Does it matter which kind of diode?

[Super-Dave]

Okay, I'm a well rounded handyman and a DIY kinda guy. I took an older car stereo, hooked it up to a 12v transformer & built a box for it to use in my mancave. I also built boxes for the car speakers too. Looks and sounds decent. Problem is, anytime the power blinks or there's a bad storm in the area, I lose all the presets, clock, and settings. (I HATE that color shifting "demo mode" that it defaults to) Every day for the last 5 days we have had thunderstorms. And every day I have to reprogram the damned thing.

So I researched battery backups & found a simple memory saver circuit.

My radio does indeed have a separate memory wire. I have all the components needed, including 9v plug wires & fresh batteries. But the original post failed to say what kind of diode is required. I have 1N4007's, 1N4735A's & 1N4148's.

2 are schottky's, the other is zener. For this circuit, does it matter which kind of diode I need? The battery is only needed to maintain power to the memory circuit when the main power blinks or goes out. A 9 volt should be worth a couple hours of backup, so I don't want a diode that has too much forward voltage drop, and when power is restored, the 12v should override the 9v battery diode and prevent it from discharging until the main power blinks again or it is unplugged and disconnected, right? If you were in my shoes with these choices, which would you pick?

I surrender to the collective wisdom and experience of the forum. Thanks in advance for your reply.

 

[crutschow]

2 are schottky's, the other is zener.

The two are standard junction diodes, not Schottky's, but either should work.
If you need a slightly lower forward drop (which I don't think you do here) you can use a Schottky type.

 

[Diver300]

Depending on the power supply you are using, there may be no need for a diode between that and the radio, so the one labelled D1 isn't needed. The only reason for a diode is if the power supply takes a lot of current from the 9 V battery, which is unlikely if it's just to support the radio's memory for a few seconds.

It would be very strange for a power supply to be damaged by subjecting it's output to a voltage between 0 and it's rated voltage, as the output voltage goes through that range every time it turns on or off.

If you do have a diode, you might need a more powerful one than a 1N4007, which is only rated at 1 A. A car radio can take a lot more than that.

Car radios will normally cut off if the voltage dips, as that is what they do when the starter motor operates, so D2 can be lower current rating.

 

[crutschow]

My radio does indeed have a separate memory wire.

Apply 9V to that wire and measure the current.
That will tell you how long the battery would last.

 

[shortbus=]

2 are schottky's, the other is zener

Just a wild guess, but any of them but the Zener should work out. I say wild guess because I'm just a hobby type guy.

 

[Super-Dave]

Well, I tested all the diodes I have, the 1N4007 only had a 0.25 forward volt drop, more for the other 2 diodes so that's what I went with. Soldered & shrink wrapped all the wires, tested it out and it works great! After I reassembled the box, I plugged it back in, reprogrammed all my stations, clock & settings, yanked the cord back out and went to have a cup of coffee and a smoke. Came back in, plugged it back into the wall outlet and all my settings were still there. I consider this a success.

Now when the next thunderstorm blinks the power again, I only have to worry about the coffeemaker, the microwave clock and the oven clock to reset. My Dave-Cave radio will be immune to short term power outages.

 

[Dick Cappels]

Measuring the current as crutschow described in post #4 will also give you an idea of the current rating you need for the backup diode. Do it with a 12V battery and you will know how much current goes through the diode in series with the power supply.