7812 Battery Charger

I made this circuit.It will charge a 12V/1.2Ah battery & it will keep power when AC loss condition.

The problem is when I speedily switch ON & OFF the switch, the 7812 regulator drops down the voltage to 1V. So every time I have to power down the circuit & restart.When I remove the 100uF capacitor & speedily switch ON & OFF the switch it works nicely.

I checked without loading the battery as well.What made the 7812 to low the voltage?

Your circuit has nothing to limit the charging current when the battery is almost dead. If the current is too high then the regulator will shut down.
Also, you do not show the input voltage which might be too high and overheat the regulator until it shuts down.
Maybe the capacitor has a partial short circuit causing it to draw a high current.

thanks audioguru.

I never connected a battery yet.I also suspect the 100uF capacitor is short circuiting the 7812.I also replaced a different type 100uF but no luck.So I removed that.
Regulator is not heating.

The datasheet for a 78xx regulator shows a high frequency capacitor connected very close to its input pin and another at its output pin. You have a low frequency capacitor at the input so maybe the regulator oscillates at a high frequency and you have a diode at its output so again maybe it oscillates. Long messy wires on a solderless breadboard can also cause it to oscillate.