For my wife’s birthday I bought an installed a car stereo system into our bath room so she could listen to music while getting ready for work in the morning. It is currently powered by an old computer 100w PSU, but it has to be on constantly or the radio losses the presets memory and settings. I think I might have found a way to make the computer PSU switchable from the stereo its self, but I am not 100% if it will work.
So here is my idea, the PSU has a +5VDC line that is always on, I think it will provide enough power to keep the stereo's memory. The stereo has a line that signals an AMP to turn on and off, I want to use a couple of diodes to connect both the +5VDC constant and the +12VDC switched to the +12VDC power and ignition Indicator so that I can provide enough power to maintain the stereo memory when off and enough power when turned on without any feed back across the different lines and voltages. The idea that the +5VDC is always on, provides the power to the memory and when the power button on the stereo is turned on, the AMP line switches on to signal an AMP to turn on, by connecting that to a relay, I can cross the PSU indicator turning on the PSU, which will then provide the full +12VDC 100W the Stereo needs. When the power button on the Stereo is turned off, the AMP switch line turns off, causing the relay to reset, therefore turning off the main power +12VDC to the stereo, but leaving the +5VDC on to provide enough power to maintain the memory.
What do you think, Is +5VDC enough to maintain the Stereo memory?
Will the two diode system allow both the +5VDC and +12VDC be connected to the same line on the stereo line in while protecting the two different Rails on the PSU?
Do you think I need to run a sperate 9VDC battery to suppelment the power from the +5VDC rail when the PSU is off?
Will I need to use some capacitors to supplement the voltage or current?
Just assume I have absolutely no clue about anything and I am a complete nofice, I promise you won't hurt my feelings.
Your idea will work if the 5 volts from the PC PS is high enough. If not, the 9 volt battery voltage should be high enough. I say this because the stereo is designed to maintain the memory during car starting where the voltage can drop to 7 volts or so. If the 9 volt battery is needed, how long will it last? A DVM set to the mA range can be used to measure the stereo current draw from the 9 volt battery. A common 6LR61 Alkaline battery stores about 500 mA-hours of charge. Divide 500 mA by the stereo OFF current and you will know how long it might last.