a question about resistors in an arc experiment

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

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

Maybe she will, if you will.

John

Regardless of what the circuit simluation does, I won't know what point he thinks he's making unless he tells me.

Sylvia.

If you think it's a bad photo to judge from, then why are you judging from it?

Sylvia.

On Sun, 18 Jan 2009 12:33:10 -0800, John Larkin wrote:

--- No, it doesn't.

Here's my circuit alongside an identical generator with an ordinary 1k resistor to ground. Probe them and see if they act the same:

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

---

--- But it's _not_ stuck at ground, it ramps up to about 190mV during the time the output from V3 is ramping up to 12V.

JF

There is not, and never was, any dispute about the possibility of a negative resistance characteristic. Nor is there, nor ever was, a dispute about the possibility of constructing something that exhibits behaviour, over a limited range, that replicates some of the behaviour that a negative resistor would have.

The issues, as I remember them, were firstly, whether an arc had negative resistance, as distinct from having a resistance characteristic that, over part of its range, leads to current dropping with rising voltage, and secondly, whether a circuit containing a power source could legitimately be called a real negative resistor.

Your circuit simulation doesn't advance matters at all.

Sylvia.

--- I've already told you what the point was, but you don't seem to get it. So, run the simulation and if you still don't get it I'll explain it to you in greater detail and in simpler terms.

Here's the latest:

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

No. It hs a power supply.

Explain why you think this wouldn't exclude it from being called a negative resistor.

Sylvia.

Well, of course it goes up a tiny bit as the current through the divider increases. It's locked up and saturated, but it's not a

You could replace the opamp with roughly a 16 ohm resistor to ground and get the same waveforms. Try it. How can three resistors in series be a negative resistor?

How could a tiny voltage change at the bottom of R3, in the POSITIVE direction, swing the whole R4+R3 impedance negative?

This is just too silly.

John