マルコウ 1 Used it on a window exhaust fan. Bought the fan to create controlled airflow out of the room inna way to prevent exposure to Corona virus. The fan was to loud, so needed something like this to reduce its speed. It works great.The speeds are backwards though. The sequence should be Off-High-Med-Low to avoid under powering the motor and damaging it. The sequence is actually Off-Low-Med-High. But by quickly going from Off to High any damage can be avoided.