# Negative resistance

1. Dec 31, 2011

### rollcast

Lets see if I understand this one correctly.

Basically negative resistance is when an object actively draws electrical current through it. The object almost encourages current to flow through it?

Thanks
AL

2. Dec 31, 2011

### paralumina

That description sounds about right to me, but I think that resistance has a lower bound of 0 because if it actively draws the current, then it does not resist the current.

In another way, the measure of conductance (1/R) describes how well a conductor allows a current flow through it. With a resistance of 0, the conductance would be infinite, but if you have a negative resistance, say -1 KOhm, that would be -.001 Siemens. This negative conductance says that it resists the flow of the current while the negative resistance says that it freely allows the current.

Maybe someone else will come along that has a more refined answer.

3. Dec 31, 2011

### Antiphon

Resistance is defined as V/I for DC. It can be positive, negative, zero or unbounded.

4. Dec 31, 2011

### Kholdstare

You're looking at it the wrong way,

Let's say you put a voltage across a material with finite and positive resistance. Now you keep replacing the material such than the resistance goes from finite positive value to finite negative value. At first the current will increase and reach positive infinity when the resistance drops to just above zero ($0^{+}$). When resistance becomes just below zero ($0^{-}$) the current will become negative infinity. When the resistance comes to finite negative value the current will be finite negative.

Now the behavior of a load is, you put a voltage across it and the current goes into it from positive terminal and the behavior of a source is you put a voltage across it and current still comes out from it from the positive terminal.

Thus the negative resistance material acts as a source. The difference between true source and this thing is in true source you don't even need to put voltage to get current. But here you have to bias the thing to get current. Like attach with a dead battery and it gets charged with additional circuitry.

Any material having bias point in 2nd or 4th quadrant will be energy source. e.g. ILLUMINATED solar cell.

5. Dec 31, 2011

### OmCheeto

ps. wiki also has entries on Faeries.

6. Dec 31, 2011

### vk6kro

Negative resistors do not exist, but negative resistance is very real and measurable.

It comes in various forms, but one is the tunnel diode.

If you pick one up and measure it for voltage, there isn't any.

However, when you connect it to a power source and start increasing the voltage, the current increases in the normal direction (positive to negative).
As you increase the voltage more, the current starts to decrease, but it never drops to zero or reverses. Then if you increase the voltage even more, the current starts to rise again and continues rising with voltage.

The region where current is dropping with increasing voltage is called a negative resistance region.

At no stage does the tunnel diode generate power itself and the dissipation in it is power that comes from the power supply.

7. Dec 31, 2011

### Antiphon

Vk6kro is right but we should properly call it negative incremental resistance. The tunnel diode has a positive resistance everywhere with a negative differential or incremental resistance in the special region.

An even more common example of this is the fluorescent lamp.

8. Jan 1, 2012

### Studiot

A quick google of 'negative resistance or negative impedance gyrator' will get you all the circuit theory and implementation you require.

PS As others have said, don't rely on Wiki.