The PU4 3U 8HP power supply unit is a plug-in device for 19" systems (e.g. VMEbus, CompactPCI®) and is especially designed for railway computer systems, being fully compliant with EN 50155. The PU4 has a nominal input voltage of 110 VDC with a wide input range of 77..137.5 V and a max. input voltage range of 66..154 V (according to EN50155). The otherwise identical PSU models PU2 and PU3 are available for the nominal input voltages of 24 VDC and 36 VDC respectively. It is is controlled by a microprocessor which supervises the voltage and the temperature, as well as three external inputs and three outputs. Thanks to the onboard intelligence the PSU can also act as an SMBus slave and communicate with the CPU board via the backplane. This allows the PSU to power-on the system at a programmed date or it allows the CPU to make status polls with regard to error messages, time-out, on/off, for example. The PSU provides a DC/DC converter to generate the isolated 5 V from the input voltage. A regulator generates 3.3 V from the 5 V isolated side. The maximum power at full load is 35 W, minimum power is 5 W. The subassembly is conformally coated, and components are secured against vibration. The PSU operates in -40 to +85°C environmental temperature. The thermal stress is extremely low due to integrated heat sinks and diversion of dissipated heat over the mounting surface.