There are various ways to control the fan rotational speed in operation, and here are the three most common methods. Varying input voltage: This is the most straightforward method, in which the fan speed varies in the same way as the input voltage is varied. This method has the poorest control on fan speed resolution, since the fan will completely stop spinning once the input voltage is dropped below the fan start-up voltage. The user only has a short range of fan speed for which to vary. Pulsing input voltage: This fan speed control method involves turning the fan ON and OFF at high frequency. The higher the frequency, the faster the speed of the fan, and vice versa. However, this effectively turns the fan ON and OFF rapidly, with power going ON and OFF to the electrical components at high frequency. This will lead to a reduced component life for the fan, but offers moderate fan speed resolution. The PWM control is the method that is now widely adopted.