The 409PSB is a Marathon-brand power supply for Apple computers. It was released in late 1991, and is compatible with the Macintosh IIsi, IIci, Classic, LC, and some other computers from the same era. The model number 409PSB refers to the type of power source needed for these machines, which is a combination of 40 watts surge, 9 watts 'ripple', and a 115v/240v input. It is a 'smart' power supply, meaning it will adjust power levels automatically for the machine to ensure optimal performance and maintainability. The 409PSB also includes a number of safety features, such as voltage and over-current protection, and is UL-certified.