Pertronic Industries Pertronic School Bell Power Supply is designed to give a 24 volt battery backed DC supply for school bell and similar applications. The unit uses an efficient switched mode power supply to float charge sealed lead acid batteries. The charge voltage is accurate and stable thus ensuring a long battery life. The School Bell Power Supply has a selectable three or four wire bell circuit for wiring to existing or new bell circuits. The bell circuit can be activated from a time switch or manual call point input, or from the bell test switch located on the front panel.