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Switches SW1 and SW2 provide a means of disconnect from the power supply and battery power sources. This part of the circuit is safety related and MUST NOT BE OMITTED. Although the switches available through Radio Shack are lighted, DO NOT ATTEMPT to connected the lamps, they are wired for an automotive "ground to activate" scheme. That would cause the power source to be directly connected to a dead short! Relay Ry2 is the workhorse that does all the work in this circuit. It provides the switching between the power supply and the battery sources. Notice that the relay is a DPDT (double pole, double throw) type and the contacts have been wired in parallel to increase the current handling capacity. During normal operation, relay Ry1 is powered by 120VAC. Contact Ry1 is closed and with switch SW3 in the automatic position, relay Ry3 is powered. Contact Ry3B is closed and relay Ry2 is powered. Contacts Ry2A and Ry2B are closed which supplies power from the power supply to the load. If AC power is lost, relay Ry1 drops out which in turn drops out relay Ry3 which drops out relay Ry2. The normally closed contacts of Ry2 then supply power from the battery to the load. This feature causes fast switchover in the event of AC power failure. Most power supplies have large filter capacitors that can cause long switchover delays as the capacitors discharge. When switch SW3 is placed in the battery position, relay Ry3 drops out and the unit switches over to battery in a similar manner to an AC power failure. This is especially useful for periodic testing of the battery under load. The unit will also try to find a valid source of power if one of the disconnect switches is turned off or if one of the power sources becomes unavailable. Diodes D1 and D2 form an OR circuit to provide power to LED1 and LED2 through current limiting resistors R1 and R2. LED1 or LED2 will light depending on which power source the unit is selecting. Current limiting resistor R3 and light emitting diode LED3 provide indication that power is available to the load. Optional switch SW4 and meter M1 provide voltage monitoring of the power supply and the battery at all times regardless of the disconnect settings or the mode of operation. | ||||||
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