a question about resistors in an arc experiment

It's a bad photo to judge from, but Sylvia looks emaciated... that's pushing "skinny" just a wee bit too far ;-)

...Jim Thompson

Obstinate? See the SIG, you're blundering along riding a collection of bad assumptions.

It's alright, around here, to admit error. Actually, it's admirable, it happens so rarely ;-)

...Jim Thompson

slope

I still don't get it. If I run the transient response, just as you set it up, the opamp output is stuck near ground and the V3 source sees a load of about 1K to near ground, plus the opamp supply current. The opamp is just an expensive ground.

So what's the function of the circuit?

John

I've run your sim, comments posted elsewhere, and I can't figure out what the circuit is supposed to do.

John

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No, it follows Ohm's law with a minus resistance value:

V =3D R * I

The value of R is negative.

You seem not to understand that there are two types of negative resistors. Both are validly negative. Try thinking about the conductance of a negative resistor. Consider passing resistance through zero and then consider passing conductance through zero. You will see that they don't both take you to the same place.

There's cause to think that neither you nor I are gorgeous burning hunks of male beauty.

John

I did a walk through the Cornell EE department. Just for fun, I counted oscilloscope screens (three) and PC screens (had to be a hundred, maybe.)

My office is a little better, three oscilloscopes (as many as Cornell!) and two PCs. I actually own about 40 scopes and 5 PCs.

John

Hardly ;-)

But does your "skinny" preference descend to "bony" ?:-)

...Jim Thompson

At work, I have an experimental setup with a total of 4 oscilloscopes on it. Several different things happen at different rates from near DC up to several MHz. To be able to see the waveform of one signal, the timing of another etc, I would either have to frantically move probes or just hook up a few old cheap scopes. The expensive one is used for the actual measurement.

Actually, yes, short of true malnutrition. Kate Moss, ballet dancer types: smart, skinny, feminine, feisty. My wife is right around 100 lbs, and she's gorgeous to me. I told her so just this morning.

Nature provides. For nearly everyone, there's someone else, somewhere.

That's an interesting thing to explore some time, the evolutionary aspects of beauty, issues of monoculture. Science Weekly had a little piece on it last week.

John

If you ground the "input", the opamp rails and it dumps a lot of current into the external short, with random sign. If you short a negative resistor, the current is zero.

It's not an accurate emulation of a negative resistor.

John

We have one of the Tek TPS-series scopes: 4 channels, 200 MHz, totally isolated channels and trigger input. It's fabulous. Anybody working on power stuff, like power amps or off-line switchers, should get one if they can.

My old ebay 11801 samplers can hold 4 sampling heads, 8 channels of 20 GHz scope.

I'm debugging some firmware in a little box. My desk is a mess... the DUT, power supplies, scopes, laptop with BDM pod, cables, probes, schematics, program listing. I swivel around to my regular PC to run the comm software that talks to the box, or to reassemble the code, or to look up datasheets and check my email. I'm so surrounded by cables and stuff it's a chore to get up and pee.

John

That sounds like the kind of how it works that i can get a kick out of. Please post (though i cannot get a.b.s.e) or email me.

She looks like one of the old ladies from 'The Villages', on her six or seventh husband, and in her late 80s. Too many years out in the hot sun, with no protection. She acts like one too. They all know everything, too. :(

Till someone loses an eye!

--

aioe.org, Goggle Groups, and Web TV users must request to be white listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in your account:

There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you're crazy.

That can be a real problem if you have a bladder infection and have to take diuretics, too. :(

--- As I posted earlier, it's because you're looking in the wrong place.

Like, why are you looking at the output of the opamp when what's important is what's happening at the input to the circuit?

Once again:

If you run the simulation and you probe for voltage at the left-hand end of R4 you should see a trace which starts at 12V and ramps down to 0V in

Then, if you probe for the current through R4 you should see a trace which starts out at -12mA and ramps up to 0mA.

Voltage falling, current rising?

That should give you a clue as to what the circuit does.

Another might be that we're talking about negative resistances.

BTW, here's another sim which starts with the input at 0V which you can use to probe the voltage and current from V3 directly; the earlier one had a nasty 12 amp spike at power-up which made the current need to be probed through R4.

Version 4 SHEET 1 880 680 WIRE 400 -112 -96 -112 WIRE 176 -32 128 -32 WIRE 320 -32 256 -32 WIRE 192 80 -16 80 WIRE -96 112 -96 -112 WIRE 192 128 192 80 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 -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 -96 384 -96 176 WIRE -288 416 -288 320 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 WIRE -96 528 -96 448 WIRE 400 528 400 -112 WIRE 400 528 -96 528 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 0 .1) 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 TEXT -8 440 Left 0 !.tran .02 uic TEXT -112 416 Left 0 ;> >

TEXT -112 144 Left 0 ;> >

JF

I think you have the current measurement sign backwards. That would explain your observations. This circuit acts like an ordinary 1K resistor to ground.

Look at the opamp output; it *does* matter. If it's stuck at ground, which is what I'm seeing, how can the circuit possibly generate negative resistance?

Plot the V3 node, and the opamp inverting input, and the opamp output. All you see is a simple voltage divider to ground. The current in R4 is flowing to the right, not to the left.

ftp://jjlarkin.lmi.net/JF.jpg

John

Maybe Sylvia will submit a photo shot from her nudist days, and change our minds ;-)

...Jim Thompson