# Hysteresis for the SW in LTspice question

Discussion in 'Electronic Design' started by Maynard A. Philbrook Jr., Dec 25, 2013.

1. ### Maynard A. Philbrook Jr.Guest

I am trying to emulate a spark gap effect using the SW component in
LTspice and the parameter Vh = xxx seems to allude me !

The help file leaves a lot to be desired and network searches
simply point me to the same help file or youtube videos with no sound
and very blurry images.

Is the value for the hysteresis absolute below the trigger or is an
offset from the trigger?

I will say this, after experimenting with the values, I find that in
this circuit I am playing with, if I don't get the Vh very close to the
Vt value, Ltspice will never complete the simulation run, it just sits
there and loops on some calculation that it never comes out of.. I
Suppose I can change the step point, I using the default for now.

P.S.
The load this is shorting is from a secondary side of a HV
transformer, much like a car coil would be.

Jamie

Den onsdag den 25. december 2013 20.31.42 UTC+1 skrev Maynard A. Philbrook Jr.:
http://ltwiki.org/index.php?title=S_Voltage_Controlled_Switch

would the neonbulb with changed voltages work ?

-Lasse

3. ### Fred AbseGuest

Level 1 Vh positive:

Switches abruptly at Vt-Vh and Vt+Vh
Hence Vt=1 and Vh=0.5 will switch at 0.5 and 1.5
Vt midway between switching points.

Level 1 Vh negative:

Switches smoothly (polynomial fit) at Vt-Vh and Vt+Vh

/Examples/Educational/Vswitch.asc demonstrates this. You need to play with
the .model directive to see the two modes As it comes, it uses Vh negative.
Play with it to see how it works. This is what most people ever need.

Putting "level=2" in the .model line, with Vh negative gives a function
dependent on log(Roff/Ron)/pi, and log(1/(Roff*Ron))/2, with a tanh limit
function. Jim would like that ;-)
You probably never need worry about level 2.

It's all in the help file, just not either intuitive, or well explained.

LTspice also appears to honor Pspice switch syntax, which is different,
too.
Par for the course.
See above
Post the relevant .asc, and let's see what can be done.
Like a spark plug?

4. ### Fred AbseGuest

Hence Vt=1 and Vh=0.5 will switch at 0.5 and 1.5V. Vt is midway between
switching points.

That's better;-)

5. ### Maynard A. Philbrook Jr.Guest

I got it, it appears that the Vh parameter plays two roles here.

- numbers creates arc intersection, much like an arc-cardinal function
which can form an arc wave in the load effect of the switch. Setting
the Vt and Vh -0..-1 sets the arc and offset for the Vt. I guess one
could use that to make selectable raise/fall rates.

Using positive numbers does the OFFSET of the Vt (trigger). This means
If i would to do this, Vh = 1, that would put the Vt at Vt+1 and Vt-1
etc, which makes sense. but the help file does not explain this to you.

Jamie

7. ### Fred AbseGuest

It actually does explain it, just not very well.

See the PDF I posted to a.b.s.e, which illustrates the various modes as
plots of switch impedance versus control voltage.