PTU is a component used between the yellow and green hydraulic systems in Airbus A320 Aircrafts to feed the other in case of either failure. Power is automatically sensed and transferred through PTU. A piston on the PTU control valve compares the yellow and green system pressures and starts working when it senses a pressure difference of 500 PSI (in systems operating with 3000 PSI).
PTU consists of a motor and a pump mechanically connected to each other. The motor and pump are used to supply each of two independent systems. As the green hydraulic system will feed the yellow system, the yellow system can feed the green system. One of the important points to be considered here is that the power transfer between systems is not hydraulic fluid transmission.It works mechanically. Each system uses its own fluid.
There is a pressure line, a return line and a case drain line for each system on the PTU operating with 28 VDC. There is also a test port on the PTU.
PTU is automatically inhibited in green or yellow system in case of overheat, low airpressure, low fluid level. (If it is in AUTO position, an amber colored fault will occur.)