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Making battery eliminator and battery cooperate

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DrSegatron:
I have an SRAM containing critical data (game saves), backed up by a CR2032. The problem is that it gobbles power and drains the battery within six months. There is also the problem with exchanging the battery as the circuit probably will discharge before I get the new battery in.

I thought of using a 3V battery eliminator which in turn is backed up by a CR2032; the power is supplied with the eliminator while it's plugged in, and whenever it is disconnected, the CR2032 takes over until the eliminator is connected again. What kind of circuitry would I need to make this work?

TerminalJack505:
You can very likely accomplish what you are trying to do with just two diodes.  Google OR-ing diodes for more information.

Basically you connect your wall adapter's positive output to the anode of a diode.  The button cell's positive terminal is connected to the anode of another diode.  The negative terminals of the wall adapter and the button cell are connected to circuit ground.

The cathodes of each diode are connected to each other and power is taken from this point.

The power source with the higher voltage will be the one that powers the circuit.  Because of this, the wall adapter's voltage needs to be a little higher than the button cell's.

There will be a voltage drop due to the diodes so Schottky diodes are typically used.  If the voltage drop isn't acceptable then they make ICs dedicated to this task.  These are called OR-ing controllers.

Edit: I originally had the anode and cathode swapped in my description.  You can find a schematic easy enough now that you know the name of it, anyway.

amyk:
Is it a standard SRAM like a 62256? If so, you can try replacing it with the so-called nonvolatile SRAM (nvSRAM). Cypress has a few offerings in this area.

DrSegatron:

--- Quote from: TerminalJack505 on May 02, 2012, 02:28:24 am ---There will be a voltage drop due to the diodes so Schottky diodes are typically used.  If the voltage drop isn't acceptable then they make ICs dedicated to this task.  These are called OR-ing controllers.
--- End quote ---

I have what I presume to be ordinary silicon diodes, and I found what I believed to be the perfect adapter to use with them: an old 3.7 V Nokia charger. But it seems to be outputting 9 V at no load.. :/

I'll have to search though my box of adapters. How high do you think I can go? 3 volts plus 10% tolerance + 0.6 volts drop over diode = 3.9 V?


--- Quote from: amyk on May 02, 2012, 08:46:42 am ---Is it a standard SRAM like a 62256? If so, you can try replacing it with the so-called nonvolatile SRAM (nvSRAM). Cypress has a few offerings in this area.

--- End quote ---

I'm afraid changing it is out of the question as it is SMD'd somewhere on the motherboard, and I'm trying to be as non-invasive as possible.

DrSegatron:
I have these, should they work?

www.fairchildsemi.com/ds/1N/1N3070.pdf

I'm getting 0.4 V drop in voltage at voltmeter currents.

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