Can anyone help me? I would like to make a simple uninterrupted power supply for a nerd kit project. My hope it to retain volatile memory variable values thru short to medium power interruptions. I am considering connecting my 12 V DC power adapter to the nerd kit board thru a diode (in series). As a battery back up I could also connect a 9 v battery to the board also thru a diode, in effect putting the two possible power sources in parallel into the regulator. The idea is that the 7805voltage reg. should handle ether input voltage and deliver a 5 volt output to the board. Since 12 volts is higher than 9 the diode in series with the battery will be reverse biased (I think that is the term) when ever the 12 volt dc is available from the adapter. The battery will not supply any current because 12 V is higher that 9 and the adapter should not supply current to the battery because of the diode. If the adapter power in interrupted for some reason the voltage on the input side of the regulator will quickly drop below 9v and current will begin to flow from the battery to supply the regulator, the diode in series with the adapter should stop any current from flowing back thru the adapter and killing the battery. What I would like to do is have a system in place to single the micro that it is now operating on backup power. By using a pin change interrupt it would be relatively simple to put the system into a power conserving mode in order to retain volatile variable data on the chip. My thoughts are that I could use a couple of high value resistors to divide the 12volt supply voltage down to 5 volts on the adapter side of the diode. With no power supplied from the adapter or the battery these resistors would pull the voltage down to the point where the micro would detect a pin change. If the 12Volt power is restored there would be a pin change and the micro would detect and then continue with its regular functions. I plan to use ether a 74XX family inverter or trigger or possible even an optical isolator (that I happen to have on hand) between the voltage divider and interrupt pin on the micro to reduce the chances of any stray high voltage getting around the regulator and effecting the micro.

Will this work? What other things should I consider? Is there a better way to accomplish the same thing?

If you have two supplies in parallel but isolated by two diodes, the higher voltage will be doing the work. This leaves the lower voltage available for backup. If you lose the higher voltage, the switch is completely seamless. So, you can have a 9 volt main supply for normal operation and a 7 volt backup.

You could always use some Super Capacitors like these. Of course you have to be really careful, and should probably get ones rated for a higher voltage, but these should run your project for some time.

Hi sask55,

Depending on your application, you might even use that signal of the 12V dropout to trigger some code that writes certain variables to EEPROM and then puts the microcontroller in a very-low-power sleep mode. That way, even with only a few hundred milliseconds of "backup" time from something like a large capacitor, you might be able to save and recover when power comes back on.

Mike