The PU3 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 PU3 has a nominal input voltage of 36 VDC with a wide input range of 25.2..45 V and a max. input voltage range of 21.6..50.4 V (according to EN50155). The otherwise identical PSU models PU2 and PU4 are available for the nominal input voltages of 24 VDC and 110 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 on-board 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.