# Equlibrium Point!

1. Nov 17, 2009

### tuebingen78

or some time the single point, in control systems they say it is the point where the internal variables of the system never change .

my question if we must have input of system zero, or it can be any value?

thanks

2. Nov 17, 2009

### vk6kro

If you had a small heating element in a large container of water, (so the water didn't boil) the heater would heat the water until the losses due to convection, conduction and radiation equalled the power being added to the system by the heater.

At that stage, the water temperature would remain the same. This would be called an equilibrium point.

If you did nothing and didn't turn on the heater, that would also be an equilibrium point but a trivial one which you would not normally consider.

3. Nov 18, 2009

### Bob S

Equilibrium point also refers to the point that a system is restored to by negative feedback.
Bob S

4. Nov 18, 2009

### tuebingen78

thanks for your answer, but can you explain more or give a reference for this.

5. Nov 18, 2009

### Bob S

Look at the first two pictures and equations for inverting and non-inverting op amps in this:
http://www.rfcafe.com/references/electrical/opamps.htm
In both cases, the gain of the differential op-amp is very high (G > 10,000), and the gain drops out of the equation. Derive these equations using

Vout = G(Vin+ - Vin-)

The op-amp output drives the feedback circuit to the point where the voltage at both inputs are equal (equilibrium).
Bob S

Last edited: Nov 18, 2009