Circuits that shouldn't work...?

[Tim Wescott]

Real oscillators start up because of noise.

Or power-on transients, and if you include that in your simulation in
LTSpice then they start just fine.

 

[Kevin Aylward]

I use Multisim 2001 and the only circuits I know it will not handle
at all are CMOS Oscillators.

You mean you actually get... EWB to like.. work at all... wow...

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Paul Burridge]

Or power-on transients, and if you include that in your simulation in
LTSpice then they start just fine.

I've never needed to use an 'external stimulus' to start an oscillator
in LTSpice. It always delivers the goods.

 

[Bob Monsen]

I've never needed to use an 'external stimulus' to start an oscillator
in LTSpice. It always delivers the goods.

It appears to have some kind of noise stimulus built in. Some other spice
variants (notably, Circuit Maker) seem not to do this, and end up with
circuits balanced on the head of a pin, so to speak.

--
Regards,
Bob Monsen

"Ignorance more frequently begets confidence than does knowledge: it
is those who know little, and not those who know much, who so
positively assert that this or that problem will never be solved by
science."
-- Charles Darwin

 

[Robert Baer]

You mean you actually get... EWB to like.. work at all... wow...

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

If you want a circuit that should not work, how about an emitter
follower with a voltage gain of almost two...

 

[Robert Baer]

It appears to have some kind of noise stimulus built in. Some other spice
variants (notably, Circuit Maker) seem not to do this, and end up with
circuits balanced on the head of a pin, so to speak.

You mean the "point" of the pin...

 

[Bob Monsen]

You mean the "point" of the pin...

Yes, that is what I meant. An unstable equilibrium. Thanks for the
clarification.

--
Regards,
Bob Monsen

"I cannot persuade myself that a beneficent and omnipotent God would
have designedly created parasitic wasps with the express intention of
their feeding within the living bodies of Caterpillars"
-- Charles Darwin

 

[Don Lancaster]

I remember someone once posted about a circuit that Spice said should
be unstable, yet when built, the circuit works perfectly fine.

What circuit was this? What other circuits don't work in simulations,
but work perfectly fine in real life???

What makes you think there is any correllation whatsoever with
simulations and real world performance?


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[John Larkin]

What makes you think there is any correllation whatsoever with
simulations and real world performance?

Simulations are valuable, provided you already understand the circuit
well enough that you don't really need to simulate it.

John

 

[john jardine]

Simulations are valuable, provided you already understand the circuit
well enough that you don't really need to simulate it.

John

And already know where the critical node is that needs the ".IC=0V"

 

[Kevin Aylward]

What makes you think there is any correllation whatsoever with
simulations and real world performance?

Err... 1000s of 1,000 transistor ic designs that work first time, and
are made in millions per month. Modern ic design wold be quite
impossible without simulation.

This *is* the reality mate. Sure, simulations *can* fail if you don't
model correctly, however, they also demonstrable work as a matter of
routine.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Robert Latest]

LTSpice, surprisingly enough, seems to do a pretty good job with
oscillators. Whatever they did to it to optimize it for switching power
supplies seems to have made it fit for this, as well.

So does LTSpice use a different or improved engine, but works
with ordinary Spice models?

All I ever use is the good old original free Spice on Unix, and I
always thought that the derivatives (like PSpice) used the
identical engine with some added GUI sugar.

Clarification welcome.

robert.

 

[Helmut Sennewald]

So does LTSpice use a different or improved engine, but works
with ordinary Spice models?

Hello Robert,
LTspice is originally based on SPICE3F4, but then it has been
practically fully recoded by Mike to make it better and also
to overcome the bugs in the original code.
It has also many new intrinsic models required for fast
simulations of SMPS regulators.

LTspice is 99% compatible to PSPICE syntax for the analog
elements of SPICE.
You can practically use any 3rd party opamp model in LTspice.
It's just a very few mouse clicks and a ".include modelfile" SPICE-line.

Digital components are of course very different, because
there is no accepted SPICE-standard for digital elements.
Every SPICE-vendor "cooks its own soup" for it.
LTspice has the A-elements which are very powerful
models for digital and mixed-signal components.

All I ever use is the good old original free Spice on Unix, and I
always thought that the derivatives (like PSpice) used the
identical engine with some added GUI sugar.

See my comments above.
Linux users can run LTspice under Wine.

Clarification welcome.

robert.

LTspice is already replacing PSPICE in education,
because LTspice is fast, very stable, unlimited and freely
available. What can we wish more?
http://www.linear.com/designtools/softwareRegistration.jsp


There is a very strong user group for questions regarding LTspice.
http://groups.yahoo.com/group/LTspice/
It has a big file's section with many circuit examples.


Best regards,
Helmut
Moderator of the LTspice Yahoo group

 

[JosephKK]

I've never needed to use an 'external stimulus' to start an oscillator
in LTSpice. It always delivers the goods.

There are several classes of circuits that often do not model well in Spice,
various (often simple like Hartley or Colpitts or three logic inverters and
a crystal) oscillators is a typical example. What irritates me is that
LTSpice badly fouls up on simple "brute force" <rectifier and big capacitor
(with or without a series inductor)> power supplies. No other Spice that
i have used fouls up in this way.

 

[Jim Thompson]

There are several classes of circuits that often do not model well in Spice,
various (often simple like Hartley or Colpitts or three logic inverters and
a crystal) oscillators is a typical example. What irritates me is that
LTSpice badly fouls up on simple "brute force" <rectifier and big capacitor
(with or without a series inductor)> power supplies. No other Spice that
i have used fouls up in this way.

Well? It's "faster" ;-)

...Jim Thompson

 

[Jeroen Belleman]

[...] What irritates me is that
LTSpice badly fouls up on simple "brute force" <rectifier and big capacitor
with or without a series inductor)> power supplies. No other Spice that
i have used fouls up in this way.

It seems to work just fine here. Where exactly does it foul up?

Jeroen Belleman

 

[Ken Smith]

a crystal) oscillators is a typical example. What irritates me is that
LTSpice badly fouls up on simple "brute force" <rectifier and big capacitor
(with or without a series inductor)> power supplies. No other Spice that
i have used fouls up in this way.

Fouls up how?

I've modeled:


---->!----
! !
(V) --- 0.1F
! ---
GND !
GND

With no problem.


I've also modeled:


K1
------ -------------+-----------+---------
) ( L2 ! !
) ( ! --- C1
) ( ! ---
L1 ) +-- GND ! !
------ ( ! GND
( !
( !
-------->!---


Where (IIRC):

L1 = 100mH + 2R
L2 = 1mH + 0.1R
K1 = 0.95
C1 = 1000uF + 0.01R

.... and got good results.

 

[Don Bowey]

Fouls up how?

I've modeled:


---->!----
! !
(V) --- 0.1F
! ---
GND !
GND

With no problem.


I've also modeled:


K1
------ -------------+-----------+---------
) ( L2 ! !
) ( ! --- C1
) ( ! ---
L1 ) +-- GND ! !
------ ( ! GND
( !
( !
-------->!---


Where (IIRC):

L1 = 100mH + 2R
L2 = 1mH + 0.1R
K1 = 0.95
C1 = 1000uF + 0.01R

... and got good results.

What are "good results" for that (useless?) circuit?

Don

 

[Joseph2k]

Fouls up how?

I've modeled:


---->!----
! !
(V) --- 0.1F
! ---
GND !
GND

With no problem.


I've also modeled:


K1
------ -------------+-----------+---------
) ( L2 ! !
) ( ! --- C1
) ( ! ---
L1 ) +-- GND ! !
------ ( ! GND
( !
( !
-------->!---


Where (IIRC):

L1 = 100mH + 2R
L2 = 1mH + 0.1R
K1 = 0.95
C1 = 1000uF + 0.01R

... and got good results.

try a standard full bridge from 120 60 Hz into a 5000 uF cap and 240 ohms
load.

 

[Jeroen Belleman]

try a standard full bridge from 120 60 Hz into a 5000 uF cap and 240 ohms
load.

Looks quite reasonable to me. What *is* your problem? Give us something to
sink our teeth into. Vague statements that something doesn't work aren't
productive.

Jeroen Belleman