DC power supply questions

**broken link removed**

suposely i hv this AC to DC converter and output is 5V to a microcontroller board.
If electricity cuts off, the board will shut down as well. What i wanna do is at the input of the LM7805, i parallel it with a 12Vdc supply. But it will still draw current from it.
How can i do in such a way that when the AC is off, then only use the supply from the 12Vdc supply??
plz help, thankx

you can use two standard diodes.

er.....can u explained more y the input from the bridge needs to be at least 0.3V higher than the battery??
FOr the Ored diode connection, the supply will stil draw current from the battery although the bridge is ON, rite?? I would like to know smth circuit connection which can automatically switch from AC(bridge) to DC(battery) when AC is OFF.

By the way, in my first thread, the stepped down Vac is let say 9Vac, after passing thru the bridge rectifier, how much will be the Vdc??
If i wan to get Vdc = 12V, wat should be the stepped down Vac??
thankx

for 9Vac rectified Vdc will be near to 12.5Vdc and on connecting the load this 12.5Vdc will decrease with increase in load current.


to get 12Vdc 9Vac is good option.



See there are two diodes cathodes of boths are shorted and anode of both get positive voltages from bridge and battery.

Now a diode is forward bias if its anode is at higher potential than cathode.
In your case when Vdc from bridge rectifier is 12V dc and battery voltage is 9V dc
then upper diode will conduct and lower diode won't as anode of lower is 9V and cathode is 12V hence lower is reverse bias and virtual no current flows from battery to ckt.


But this ckt won't work with 12Vdc battery
if u need u have to change Vac to higher like 10Vac or 12Vac.


Wrong: lower diode is reverse bias when bridge is on so no current drawn from battery.


this will work fine.

You can use a 12 volt battery (full charged voltage is around 13.8 ), select your power supply voltage at 14.2 volt.
when the input from adapter falls below 13.8, the battery will start delivering the power.

for 14.2 Volt dc or more you have to use 10VAc or 12Vac transformer.