Battery converter 1

[CUTiger]

Hello,

I am looking for help with a high school class science olympiad that I am coaching. We need to power a number of different electronics off of the same battery. How complicated is it to run something that was originally powered from AA's, AAA's, 9v, or C batteries, to now run off of a larger battery? The high school has minimal money to spend, so something simple would be ideal.

Thanks!

 

[cbarn24050]

You would think that in a high school there would be a teacher who knew the answer to that question.

 

[IRstuff]

Depends on the number of actual voltage levels required. You obviously have 9V, but based on the other batteries, you could have 1.5, 3.0, 4.5, 6.0, etc.

If the power draw from any given project is not excessive, you can probably get adjustable 3-terminal regulators with the associated variable resistors and filter capacitors for about $5 per voltage required. It may also be possible to run an inverter to run some off-the-shelf AC adapters with selectable outputs. These are a probably in the $10-$20 range per voltage required.

You might also look to see if the project can be reconfigured to run from a particular voltage to cut down the number of simultaneous voltages required.



TTFN

 

[CUTiger]

Thanks for the help IRstuff. I will check into the $5/voltage option.

 

[OperaHouse]

Just want to mention that there can be problems if these seperate devices are connected to each other as they might not share the same common "ground" of the battery. Look for a LM7805 three terminal regulator. A 470 ohm resistor can be connected from the output to the common terminal. Then a 500 ohm pot is connected from this common to the negative supply. You will need small caps on the input and output. This will give you an adjustable supply from 5V to a little over 10V to power thos 6 and 9V applications. For lower voltages, one or more diodes can be used to drop the voltage from the 5V minimum of the regulator. Just figure 0.6V for each diode. Figure 3 diodes in series for a 3V supply.

 

[VolkerR]

If you want to go with an adjustable 3-terminal regulator, the LM317 in TO220 is quite popular

Datasheet:

http://cache.national.com/ds/LM/LM117.pdf

Digikey link:

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=147559&Row=260343&Site=US

But keep the current consumption in mind. 3-terminal regulators just burn the excess power and are not very efficient (If you drop from 12V to 6V then the regulator wastes as much power as your gadget uses.) You might need a heatsink.

If you need efficient power conversion, go with "Simple Switchers". (The circuitry is more complicated, you need an extra coil and a Schottky diode, but the efficency is around 80-95%)

 

[richs]

Hi-

If these are "stationary" projects (not like robots),
then could you consider running them off a small transformer
and power supply?

If you have a "broken" appliance with a "wall wart"
transformer, then you have the most expensive part of
the variable power supply.

If it is an AC supply and around 9-12Volts or so, you
are in business. I would suggest looking for a bridge
rectifier (4 diodes) and a fairly large filter cap with
at least a 25V rating. Or, maybe a couple in parallel
again, these might be salvages from broken or other
cast off electronic circuits (Things like plug in
boom boxes and the like).

Most likely, the things that you are powering aren't
going to draw more that a 1/3 of an amp or so (I'm guessing)
but, what the heck. See if the wall wart will draw that.

After the filter caps, you will definitely need a
voltage regulator of some kind. Either one mentioned
will work, my preference would be the LM317, but what the
heck. I think that this part is available at Radio Shack,
but if you have another electronic parts outlet in the
area, you might find it cheaper there. Between the
filter caps and the regulator, you might want to switch
in some series resistance. And throw a heat sink on
the LM317. As mentioned, any power not passed to the
device will be dissipated as heat by the regulator. This
could be something as simple as a cut up tin can,
preferably with some heat sink compound between the
regulator and heat sink. A simple 4-40 screw and nut
can be used to mount the heatsink (please note that one
of the regulators inputs is connected to the metal tab,
so there will be a potential between the heatsink and
ground. I'm sorry I forget which voltage it is......

The schematic for the LM317 for can be found on the
internet. As an example you can go to:

http://cache.national.com/ds/LM/LM117.pdf

and get it. The example in there will guide you through
the rest of it.

Good luck with the project. You might get one of
your contestants to work on the project, or if possible,
use it as part of the contest?

Cheers,

Rich S.

 

[Heydave]

Is this a single project or a group of seperate projects? If it is a single project, I suspect there may be other signal connections between the electronics. Using a common power source can cause problems if this is the situation.

One problem that I see very likely in a multiple project situation is errors. Wiring errors and other problems as well as plain clumsyness can result in short circuits that can drag down power supplies and even damage them.

I am guessing that rules require only battery power with low voltage and that is why you said one large battery. The easiest thing you can do is the set up an array of 6 "D" cells in series to produce 9 volts. This is your highest voltage. You can tap voltages between the cells to get the other voltages. "AA", "AAA", "C", and "D" cells all have the same voltage. Adding 6 of these in "series" is 9 volts. In this way, you have one set of large batteries to power everything. To extend the life of the pack, rotate the cells after every day's use shifting the bottom battery to the top and moving all others down. Unlike any electronic regulator scheme, this is the most energy efficient and least expensive. You might consider using 1 amp in-line automotive fuses on each voltage tap to avoid burns or fire from accidents and errors. Do not use a fuse in the negative lead which is shared with several projects and be sure it is a solid connection. If it breaks or is opened when several projects using different voltages are in use, some projects will get reverse polarity and may be damaged.

 

[automatic2]

the mention of current is of all importance. If you are distrubiting power to various workstations off of a larger storage battery (automobile), you'll want to pay close attention to limiting current prior to output termination. Again as mentioned, you wouldn't want someone accidentially playing with welding currents. If you are using automotive batteries, make absolutely sure they are properly stored(vented, enclosed). Include a disconnect external to the enclosure, along with a fuse for short circuit protection. Use appropiate wiring practices for branch distribution, considering fusing on the legs. The 3 terminal voltage regulators are great, and provide current limiting/shutdown for final output.