it necessary to use a toroid, why not ordinary EI transformer with
possibly air gap ?

A toroid would be about half the footprint and half the weight of a
regular transformer, and would have lower output impedance. I know a
couple of guys who do nice toroids. One did this for us, lower power.

https://www.dropbox.com/s/0bl6jdkw4ljx4e2/400_Hz_Toroid.JPG?dl=0

Or how about a high inductance but low resistance external solenoid
across toroid primary ? Apparently the frequency is in the 400 Hz
ballpark, so even less solenoid inductance should suffice, compared to
50/60 Hz.

I'm guessing that the TI amp would fight to maintain its output offset
voltage. A series cap would be a lot easier. I could quit bitching and
use some big caps. Probably don't need the diodes.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Fri, 31 May 2019 22:49:02 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Friday, May 31, 2019 at 7:56:00 PM UTC-7, 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
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.

I'd just capacitor-couple the output, with a medium-size resistor parallel to the
capacitor. The digital amps I've seen want to self-bias, do NOT like
injected DC.

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.

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.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Sat, 1 Jun 2019 09:34:19 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Fri, 31 May 2019 16:00:33 -0700, bloggs.fredbloggs.fred wrote:

There's nothing special about audio:

https://www.pes-publications.ee.ethz.ch/uploads/
tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf


Ah, so that's how flux capacitors arose. I've always wondered. ;-)

The problem is that the DeLoreans are getting hard to find.

ebay has an '82 for $40K.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

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