LT Spice diode C-V graph

On Mon, 30 Jun 2014 18:12:11 -0400, Phil Hobbs
<hobbs@electrooptical.net> wrote:

>On 6/30/2014 5:07 PM, RobertMacy wrote:
>> On Mon, 30 Jun 2014 12:04:39 -0700, Phil Hobbs
>> <hobbs@electrooptical.net> wrote:
>>
>>>>> ...snip...
>>> Well, the Earth is nearly spherical already, which helps. ;)
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>>
>>
>>
>> Hmmm....let's see Mount Everest(sp?) is around 28,000 ft above sea
>> level, and the earth's diameter approx 8,000 miles
>> That's like having a ball bearing 1 cm in diameter polished to 6 microns
>> or so. Yep, pretty good sphere.
>>
>> Turn that around a 1 cm ball polished to 1 micron is like having an
>> earth with the highest peak around 4200 ft.  hmmm
>
>Spherical cow joke alert. ;)
>
>Cheers
>
>Phil Hobbs

Had to look that one up...

https://en.wikipedia.org/wiki/Spherical_cow

You physicists are such cards.


-- 

John Larkin         Highland Technology, Inc

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

On Mon, 30 Jun 2014 15:26:41 -0700, John Larkin  
<jlarkin@highlandtechnology.com> wrote:

>> ...snip....
>
> Had to look that one up...
>
> https://en.wikipedia.org/wiki/Spherical_cow
>
> You physicists are such cards.
>
>

INCLUDED AT THAT URL:
In the 1997 British Channel Four series "Brass Eye", host Chris Morriss  
campaigns against spherical cows, in a reality-meets-comedy segment, he  
distributes flyers and gets public opinion on the use of spherical cows as  
food, implying that such cows exist and are secretly being sold in British  
shops. Response was universally negative, but not one respondent  
questioned the veracity of the spherical cow's existence.


Thanks for posting that URL. I had never heard of this reference before.

The humour still eluded me as I read the description and thinking, "Of  
course cows in a vacuum would be spherical!" but then I realized that  
wasn't the point. sigh, it's been a long day.

On 6/30/2014 6:24 PM, RobertMacy wrote:
> On Mon, 30 Jun 2014 15:12:11 -0700, Phil Hobbs
> <hobbs@electrooptical.net> wrote:
>
>>> ...snip....
>>
>> Spherical cow joke alert. ;)
>>
>> Cheers
>>
>> Phil Hobbs
>>
>
> uh, very dense here. hand waving over top of forehead front to back.

https://en.wikipedia.org/wiki/Spherical_cow

I've been talking about LTspice's spherical cow universe on and off for 
the last couple of weeks.

Cheers

Phil Hobbs




-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

Phil Hobbs <hobbs@electrooptical.net> wrote:

> https://en.wikipedia.org/wiki/Spherical_cow
 
> I've been talking about LTspice's spherical cow universe on and off for 
> the last couple of weeks.
 
> Cheers
 
> Phil Hobbs

No simulation is perfect. Are you dissatisfied with the results of the 
LTspice simulations, or does the spherical cow reference apply to all 
simulations?

I believe the value of LTspice is you get answers that are close enough 
for further study - breadboarding, or going directly to pcb. But you have 
to be aware of the limitations of the models used, and how to get around 
them. This is the same as being aware of the limitations of your test 
equipment - probes, scopes, dvms, etc. Nothing is perfect - you simply 
have to work between or around the limits.

Used properly, simulation can save a great deal of time and effort, 
especially if there is a flaw in the original design and it is picked up 
in LTspice. For example, I just spent considerable time trying to make a 
forward converter PWM power supply that would generate a variable 
positive voltage when needed, then a negative voltage at low output. The 
negative voltage would be produced when the PWM duty cycle was low, since 
the load was a sink to a negative voltage.

After fighting the various problems getting a TL494 regulator to function 
due to major problems with the model, I finally got the system working. 

It instantly showed there was a major flaw in my idea. As the output 
voltage decreased, it would eventually hit the point where the output 
rectifier diodes started conducting due to the sink current, and the 
output voltage could not go below that point.

This saved a great deal of time and effort wasted breadboarding a flawed 
design.

However, this led to a very simple solution. I now had a functioning 
TL494 model, and it only took a few minutes to convert the circuit to a 
series PWM switch running from a positive voltage. Since it had no 
rectifier diodes, it did not suffer from the diode turnon, and could 
easily swing between positive and negative output voltages.

This effort also showed me how to balance the TL494 feedback compensation 
against the output filter inductor to give a well-damped response. This 
information is completely lacking in all the TL494 references I could 
find, and the procedure is much simpler than going through pages of math 
that could not possibly give the detailed insight that working with the 
model gave. You cannot see the waveforms by looking at the equations.

So you may call it a spherical cow universe. I find it to be a very 
useful and functional tool.

On 6/30/2014 8:58 PM, Steve Wilson wrote:
> Phil Hobbs <hobbs@electrooptical.net> wrote:
>
>> https://en.wikipedia.org/wiki/Spherical_cow
>
>> I've been talking about LTspice's spherical cow universe on and off for
>> the last couple of weeks.
>
>> Cheers
>
>> Phil Hobbs
>
> No simulation is perfect. Are you dissatisfied with the results of the
> LTspice simulations, or does the spherical cow reference apply to all
> simulations?

There are two general kinds of simulations, I think.  The first kind is 
where the physics is well known but the boundary conditions are 
complicated.  Most kinds of electromagnetic simulations are like that, 
and so are lens designs, FEM simulations of mechanical systems made of 
rigid parts, static stellar structure calculations, and stuff like that.

Most of my simulators have been like that.  I've written a large-scale 
optimizing clusterized FDTD simulator, as well as simpler ones for 
thermal transfer and ray tracing.

The second kind is much squishier, and includes circuit simulations, 
oceanography, weather, climate, dynamic stellar structure, predator/prey 
studies, yada yada.  Those require an enormous amount of ground truth to 
calibrate them, and even then you can't take anything for granted.

IC fabs work pretty hard to make their models match reality. 
Semiconductor companies don't take anything like that level of care in 
making models for board-level designs.  And that's a very, very simple 
case compared with dynamic stellar structure and *a fortiori* climate, 
which is still very poorly understood and has a zillion adjustable 
parameters.

>
> I believe the value of LTspice is you get answers that are close enough
> for further study - breadboarding, or going directly to pcb. But you have
> to be aware of the limitations of the models used, and how to get around
> them. This is the same as being aware of the limitations of your test
> equipment - probes, scopes, dvms, etc. Nothing is perfect - you simply
> have to work between or around the limits.

Yup.

>
> Used properly, simulation can save a great deal of time and effort,
> especially if there is a flaw in the original design and it is picked up
> in LTspice. For example, I just spent considerable time trying to make a
> forward converter PWM power supply that would generate a variable
> positive voltage when needed, then a negative voltage at low output. The
> negative voltage would be produced when the PWM duty cycle was low, since
> the load was a sink to a negative voltage.
>
> After fighting the various problems getting a TL494 regulator to function
> due to major problems with the model, I finally got the system working.
>
> It instantly showed there was a major flaw in my idea. As the output
> voltage decreased, it would eventually hit the point where the output
> rectifier diodes started conducting due to the sink current, and the
> output voltage could not go below that point.
>
> This saved a great deal of time and effort wasted breadboarding a flawed
> design.
>
> However, this led to a very simple solution. I now had a functioning
> TL494 model, and it only took a few minutes to convert the circuit to a
> series PWM switch running from a positive voltage. Since it had no
> rectifier diodes, it did not suffer from the diode turnon, and could
> easily swing between positive and negative output voltages.
>
> This effort also showed me how to balance the TL494 feedback compensation
> against the output filter inductor to give a well-damped response. This
> information is completely lacking in all the TL494 references I could
> find, and the procedure is much simpler than going through pages of math
> that could not possibly give the detailed insight that working with the
> model gave. You cannot see the waveforms by looking at the equations.
>
> So you may call it a spherical cow universe. I find it to be a very
> useful and functional tool.

Spherical cows aren't useless, they're just approximate.  Board-level 
SPICE models are like spherical cows because the models are poor and 
because one rarely has a really accurate model of circuit strays.  Many 
folks seem to have an inordinate amount of faith in whatever comes out 
of a computer simulation.

If you go back and look at what I actually said about spherical cows, it 
was in the context of somebody liking one of my simulated designs more 
than I thought it deserved.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

On Mon, 30 Jun 2014 15:37:17 -0700, RobertMacy <robert.a.macy@gmail.com> wrote:

>On Mon, 30 Jun 2014 15:26:41 -0700, John Larkin  
><jlarkin@highlandtechnology.com> wrote:
>
>>> ...snip....
>>
>> Had to look that one up...
>>
>> https://en.wikipedia.org/wiki/Spherical_cow
>>
>> You physicists are such cards.
>>
>>
>
>INCLUDED AT THAT URL:
>In the 1997 British Channel Four series "Brass Eye", host Chris Morriss  
>campaigns against spherical cows, in a reality-meets-comedy segment, he  
>distributes flyers and gets public opinion on the use of spherical cows as  
>food, implying that such cows exist and are secretly being sold in British  
>shops. Response was universally negative, but not one respondent  
>questioned the veracity of the spherical cow's existence.
>
>
>Thanks for posting that URL. I had never heard of this reference before.
>
>The humour still eluded me as I read the description and thinking, "Of  
>course cows in a vacuum would be spherical!" but then I realized that  
>wasn't the point. sigh, it's been a long day.

It's hard to raise spherical cows; they keep blowing away (except in a vacuum.)


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation

Phil Hobbs <hobbs@electrooptical.net> wrote:

> Spherical cows aren't useless, they're just approximate.  Board-level 
> SPICE models are like spherical cows because the models are poor and 
> because one rarely has a really accurate model of circuit strays. 
> Many folks seem to have an inordinate amount of faith in whatever
> comes out of a computer simulation.

Yes, they also believe or misinterpret what their scope is telling them. 

Circuit strays are a problem in simulation. They are an even worse 
problem in pcb layout. But with LTspice, you can quickly find which nodes 
are sensitive, and how much stray capacitance or inductance can be 
tolerated. You did this in your 1nA calibrator, and it helped you to 
complete the project successfully. It would be difficult to do that with 
conventional methods, since you are facing ground bounce, probe problems, 
and low signal levels.
 
> If you go back and look at what I actually said about spherical cows,
> it was in the context of somebody liking one of my simulated designs
> more than I thought it deserved.

Which one was that?

Maybe you do not give yourself enough credit. I find your designs to be 
uniformly superb. Maybe there might be some nits about minor details, but 
please be assured I am in awe of your design concepts.

Nobody here designs the way you do. I can look at your designs from all 
kinds of angles, and learn more with each approach. Nobody else has 
posted such rich designs.

> Cheers
 
> Phil Hobbs

On 6/30/2014 10:16 PM, John Larkin wrote:
> On Mon, 30 Jun 2014 15:37:17 -0700, RobertMacy <robert.a.macy@gmail.com> wrote:
>
>> On Mon, 30 Jun 2014 15:26:41 -0700, John Larkin
>> <jlarkin@highlandtechnology.com> wrote:
>>
>>>> ...snip....
>>>
>>> Had to look that one up...
>>>
>>> https://en.wikipedia.org/wiki/Spherical_cow
>>>
>>> You physicists are such cards.
>>>
>>>
>>
>> INCLUDED AT THAT URL:
>> In the 1997 British Channel Four series "Brass Eye", host Chris Morriss
>> campaigns against spherical cows, in a reality-meets-comedy segment, he
>> distributes flyers and gets public opinion on the use of spherical cows as
>> food, implying that such cows exist and are secretly being sold in British
>> shops. Response was universally negative, but not one respondent
>> questioned the veracity of the spherical cow's existence.
>>
>>
>> Thanks for posting that URL. I had never heard of this reference before.
>>
>> The humour still eluded me as I read the description and thinking, "Of
>> course cows in a vacuum would be spherical!" but then I realized that
>> wasn't the point. sigh, it's been a long day.
>
> It's hard to raise spherical cows; they keep blowing away (except in a vacuum.)
>
>
That's actually a feature--remember the theme from Rawhide?  (I actually 
know it only from the Blues Brothers.)

"Keep 'em rollin, rollin, rollin,
Though the streams are swollen,
Keep those dogies rollin, rawhide!
All kinds of weather, hell bent for leather,
Keep them dogies rollin, rawhide!"

Much easier with spherical cows, at least if you're going downhill.
;)

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

On Mon, 30 Jun 2014 15:49:00 -0700, Phil Hobbs <hobbs@electrooptical.net>  
wrote:

>> ...snip...
> https://en.wikipedia.org/wiki/Spherical_cow
>
> I've been talking about LTspice's spherical cow universe on and off for  
> the last couple of weeks.
>
> Cheers
>
> Phil Hobbs
>
Larkin posted a Wikipedia URL for the round cow, but now I seem to have  
missed how that applies to LTspice ?? as a round cow universe ?? Arrrggg I  
think I'll give up subtle thought.

On Mon, 30 Jun 2014 22:57:02 -0400, Phil Hobbs <hobbs@electrooptical.net> wrote:

>On 6/30/2014 10:16 PM, John Larkin wrote:
>> On Mon, 30 Jun 2014 15:37:17 -0700, RobertMacy <robert.a.macy@gmail.com> wrote:
>>
>>> On Mon, 30 Jun 2014 15:26:41 -0700, John Larkin
>>> <jlarkin@highlandtechnology.com> wrote:
>>>
>>>>> ...snip....
>>>>
>>>> Had to look that one up...
>>>>
>>>> https://en.wikipedia.org/wiki/Spherical_cow
>>>>
>>>> You physicists are such cards.
>>>>
>>>>
>>>
>>> INCLUDED AT THAT URL:
>>> In the 1997 British Channel Four series "Brass Eye", host Chris Morriss
>>> campaigns against spherical cows, in a reality-meets-comedy segment, he
>>> distributes flyers and gets public opinion on the use of spherical cows as
>>> food, implying that such cows exist and are secretly being sold in British
>>> shops. Response was universally negative, but not one respondent
>>> questioned the veracity of the spherical cow's existence.
>>>
>>>
>>> Thanks for posting that URL. I had never heard of this reference before.
>>>
>>> The humour still eluded me as I read the description and thinking, "Of
>>> course cows in a vacuum would be spherical!" but then I realized that
>>> wasn't the point. sigh, it's been a long day.
>>
>> It's hard to raise spherical cows; they keep blowing away (except in a vacuum.)
>>
>>
>That's actually a feature--remember the theme from Rawhide?  (I actually 
>know it only from the Blues Brothers.)
>
>"Keep 'em rollin, rollin, rollin,
>Though the streams are swollen,
>Keep those dogies rollin, rawhide!
>All kinds of weather, hell bent for leather,
>Keep them dogies rollin, rawhide!"
>
>Much easier with spherical cows, at least if you're going downhill.
>;)
>
>Cheers
>
>Phil Hobbs

A very young Clint Eastwood played Rowdy Yates. I remember that, back when half
the shows on TV were Westerns.

I met Clint once. He was mayor of Carmel, and owned the Hog's Breath saloon.
He'd tend bar now and then. I also met Johnny Weismuller (in a bar in New
Iberia) and was kissed by Hopalong Cassidy (at a Mardi Gras parade, when I was
just a baby.)


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation