Making a battery backup for an AC adapter

[Tom Doolittle]

Hi,

I have a home electronics project that I was hoping to get a little help
with. I have a battery operated toy (a Lego RCX programmable brick - part
of Lego's Mindstorm robotics set) that allows you to code simple robotic
programs on your computer and download them to the programmable brick via an
infrared connection. The particular version of the programmable brick that
I have operates on DC power (6 AA batteries, or 9V of power) and does not
have an AC adapter. However, I need it to run off of an AC adapter so the
batteries don't constantly go dead. I've managed to rig the unit so that it
runs off a 12V AC wall-wart adapter (I used a voltage regulator to get the
power to a consistent 9v), so I have that part of the problem solved.

However, the problem that I have now is that I need to be able to move the
programmable brick from an AC outlet in one part of the house to an AC
outlet in another part of the house. This is a challenge since the
programmable brick stores the downloaded programs in RAM, which is erased as
soon as the unit is unplugged, which means that when I plug the unit in
again, the program has been erased and the programmable brick is pretty
useless : (

So what I want to do is make a temporary battery backup from a 12V camera
battery (or some other type of battery pack) so that I can unplug the unit
for a minute, have the programmable brick run off of battery power for a
minute while I move it to another AC outlet, then plug the unit back in and
have the power switch back to the AC adapter. This will allow me to move
the unit from room to room and not have the RAM erased.

I'm not an electrician or an engineer, but am capable of figuring most
simple electronics out (at least I think I'm capable - I could be wrong : ).
So I have a conceptual idea of what I need to do, but am not sure how to
make it work from a practical standpoint.

I have drawn some simple diagrams that I hope illustrate the problem and
would be grateful for any suggestions. The diagrams are here:

http://home.attbi.com/~leifpearson/projects/BatteryBackup.jpg

Thanks alot for any help,

- Tom

[Dave M]

This problem should be simple to solve. Just wire a diode in series with
the battery to the robot. Wire another diode in series with the output from
the wall wart to the robot. Tie the cathodes of the diodes together and
join them together to the robot. Just be sure that the diodes are capable
of handling the current drawn by the robot. Judging from your use of AA
sized cells, I would suggest 1N4001 diodes, which are rated at 1A.

Battery ---------->|--------+
|
|
Wall Wart ---------->|--------+------------- Robot

The power will be supplied by the device that has the larger output voltage.
For instance, if the wall wart output is 9.5VDC, and the battery pack output
is 9VDC, the wall wart will supply all power to the robot until its output
drops below the 9VDC level of the battery pack. When the wall wart output
drops below 9VDC, the battery pack will supply all power to the robot until
the wall wart output is restored to a higher level.
You may need to readjust the wall wart's output to a level that is slightly
above the output voltage of a fresh battery pack.
--
Dave M dgma...@atxxt.net
Remove the xx for my correct email address

"Tom Doolittle" <tom_doo...@excite.com> wrote in message
news:0LSc9.274572$983.532296@rwcrnsc53...

[Tom Doolittle]

Thanks alot, Dave - that's a huge help. I think I have it, but what happens
to the ground (neutral) wire in this case? Can I run a ground from the
negative terminal on the battery, join it with the neutral wire from the
wall-wart, and then run it to the robot? Here's my diagram of what I think
the thing should look like - do you think this will work (all the normal
disclaimers apply, I'm doing all of this at my own risk, nobody but myself
can be held responsible if I electrocute myself, etc...):

http://home.attbi.com/~leifpearson/projects/BatteryBackup3.jpg

Thanks,

- Tom

"Dave M" <dgma...@atxxt.net> wrote in message
news:WtTc9.7565$jG2.5...@bgtnsc05-news.ops.worldnet.att.net...

[Ned Konz]

Tom Doolittle wrote:

> Thanks alot, Dave - that's a huge help. I think I have it, but what
> happens
> to the ground (neutral) wire in this case? Can I run a ground from the
> negative terminal on the battery, join it with the neutral wire from the
> wall-wart, and then run it to the robot?

You don't have grounds that I can see (at least in your diagram), but you do
have to make a complete circuit, so you have to hook up the negative ends
of the battery and wall wart as well.

> Here's my diagram of what I
> think the thing should look like - do you think this will work (all the
> normal disclaimers apply, I'm doing all of this at my own risk, nobody but
> myself can be held responsible if I electrocute myself, etc...):
>
> http://home.attbi.com/~leifpearson/projects/BatteryBackup3.jpg

Looks good to me.

[Tom Doolittle]

It works! The last diagram I published is the design I used
(http://home.attbi.com/~leifpearson/projects/BatteryBackup3.jpg), and I got
it all cobbled together last night.

Thanks to everyone for their help.

- Tom

"Ned Konz" <n...@bike-nomad.com> wrote in message
news:un9k26c...@corp.supernews.com...