Uninterruptible power supply

The un-interrupted power supply has transformer and rectifier system for supplying electrical energy to the network (4) in the format required of the network. The conversion circuits are reversible, so that when supply is available, the battery may be recharged from the electrical supply. The un-interrupted power supply includes a battery (5) for energy storage, and a transformer system for supplying electrical energy to the network (4) in the format required of the network. The transformer system includes first high frequency transformer (40) connected to the battery, and providing an output to a rectifier (42). The rectifier includes a further high frequency transformer which is linked to a synchronization circuit (45). This provides synchronization signals to the first high frequency transformer and the rectifier, controlling the operation of these two elements of the circuit in order to provide a low frequency voltage which is attuned to the electrical network. The conversion circuits are reversible, so that when supply is available, the battery may be recharged from the electrical supply.