Making battery eliminator and battery cooperate - Page 1
Forum section: electronics / beginners

author: DrSegatron
author_rank: Contributor
timestamp: May 02, 2012, 01:16:11 am
content: 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?

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 02, 2012, 02:28:24 am
content: 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.

author: amyk
author_rank: Super Contributor
timestamp: May 02, 2012, 08:46:42 am
content: 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.

author: DrSegatron
author_rank: Contributor
timestamp: May 02, 2012, 11:33:49 am
content: Quote from: TerminalJack505 on May 02, 2012, 02:28:24 amI 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 amI'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.

author: DrSegatron
author_rank: Contributor
timestamp: May 02, 2012, 11:57:52 am
content: I have these, should they work?www.fairchildsemi.com/ds/1N/1N3070.pdfI'm getting 0.4 V drop in voltage at voltmeter currents.

author: DrSegatron
author_rank: Contributor
timestamp: May 02, 2012, 03:41:29 pm
content: This is my test setup. I found a variable supply I could set to 4.5 V, and added three diodes more to drop the voltage.With only the wall supply I get 2.81 V over the 40k? load resistor.With only the battery I get 2.69 V.With both I get 2.82 V.I'll see if I can find a suitable resistor to get the component count down, and then see if the voltage is enough for the SRAM.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 02, 2012, 03:50:15 pm
content: QuoteThat should work.To answer your other questions...QuoteYou can likely just put a dummy load between the two terminals of the Nokia charger to get the correct voltage. A 10k or 4.7k resistor, for example. Just find the largest value resistor that works. The battery won't be affected by the dummy load since the resistor is on the wall adapter side of the OR-ing diode.How high a voltage the circuit can tolerate is hard to say. 3.3V (after the diode drop) is as high as you probably want to go.QuoteYes, those diodes should work fine.

author: DrSegatron
author_rank: Contributor
timestamp: May 04, 2012, 07:38:27 am
content: I didn't get the Nokia charger to cooperate, so I used a 3V adapter at 5V instead.I exchanged the button cell with two AAs. I'm getting 3.2V from the wall and 2.9V from the batteries. I tested the SRAM successfully with 2.2V, so it will hold the memory with a bit discharged batteries as well.OK, going to try to solder this thing together!

author: DrSegatron
author_rank: Contributor
timestamp: May 04, 2012, 11:39:10 am
content: I built it up and it worked on the bench, but once installed the adapter part gave only 0.65V. Probably my shoddy soldering; the adapter still outputs 5V outside of the circuit.I'm going to use it with only the two AAs a while, see how long they will last. Probably a lot longer than 6 months!Thanks anyway for your help!

author: 6502nop
author_rank: Regular Contributor
timestamp: May 04, 2012, 01:11:59 pm
content: If I may offer a solution that has been around for decades?http://www.jameco.com/webapp/wcs/stores/servlet/StoreCatalogDrillDownView?langId=-1&storeId=10001&catalogId=10001&categoryName=cat_2030&subCategoryName=Passive%20Components%20%2F%20Capacitors%20%2F%20Memory%20Backup&category=203075&refine=1&position=1&history=2vqckbbs%7CsubCategoryName~Passive%2BComponents%5Ecategory~20%5EcategoryName~category_root%5EprodPage~15%5Epage~SEARCH%252BNAV%40xfo5wh3a%7Ccategory~2030%5EcategoryName~cat_20%5Eposition~1%5Erefine~1%5EsubCategoryName~Passive%2BComponents%2B%252F%2BCapacitors%5EprodPage~15%5Epage~SEARCH%252BNAVThese caps work wonders. They are an essential part of the Symbol PDT-3110s I used to use (and fix) at my last job. I could take my sweet time swapping the NiMH batteries, and I would guess they allowed for about a good 5 minutes (I only took about 20-30 seconds to swap, so I'm not sure).I'm not familiar with the cartridge/SRAM (Nintendo?), but if it uses a battery socket, and not the solder-tab types, then this should be a one-off solution. Just solder a cap in parallel with the socket, or pre-charge a cap with 3V, and use micro-clips to the SRAM while swapping the battery.nop

author: DrSegatron
author_rank: Contributor
timestamp: May 04, 2012, 01:23:33 pm
content: Quote from: $ea on May 04, 2012, 01:11:59 pmI even got a few of the first one in the list! Definitely worth trying if I ever discharge the AAs.It's a Sega by the wayhttp://en.wikipedia.org/wiki/File:Sega-Saturn-Motherboard.jpg