Transistors for switching between the powers source and battery backup

Hey guys, you helped me out several months ago with one of my problems, so I hope that you can help me with this also (it shouldn't be that hard).

So I am using the chip DS2016 for some external memory in my small project. When in operation, it will be plugged into the wall, and through a converter which will give the chip 5 Volts to run on. This is fine. But the chip also has the ability to run on lower power mode and so I have a smaller battery (3V) which I want to use as a battery backup when the board is unplugged from the wall. The problem, though, is that I don't want to keep the backup battery hooked up the entire time (since that would drain it).

It seems to me that a simple transistor would switch between the two power sources, but i can't get it to work out. I think my PNP transistor is not the correct type, and I don't think NPN transistors can do what I want. Both the NPN and PNP are from Radio Shack (for convenience) and the link to them are below.

h**p://www.radioshack.com/product/index.jsp?productId=2062586&cp=2032058.2032230.2032279&parentPage=family
h**p://www.radioshack.com/product/index.jsp?productId=2062585&cp=2032058.2032230.2032279&parentPage=family

Thanks for any help.

-nshemon

also here's a link to the chip:
h**p://www.maxim-ic.com/quick_view2.cfm/qv_pk/2835

Re: Battery Backup

Hi nshemon,

Why don't you connect your RAM to the 5 volt power supply and to the battery
via two scottky diodes like BAT85. You could even replace your battery by a
rechargeable battery if you bridge the diode with a suitable resistor so that it
can recharge when the 5 volt power supply is switched on.

on1aag.

Re: Battery Backup

Thank you on1aag. That's a good idea, and I will probably end up doing that. But is there a reason why you mention the Schottky barrier diode? Is there a reason why any old diode wouldn't work?

Also, I wouldn't mind hearing a way to switch back and forth between the two power supplies like I mentioned in the beginning. If not for this project, I think it could be useful for my general knowledge.

Thank you again.
nshemon

Re: Battery Backup

Ordinary diodes have about 0.6 to 0.7V drop.
To further reduce voltage drop to a minimum, I would recommend
p-channel FET's. If you visit Fairchild, IR, or Vishay, you will be
able to find more information on FET switches.
Regards,

Re: Battery Backup

Hi nshemon,

An ordinary diode could also be used, but the voltage drop will be much higher
than when you use a Scottky barrier diode. A p-channel FET can not be used
because the internal structural diode of the FET would allow the 5 volt supply
to fry your battery. :!:

on1aag.

Re: Battery Backup

ok cool, this all sounds pretty good. Just a couple questions. If I were to go for the rechargeable battery thing, how would I find out what size resistor I needed? Also, I grabbed this battery off of an old motherboard, and I'm not sure if it's rechargeable, but I don't want to go out and buy another battery if this one is rechargeable. Is there any way for me to find out if it's rechargeable or not?

Thanks for all the help so far.
nshemon

edit: So after thinking about this some more I realized that there is still one problem that needs to be solved. When the main power is unplugged, I need the memory to go into "standby" mode. To put the chip into standby mode I need to bring one of the pins to high, while I need that same pin to be low when plugged into the wall.

sorry for not mentioning this in the beginning...

Re: Battery Backup

Hi nshemon,

When it's a rechargeable battery, the capacity is usually marked on the battery
which makes it easier to identify. But if it comes from a motherboard it's gonna
be at least ten years old, you've got to be a little realistic about it's performance.
Nevertheless it can still be used for testing. A battery from a motherboard is usually a 3,6 volt/60mAh type and the charge current is one tenth of it's capacity. The time to charge the battery with 6 mA to it's full capacity will be between 14 and 16 hours. But in this case it will be considerable longer because the charge current wil drop as the battery voltage increases.
But I didn't realize that your RAM was so hungry, therefore I had to make a few changes. Due to the current consumption of the RAM (and the charge current for the battery), the voltage drop caused by the diode would increase to 470 mV.
This would mean that it would take a very long time to charge the battery. You could add an extra diode in series with a 220 ohm resistor from the 5 volt supply directly to the battery, reducing the voltage drop to 300 mV because
the charge current is only 6 mA.

on1aag.

Re: Battery Backup

Make sure it is not NiCad battery as charging it like that (almost continouosly) wouldn't make much good to it .. i.e. it won't last long ..

Also, there are special Nonvolatile Controller Chips, for example the DS1210, so keep this option in mind, too ..
**broken link removed**

Regards,
IanP

Re: Battery Backup

wow, that's awesome guys. I think I am going to go with the NV controller. I hadn't heard of these before.

Also thank you on1aag, the rechargeable battery sounds great but I think it might be best for me to go with a pre-packaged chip.

Thank you all,
nshemon

Re: Battery Backup

A question to on1aag, or anybody who knows

How do you size the resistor?
I guess it depends on the batery capacity, but what is the formula?

Cheers