# Unstable points in a basic memoryless circuit

I have been told that there can be more than a single solution to a circuit, and that some of those solutions might be unstable. As a simple example, a resistor with a constant power source, connected to a diac (which, I've also been told, has the "S"-like I vs V characteristic.

The big blue dot I drew was the point that supposedly is unstable. I can't understand why clearly from the explanation attempts I got from the teachers (I don't think the ones I asked understand it clearly either). I wonder if anyone in here can help out

One way to see that the solution given by the big blue dot is unstable is to consider the effect of perturbing the voltage across the diac; perturb the voltage across the diac $\epsilon$ more positive.
• Thanks +1. Let's see, increase Vd by e. That decreases Id, and that means less Vr than what we had at the beginning, which means higher Vd than what we had at the beginning. But does that mean it's higher than Vd+e? I think looking at the plot that it's lower – Rojo Aug 31 '12 at 14:16