Hot swap controllers are necessary to facilitate the controlled removal and insertion of a PCB or module from a live backplane. The hot swap controller carefully controls the power-up in order to avoid the possible large current and voltage transients and continually protects the system. For instance, large capacitors can often demand as much current as is available on power-up. In a large system this could be hundreds of amps or more. The hot swap controller will allow the capacitors to charge up slowly with a controlled current. This will prevent dangerous current and voltage transients in the system. The hot swap controller usually controls the main pass MOSFET on a system board with current being sensed through a series very low resistance sense resistors.