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another LT Spice question

Started by John Larkin June 13, 2013

I have this circuit driving a pulsed laser. I want to compute the
energy dumped into the laser and the energy stored in the power supply
capacitors. The ballpark is 120 amps into a 20 volt laser for a few
hundred microseconds.

What I did was create a "B" behavioral current source whose equation
is   I = V(LASER) * I(LASER)   which represents power, and dump that
into a 1 farad cap. The voltage on the cap is then energy in joules,
and I can probe/plot that just like any other node. This works [1].

Then, for the cap energy, I made a behavioral voltage source

   V = 0.5 * 1m * V(VCC)**2

where the 1m is because it's a 1000 uF cap. That gives me a probe-able
node scaled 1 volt per joule. That works, but if I change the cap
value I have to edit the equation. The nicer version is

   V = 0.5 * C1 * V(VCC)**2

but Spice barfs on the C1 bit. Is there a way to put the cap value
into the equation?


Too bad there's no integration operator available in the equations.

[1] except that it initializes to -250 megavolts. Putting a 1 ohm
resistor across the cap fixes that.


-- 

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation
On Thu, 13 Jun 2013 16:22:15 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

> > >I have this circuit driving a pulsed laser. I want to compute the >energy dumped into the laser and the energy stored in the power supply >capacitors. The ballpark is 120 amps into a 20 volt laser for a few >hundred microseconds. > >What I did was create a "B" behavioral current source whose equation >is I = V(LASER) * I(LASER) which represents power, and dump that >into a 1 farad cap. The voltage on the cap is then energy in joules, >and I can probe/plot that just like any other node. This works [1]. > >Then, for the cap energy, I made a behavioral voltage source > > V = 0.5 * 1m * V(VCC)**2 > >where the 1m is because it's a 1000 uF cap. That gives me a probe-able >node scaled 1 volt per joule. That works, but if I change the cap >value I have to edit the equation. The nicer version is > > V = 0.5 * C1 * V(VCC)**2 > >but Spice barfs on the C1 bit. Is there a way to put the cap value >into the equation? > > >Too bad there's no integration operator available in the equations. > >[1] except that it initializes to -250 megavolts. Putting a 1 ohm >resistor across the cap fixes that.
You probably need to declare C1 as a parameter... see page 85 of the SCAD4 manual. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On 6/13/2013 6:22 PM, John Larkin wrote:
> > > I have this circuit driving a pulsed laser. I want to compute the > energy dumped into the laser and the energy stored in the power supply > capacitors. The ballpark is 120 amps into a 20 volt laser for a few > hundred microseconds. > > What I did was create a "B" behavioral current source whose equation > is I = V(LASER) * I(LASER) which represents power, and dump that > into a 1 farad cap. The voltage on the cap is then energy in joules, > and I can probe/plot that just like any other node. This works [1]. > > Then, for the cap energy, I made a behavioral voltage source > > V = 0.5 * 1m * V(VCC)**2 > > where the 1m is because it's a 1000 uF cap. That gives me a probe-able > node scaled 1 volt per joule. That works, but if I change the cap > value I have to edit the equation. The nicer version is > > V = 0.5 * C1 * V(VCC)**2 > > but Spice barfs on the C1 bit. Is there a way to put the cap value > into the equation? > > > Too bad there's no integration operator available in the equations. > > [1] except that it initializes to -250 megavolts. Putting a 1 ohm > resistor across the cap fixes that.
.param c1 1m V = 0.5 * {c1} * V(VCC)**2
On Thu, 13 Jun 2013 19:46:38 -0500, John S <Sophi.2@invalid.org>
wrote:

>On 6/13/2013 6:22 PM, John Larkin wrote: >> >> >> I have this circuit driving a pulsed laser. I want to compute the >> energy dumped into the laser and the energy stored in the power supply >> capacitors. The ballpark is 120 amps into a 20 volt laser for a few >> hundred microseconds. >> >> What I did was create a "B" behavioral current source whose equation >> is I = V(LASER) * I(LASER) which represents power, and dump that >> into a 1 farad cap. The voltage on the cap is then energy in joules, >> and I can probe/plot that just like any other node. This works [1]. >> >> Then, for the cap energy, I made a behavioral voltage source >> >> V = 0.5 * 1m * V(VCC)**2 >> >> where the 1m is because it's a 1000 uF cap. That gives me a probe-able >> node scaled 1 volt per joule. That works, but if I change the cap >> value I have to edit the equation. The nicer version is >> >> V = 0.5 * C1 * V(VCC)**2 >> >> but Spice barfs on the C1 bit. Is there a way to put the cap value >> into the equation? >> >> >> Too bad there's no integration operator available in the equations. >> >> [1] except that it initializes to -250 megavolts. Putting a 1 ohm >> resistor across the cap fixes that. > >.param c1 1m >V = 0.5 * {c1} * V(VCC)**2
Yeah, that works. Of course, I have to set the value of the cap too, to {C1}. Kinda klunky, but works. I have a 20 uF ceramic in parallel with the 1000uF alum, but it's easier to ignore it and tolerate the 2% error. It's churlish to complain about something free and as good as LT Spice, but some sort of Valof(C1) operator would be nice, and an integration operator. I often wind up building my own test equipment on my schematic, rather than doing math. Thanks -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
On Thu, 13 Jun 2013 18:26:09 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

>On Thu, 13 Jun 2013 19:46:38 -0500, John S <Sophi.2@invalid.org> >wrote: > >>On 6/13/2013 6:22 PM, John Larkin wrote: >>> >>> >>> I have this circuit driving a pulsed laser. I want to compute the >>> energy dumped into the laser and the energy stored in the power supply >>> capacitors. The ballpark is 120 amps into a 20 volt laser for a few >>> hundred microseconds. >>> >>> What I did was create a "B" behavioral current source whose equation >>> is I = V(LASER) * I(LASER) which represents power, and dump that >>> into a 1 farad cap. The voltage on the cap is then energy in joules, >>> and I can probe/plot that just like any other node. This works [1]. >>> >>> Then, for the cap energy, I made a behavioral voltage source >>> >>> V = 0.5 * 1m * V(VCC)**2 >>> >>> where the 1m is because it's a 1000 uF cap. That gives me a probe-able >>> node scaled 1 volt per joule. That works, but if I change the cap >>> value I have to edit the equation. The nicer version is >>> >>> V = 0.5 * C1 * V(VCC)**2 >>> >>> but Spice barfs on the C1 bit. Is there a way to put the cap value >>> into the equation? >>> >>> >>> Too bad there's no integration operator available in the equations. >>> >>> [1] except that it initializes to -250 megavolts. Putting a 1 ohm >>> resistor across the cap fixes that. >> >>.param c1 1m >>V = 0.5 * {c1} * V(VCC)**2 > >Yeah, that works. Of course, I have to set the value of the cap too, >to {C1}.
.STEP PARAM C1...
> >Kinda klunky, but works. I have a 20 uF ceramic in parallel with the >1000uF alum, but it's easier to ignore it and tolerate the 2% error. > >It's churlish to complain about something free and as good as LT >Spice, but some sort of Valof(C1) operator would be nice, and an >integration operator.
It's probably "S", RTFM >:-}
> >I often wind up building my own test equipment on my schematic, rather >than doing math. > >Thanks
We all do. It's the only way to conveniently display such information. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
I don't know if it matters in your analysis, but spice doesn't conserve 
charge.

On Thu, 13 Jun 2013 20:34:23 -0700, miso <miso@sushi.com> wrote:

>I don't know if it matters in your analysis, but spice doesn't conserve >charge.
It did initialize a 1 farad cap to 225 megavolts, which is more joules than my PC power supply can deliver. https://dl.dropboxusercontent.com/u/53724080/Truckee/Exit201.jpg -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Thu, 13 Jun 2013 18:26:09 -0700, John Larkin wrote:

> It's churlish to complain about something free and as good as LT Spice, > but some sort of Valof(C1) operator would be nice, and an integration > operator.
LTSpice does have an integration operator for it's behavioral supplies. In fact, it has a few -- one Laplace transform operator, and two time- domain integrators (one rolls over, the other limits). I've used them to simulate microprocessor control of a PWM generator in a power supply. I can't remember more details than that, though -- try searching the manual on "integrat": that'll get you both "integrate" and "integrator". -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On Fri, 14 Jun 2013 08:34:07 -0500, Tim Wescott <tim@seemywebsite.please> wrote:

>On Thu, 13 Jun 2013 18:26:09 -0700, John Larkin wrote: > >> It's churlish to complain about something free and as good as LT Spice, >> but some sort of Valof(C1) operator would be nice, and an integration >> operator. > >LTSpice does have an integration operator for it's behavioral supplies. >In fact, it has a few -- one Laplace transform operator, and two time- >domain integrators (one rolls over, the other limits). I've used them to >simulate microprocessor control of a PWM generator in a power supply. > >I can't remember more details than that, though -- try searching the >manual on "integrat": that'll get you both "integrate" and "integrator".
.MEASURE includes an integration function, but I don't understand .MEAS. I put one in my sim, but I don't see its output. .MEAS TRAN res1 FIND V(VCC) AT=50u It's supposed to "print" the result. Where? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Fri, 14 Jun 2013 06:48:40 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Fri, 14 Jun 2013 08:34:07 -0500, Tim Wescott <tim@seemywebsite.please> wrote: > >>On Thu, 13 Jun 2013 18:26:09 -0700, John Larkin wrote: >> >>> It's churlish to complain about something free and as good as LT Spice, >>> but some sort of Valof(C1) operator would be nice, and an integration >>> operator. >> >>LTSpice does have an integration operator for it's behavioral supplies. >>In fact, it has a few -- one Laplace transform operator, and two time- >>domain integrators (one rolls over, the other limits). I've used them to >>simulate microprocessor control of a PWM generator in a power supply. >> >>I can't remember more details than that, though -- try searching the >>manual on "integrat": that'll get you both "integrate" and "integrator". > > >.MEASURE includes an integration function, but I don't understand .MEAS. I put >one in my sim, but I don't see its output. > >.MEAS TRAN res1 FIND V(VCC) AT=50u > >It's supposed to "print" the result. Where?
Oh, it goes into the log file. It's computed post-simulation. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators