But it is not always possible to prevent instability by these measures. An often-effective solution is to use positive feedback to introduce a small amount of hysteresis. Noisy signals can occur in any application, and especially in industrial environments. As the signal crosses the threshold region, the noise is amplified by the open loop gain, causing the output to briefly bounce back and forth. This is unacceptable in most applications, but it can generally be cured by introducing hysteresis.
Noise causes multiple transitions.
Where to use hysteresis Besides comparator noise reduction, system hysteresis is used in on-off control to avoid overly frequent cycling of pumps, furnaces and motors. In the simplest applications a controller turns an actuator on and off as a system parameter falls below or rises above a reference set point. With hysteresis, the actuator remains on until the parameter rises somewhat above the set point, switches, and then remains off until the parameter falls to a value below the set point. The levels at which switching occurs are called the high and low threshold voltages, V th and V tl . Allowing hysteresis for a few degrees of temperature change reduces unnecessary cycling adequately for home environments.