garage stereo

I have a garage stereo built off an Alpine deck and two amplifiers with 3 computer power supplies. Currently I turn off the power supplies when not in use to help them last longer. The problem with this is that all my presets and audio configuration gets erased every time. I need help designing something that will act as a backup power source so when I turn off my power supplies it will take over and provide enough power to store the presets. I have tried doing this with a relay and it didn't switch fast enough I tried using a capacitor with the relay and still couldn't get it to work. What would you recommend doing to make this work? Would a simple diode inline with both circuits work so the power supplies don't feed into each other? Will this hurt the deck? Did I possibly do something wrong with the capacitor to where it wouldn't work? Also open to other ideas.

find a wall adapter that outputs 12v. most will only output less than an amp of current but thats all you need to keep your memory. connect it to the yellow wire of your alpine with a diode with the stripe facing away from the wall adapter just to be safe and that should get you done cheaply.

Do I need a Diode in line with the computer power supplies also to keep the voltage from the adapter trying to flow to them?

yeah thats probably a good idea too

How can I determine the size of diode I need?

Almost any diode - my Alpine measures just under 10mA on standby (12.8V). I was going to suggest a 9V battery (thru a diode), but that will only last ~2 days at 10mA. They are often used in cars (for short-term battery disconnects). Maybe a small battery is an alternative - a 12V 1.2AH for 5-6 day reserve else larger. I'd probably just connect via a resistor (sized to limit the charge current from the supply) making sure a diode prevents it powering the power 12V feed and the PSU.

Will any diode work on both the bigger power supply and the smaller one?

I don't want to deal with battery

//POST EDIT (Z+1hr) - I'm not sure if the constant +12V does or can carry full unit current. Please confirm before fitting diode. end POST EDIT// Diodes are sized by forward carrying current, and reverse blocking voltage. For the memory function, only ~10mA should be required; certainly well under 1A. Hence the IN400x series should be fine. A 50V rating is sufficient (IN4001?) though usually the IN4004 (400v) or 1N4007 (1kV) are the commonly available versions. Just make sure you only use the diode for the memory (constant) power - not the main power (often 10A - 15A).

Do I not need to worry about the smaller power supply feeding into the computer power supply and damaging it?

Yes. But you should check the max current first. Else get 10A or 15A diodes (whatever), and probably a circuit breaker for to protect plugpack (or sacrificial diode instead).