The PU2 3U 8HP power supply unit is a plug-in device for 19" systems (e.g. VMEbus, CompactPCI®) and is especially designed for computer systems in public transport vehicles, being fully compliant with EN 50155. The PU2 has a nominal input voltage of 24 VDC with a wide input range of 16.8..30 V and a max. input voltage range of 14.4..33.6 V (according to EN50155). The otherwise identical PSU models PU3 and PU4 are available for the nominal input voltages of 36 VDC and 110 VDC respectively. It 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 Volts 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.