Still waiting for a Ltspice expert answer.

Does the wires assume real life situations to some degree or do we need to add stray inductance, Resistors and caps to act like a board?

I would of thought it would assume an average lay out of one persons worse nightmare and the actual real live circuit would just work better, seeing that this designer would engineer proper protocols on the board.

It just seems that I don't see many of the small defects the sim may show, when it is on a real board assembled using common practices.

Which leads me to my question, does the sim assume a worse case scenario or maybe the sim just over exaggerates now and then?

This would be LTspice I am referring to. I was thinking of looking at Pspice to see how that goes.

Jamie

I've heard of a few simulators which assumed the designer indicated wire resistance proportional to the length of the traces on the schematic drawing. Which often have no relation to actual board traces, and certainly have no relation to their widths.

As far as I know, most professional environments (SPICE based) assume all nets are ideal. If the models themselves include parasitics, they will necessarily be limited to package parasitics. It's always the user's responsibility to include realistic wiring parasitics.

I don't remember if LTSpice includes package parasitics. I seem to recall they mostly use MODELs rather than SUBCKTs, but they may've expanded the MODEL definition to include simple series or parallel elements.

In short, no, SPICE tends to assume ideal conditions. It's a model, not a reality simulator, and it's your duty to construct a model that has any useful representation at all.

If you insist on reality simulation, there are suites out there which attempt to do such things. Ansoft makes such a system, including electromagnetic (which can do static, AC and transient 3D simulations on real board layouts, and generate approximate SPICE models for the circuit), mechanical (stress/strain) and thermal (including fluid flow, expansion, tempcos to feed back into the circuit). A few others make similar suites; all are horrifically expensive, only something worthwhile if you're either making millions of top-quality, moderately complex products (cars maybe??), or a few very, VERY critical systems (avionics, aircraft, spacecraft?).

Tim

Well that just sounds counter productive.

Why waste my time using a sim if I am going to do it by hand in the first place?

You can haggle all you want Jim, It does not work on me..

I am not one of your goal post. You have JL for that :)

Jamie

Wires in LTSpice (and PSpice, as far as I know), are made of Magic Stuff. A node is a node is a node (in Spice-land) and has no voltage variations across it.

Jim's right, although I would state it differently: the simulation is help for those who already know what's up; try something that the simulation isn't prepared to give a correct answer for, and you'll get a wrong answer.

It _is_ handy for complex, nonlinear circuits (like switchers), but you still have to pose questions that it can answer sensibly.

Don't forget to add, "delay" All these simulators keep forgetting about that pesky quirk of reality, "It takes time for a signal to go from here to there." And, oh, yes, don't forget to add that even peskier term, "radiation loss" That way when you simulate charging a cap from another cap, you can reconcile the apparent loss of energy.

"goal post"? Bwahahahahahaha! So show us your by-hand math. You're showing your inexperience by continuing to argue. ...Jim Thompson

If you don't understand your circuit maths, how do you know if the simulation is close to reality???? Many times, you can do this in your head, but sometimes you need to write down a bunch of maths to understand issues you may run in to. As an excercise, try to temperature stabilize a simple one transistor amplifier without understanding the basic transistor equations.

Do you blindly trust your models??? If you do, beware, they are often wrong. With experience, you will learn a healthy distrust in your simulator and test equipment.

I don't think Jamie is even a technician, but he _is_ successfully becoming a reductive emulation of John Larkin ;-) ...Jim Thompson

No.

One persons' nightmare is not "average." WTH?

No, what you suggest -- "average" is whackadoodle. There is no "average" layout.

There may be standards and usual practice for a variety of tasks -- the more simple the more likely it can be handled by standards. "Simple" is limited. The harder the job, the more diverse opinion you'll get regarding what is a proper approach.

"Worst case" and "one persons' nightmare" doesn't mean "average" or "common." If your wrong views were more coherent, then they would be easier to pick apart!

You have 1 pF of pad capacitance, and you think it may matter? then put it in You care about 0.7 nH of via inductance? then put it in You care about the inductance of a long trace? then put it in

The models are just *models* -- not the real thing. How good are they? After all, that is all spice has to work with. Models are never perfect -- what should spice assume as "average" about them? Caveat emptor: you need to have some idea of what you're doing in the first place.

I think your question is really: Can I get SPICE to think for me? No, you can't.

Well no problem, I'll just continue with how I've been doing it all these years. Calculator,years of foot notes, engin books, equipment use, CAD steps and then boards.

I guess spice is good for concept circuit experimenting. Saves on paper.

I am experimenting with a little tool app I am writing that monitors the clipboard of a BMP image arriving into it. This tool has FTP code where I can then hit the default button and send it some where with some local index to manage them. So far it looks ok. I was thinking to use it as an uplink tool to send schematics to your web site for the general public audience instead of dumping spice in the chat areas.

Was thinking of supporting the WMF format too, it would save on network traffic.

Jamie

Yeah, you are correct as usually Jim. I mean, are you ever wrong?

Yes, I am just a back woods inbreed hill billy. Most of the time I run around with no clothes on, and when I do have them on, they are hand me downs, full of holes and have not seen what would represent laundry detergent or anything close to that. Yes, we can't even afford making our own soaps out of lard from road kill. etc..

I use a chinese abacus because I have found that I can actually exceed the math skills that requires more than 10 fingers. Yes, I still have all my fingers. I have not yet lost any of them while fighting with my sisters, in the sack.

Nothing like having a hole down, especially when its your lady or boyfriend, what ever your party maybe.

Jamie

Spice is an absolute necessity for chip design...

(1) You can't breadboard much, if anything.

(2) The expense of building a chip requires checks to make sure you don't make a dud.

Of course, in the chip-making world, the models are extremely accurate.

In the PCB-level world, unfortunately, whole-chip models are often not that wonderful... mostly because the chip manufacturers are so paranoid of copying that they devise behavioral models that are mostly created by cretins.

Likewise, in discrete devices, particularly MOSFET's, they don't spend enough effort to create more than school-boy-level models. You're lucky if you get a Level=3 representation. In my chip-design world we're at BSIM3v3 or higher (Level=49+). ...Jim Thompson

So your ladies and boyfriends have their holes down? I guess that would promote the inbreeding. Try to resist.

Yes, the sim is good for concept sketching but not very good for the refinements. I find the bench model+Test gear tells the story.

Like I've said before, I never used a Sim until the last few months and only here basically for general chatter.

Jamie

I believe that this is one of the stupidest questions I've ever seen on this group. Got any more?

That's true (with LTSpice) but all connections on a wire are not equal. You can measure the current in a wire at any place along the wire. LTSpice looks left and right from that point to get the current at that point. The topology matters.

Like transmission line speed > C. ;-)

It's also handy for statistical or worst-case analysis on complex designs.

at

he

It tells the story of the parts you have on your bench, sims can help you gain some confidence, if you have the right models, that it will also meet specs if the parts are at the limits of their specifications

hs

-Lasse

Often, and I say so... check my posts. I've tossed far more designs than I've kept... I have no problem deciding a design is going the wrong direction, or has too many patches... ask my clients... sometimes they get a little irritated when I restart a design, but they end up happy. (Ask qrk, a poster in this thread :-)

You're no hillbilly. _I'm_ a real hillbilly, raised in WV ;-)

I have used homemade soap, slopped the hogs and brought in the cows every evening for milking (with the aid of some very smart dogs :-), and helped my grandfather choose chickens for dinner by grabbing them by the head, spinning them a few turns then letting them run loose to bleed out. (It always fascinated my engineering mind trying to figure out how they managed to stay upright as they ran, with their head dangling to the side :-)

"Hole" down? What's that?

But, indeed, you have an attitude problem. Just because one of your amplifier abortions survived, doesn't make the architecture a good idea.

I do wish you'd run the math rather than your mouth. ...Jim Thompson

Well, that's what I hoped he'd get -- wires in LTSpice are made of magic stuff, wires in this humdrum mundane world we live in are made of real stuff.

If you want to simulate the real stuff with the magic stuff, you need to add detail to your simulation.

True, although even there you can often go much farther with pencil & paper methods than most folks go.