Dynamic controller

[tangjh88]

I want to control voltage under 245V. When the voltage exceed 245V, i will use NI 9403 to turn on one resistor to pull down the voltage, and the resistor should turn off when the voltage back to normal.

I face a problem with my controller. For example, when voltage is 246V, the resistor is turned on. After the resistor turn on, the voltage drop to 243V, then my resistor turn off. This cause the voltage go up to 246 again. So the resistor keep on and off. How can i solve this problem? 

[Alain_S]

Using a so called "Dead Band" around the 245V.

Level of dead band will depend on requirements.

http://en.wikipedia.org/wiki/Dead_band

[GerdW]

Or use the concept of "Schmitt triggers" (switches with a hysteresis)...