Try an experiment. Next time you are in a room with someone, ask them to name everything in the room. Only certain kinds of people will say “air” or “light.” For most people, those are just givens, and you don’t think about them unless, for some reason, you don’t have them. Resistance is like that in electronics. You use it constantly, but do you ever think much about what it is? For a resistor, the value in ohms really represents the slope of the line that describes the amount of voltage you’ll see across the component when it carries a certain amount of current. For resistors, that slope is — at least in theory — constant and positive. But [Void Electronics] made a video exploring negative resistance, and it is worth watching, below.
If you haven’t seen negative resistance before, you might wonder how that is possible. Ohm’s law is just a shorthand for calculating the slope of a graph with voltage on the Y axis and current on the X axis. It works because the voltage and current are always zero at the same time, so the slope is (V-0)/(I-0), and we just shorten that to the normal Ohm’s law equation.
But not everything has a linear response to current. Some devices will have different slopes over different current regions. And sometimes that slope can be negative, meaning that an increase in current through the device will cause it to drop less voltage. Of course, this is usually just over a narrow range and, as [Void] points out, most devices don’t specify that parameter on their data sheets. In fact, some transistors won’t even work in the circuit.
The circuit in question in the video below the break is an odd one. It uses two resistors, an LED, and a transistor. But the transistor’s base is left disconnected. No 555 needed. How does it work? Watch the video and you’ll see. There’s even a curve tracer if you don’t like to see hand-drawn graphs.
We’ve looked at negative resistance more than once. There are a few exotic devices, like tunnel diodes, that are explicitly used for the negative resistance property. When the gas in a neon bulb breaks down, you get the same effect.
“It works because the voltage and current are always zero at the same time, so the slope is (V-0)/(I-0), and we just shorten that to the normal Ohm’s law equation.”
Really going to need some explanation on that. Do it like I’m slow if you don’t mind. Sounds not quite correct.
So… if we have a superconductor… according to that… there will never be a current running through it…
Correct. Since a magnetic field can’t enter a superconductor, any current stays on the surface (would it try to enter, the magnetic field would cause an eddy current compensating for that, so the sum of current and eddy vanishes everywhere except on the surface).
And then there are some superconductors with “holes” in them, which allow to freeze a magnetic field present when the conditions for superconduction are met.
We’re used to writing R=U/I. But it is R=change I. U / change in I. Normally that’s the same. But in a Diode for example it doesn’t work. Now imagine something similar to a diode curve, but with bends that make an S shape. Which voltage gets you a specific current depends on from which side you approach the intended working point!
Dear Al, you miss one very important detail:
Differential Resistance.
Youndo not even mention the word “Differential” in you text. It makes all the difference to understanding how that works. I heard so many incomplete and thus confusing explanations before someone explained it completely and correct. Please do not contribute to these incomplete explanations.
Negative resistance is not possible for an passive linear component with ohmic behaviour.
For active components or circuits with active parts, negative resistance is possible, but only with an energy source: Positive voltage results in negative current.
What this video describes is negative differential resitance, that is, where the current increases when you decrease the voltage – in a specific and limited range. And that is the important part not to confuse.
Please stay precise in the wording, it matters.
Can you describe it as it was described to you? I’d also like to understand.
he whole thing is about the non-linear behavior of components. The actual resistance may be positive all the way, but the slope of rise of current per volt is not the same all the way. You may find a point where the slope becomes steeper when the voltage goes down, thus lower differential resistance, but it’s still a positive resistance in the absolute.
For example, suppose you have a device that passes 1 Amp per 1 Volt, which is 1 Ohms.
But then after 1 Volts, it passes 2 amps over the next volt, so the slope is different. Then from 2 volts and above it goes back to 1 more amp over 1 more volt. Therefore, between 0-1 volts the differential resistance is 1 Ohms, between 1-2 volts the differential resistance is 0.5 Ohms, and then again above 2 volts it is 1 Ohms again.
So when you’re applying a voltage that falls down towards 2 Volts and below, it seems like the device exhibits “negative resistance” because the slope of the resistance function changes, but the actual resistance remains positive.
Trace U and I, the slope of the line between (0, 0) and your operating point is the “normal” resistance U/I. Now draw a tangent on your operating point, and the slope of that is the differential resistance dU/dI.
Agreed. Negative resistance would create a current (and a corresponding voltage) where no external voltage is applied. What is described here is a negative resistance response to current.
It would not. If V=RI then for zero V there is no I whether R is positive or negative, because the product RI must be zero.
It looks to me like they didn’t clean their PCB blank well enough before etching. Those little dots of copper all over the place are what you get for not scrubbing with isopropyl alcohol and lightly sanding first
Name everything in the room? Easy, simply make the fundamental particles in the standard model.
Alternatively, just say “protons, neurons, electrons, photons, and probably some neutrinos”.
You’re in the third category. ;)
a bit more than probably, check out the neutrino flux from the sun .
also if you include virtual particles you (probabilistically) get the whole zoo :-)
I can name that tune in one note: contents.
neurons eh.. OK then.
I agree, they are somewhat fundamental for the observer.
Well, now I’ve decided I really need to build a curve tracer.