That's pure obvuscation.
For establishing the resistance of the circuit, the only relevant current is that going into, or out of, the circuit. The current through R4 is not that current. The offset of which you speak could change the sign of the current into the circuit, which in this context would be hugely significant. You can't right it off as being something that's of no concern.
Sylvia.
It wouldn't matter if the opmp was powered from a separate supply. It would still be railed low, and the input to R4 would still look like a positive, about 1K, resistor to ground, and the circuit would still be useless.
You measured the current backwards.
John
On Sun, 18 Jan 2009 17:41:02 -0800, John Larkin wrote:
--- That seems to make sense, and I think the problem is the way LTspice handles currents.
Referring to the last schematic I posted:
Version 4 SHEET 1 880 680 WIRE 176 -32 128 -32 WIRE 320 -32 256 -32 WIRE 192 80 -16 80 WIRE 192 128 192 80 WIRE -512 144 -672 144 WIRE -16 144 -16 80 WIRE -16 144 -288 144 WIRE 0 144 -16 144 WIRE 128 144 128 -32 WIRE 128 144 80 144 WIRE 160 144 128 144 WIRE 320 160 320 -32 WIRE 320 160 224 160 WIRE 160 176 128 176 WIRE 192 208 192 192 WIRE 192 208 16 208 WIRE -672 240 -672 144 WIRE -512 240 -512 144 WIRE -288 240 -288 144 WIRE 128 256 128 176 WIRE 176 256 128 256 WIRE 320 256 320 160 WIRE 320 256 256 256 WIRE 128 304 128 256 WIRE -672 416 -672 320 WIRE -512 416 -512 320 WIRE -512 416 -672 416 WIRE -288 416 -288 320 WIRE -288 416 -512 416 WIRE 16 416 16 208 WIRE 16 416 -288 416 WIRE 128 416 128 384 WIRE 128 416 16 416 WIRE 128 464 128 416 FLAG 128 464 0 SYMBOL res 112 288 R0 SYMATTR InstName R1 SYMATTR Value 100k SYMBOL res 272 240 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL res 272 -48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 490 SYMBOL voltage -288 224 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(0 12 0 .01) SYMATTR InstName V3 SYMBOL res 96 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 490 SYMBOL Opamps\\\\LT1797 192 96 R0 SYMATTR InstName U1 SYMBOL voltage -672 224 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(0 12 0 .01) SYMATTR InstName V1 SYMBOL res -528 224 R0 SYMATTR InstName R5 SYMATTR Value 1000 TEXT -8 440 Left 0 !.tran .02 uic
If you lay the cursor over V1, even though current is going out of V1 it's shown as going to ground. Then, if you lay it over R5 it'll show it as going to ground also, the scheme seeming to be that charge flows in the direction of the highest voltage to ground.
Plot the currents and they're shown as going in different directions with charge flowing out of a device being designated as negative and into a device as positive.
More to the point though, plot the voltage across R5 and the current through it and they'll be in phase, just as expected for a normal resistor. Congruent, even, so if you want to see separate slopes, change the value of R5 to 750 ohms or so.
Now, if you plot the voltage out of V3 and the current through R4 you'll see that the relationship between current and voltage is definitely _not_ the same as in the V3 R5 case.
What's happening is that LTspice is plotting the current _into_ R4 as though E3 were a sink with charge flowing _out_ of R4.
Make sense?
JF
JF's circuit is weird, power tied to an input... not worthy of a look.
The Howland pump version, posted very early on WORKS!
...Jim Thompson
If I hover the probe over the V3 source, the "current meter" arrow points down, and the graphed current is ramping negative, which I suppose means the current is actually positive and ramping UP into R4.
If the arrow were reversed (pointing UP inside of V3) and the graph ramped positive and up, the situation would be more obvious.
Ditto R4; arrow points left, current graphs negative. One crosscheck as to the current direction is the voltage at the ends of R4: +12 on the left, +6 on the right, so the current is flowing to the right.
I like to check my circuits two or three (or as many as possible) different ways and make sure they're all consistent. And the next day, assume that an idiot designed the circuit and do it again. I can't make myself any smarter than I am, but I can make myself be very careful.
Is there any way to get a netlist from a newsgroup post directly into LT Spice? I've been highlighting, copying, opening an editor, pasting, saving as .ASC somewhere, finding the file, and dragging onto the LT Spice shortcut. All that is a nuisance.
John
What's it take, maybe a minute?
As for importing netlists, I'm sure LTspice can do it. You sure can do it in PSpice... actually it's "Run" circuitname.cir
...Jim Thompson
I enjoy the Folsom Street S/M Fair. And lesbian weddings. Skiing out of control. Brainstorming wild ideas in front of customers. Reading outrageous stuff, like history. Eating strange food. I live in one of the wildest and most diverse cities on the planet, by choice. I must not understand your definition of "prissy."
Don't be silly. Pop psychology isn't in your skill set. Circuit design seems to be enough of a challenge.
Personally, I've adapted my personality to do good engineering. It's working.
I seek no followers, or anyone to follow. I can, and do, turn off the TV set and don't subscribe to People Magazine. The trash that modern culture has become saddens me, but doesn't affect me much.
He managed to kill himself, too, depriving us of his brilliance.
As someone in the entertainment biz said, killing yourself can be a good career move.
John
Hey, prongie, post some circuits. Maybe even ones that work.
John
Maybe because he is coarse, rude, has a scat fetish, and he's Always Wrong. But there's no accounting for taste.
John
I've met a couple of COBOL programmers - the one seemed quite level-headed, and after all, how hard is it to write COBOL? The other one asked me to tutor her, but that was just a ruse to get into my knickers. >:->
Cheers! Rich
OK, I can nitpick too - we're not even talking about a "negative resistor"
- obviously, such a thing can't exist. What we're talking about is "negative resistANCE", which is a characteristic of various things, notably, and arc, a UJT, or a tunnel diode.
Hope This Helps! Rich
If it's posted as an attachment (probably have to be in a binaries group) I can just click on it from Agent (I have .asc or whatever it is associated with LTSpice).
But please don't do that. It's great that, partly thanks to LTSPice and AACircuit, we can still easily exchange circuits in these non-binary groups.
Don't you just hate it when that happens?
John
As far as I can tell, the current meter in LTspice tells which way the resistor is oriented. It doesn't say which way the current is flowing. If you flip the resistor 180 degrees, the current meter points the opposite direction, even though nothing else has changed.
I am unable to find a reference in the LTspice documentation that explains how this is supposed to work, but that doesn't mean it is not there.
For netlists, if someone is using XNews and uses the attachment option for the ASC and PLT files, XNews will save them to the directory you specify with the original filenames. You can then run the file normally as you would any other LTspice file.
This has a number of drawbacks. XNews changes the thread title, so your post disappears from the current thread and people are not likely to see it. Also, this behavior may be unique to XNews and the other news clients may behave differently.
For saving netlists, I wrote my own editor and have a function that selects the entire block with one keystroke, then a second keystroke allows me to enter a fileame and save it. But my editor won't run on your system.
Mike
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