On Wed, 14 Feb 2007 10:21:27 -0500, "HK" wrote:

I'm trying to help my daughter with a school project and while I know my way
around house wiring, I'm a relative newbie when it comes to low voltage.

Let's say I have a 6 volt DC power supply (4 "C" batteries). If I want to
drive a small motor and some lights, I just find 6 volt motors and lights
and wire them in parallel or series.

Parallel. If you put 6 volt items in series, the required voltages
are the sum of the individual voltages. Just like xmas tree lights
that only need a couple volts, but times 50 and you get 110 volts and
they plug into the wall directly. If you tried to use a 2.5 volt
power supply with 50 xmas tree bulbs in a row, you'd get no light.
Same with nmotors.

Why is it that I can't light up a 12v light with a 6 volt power supply? I
always test my batteries using a multi-meter and as the battery ages, the
voltage drops. With low voltage, the device (flashlight, etc.) still works
but the light is weak. So, wouldn't a 12v light just be weak if I use a 6v
power supply?

Is there some kind of voltage threshold at which a device won't work?

Like I said, basic question.

mY aNswer is going to conflict a little, maybe, with the first two,
but not directly.

When a battery is running low, it still has most of the voltage it did
when new. It was surprising to me, but the chem teacher did the
arithmetic in front of us, and he didn't make any errors.

So when a battery is 80% dischagred it still has 80% of the original
voltage. It might even have been that when 90% discharged it had 90%
of the orignal voltage. So by the time a 12 volt battery is down to
6 volts, it must be 98+ discharged, and it's not going to have enoug
output to actually make light from a 12 volt bulb. Frankly, I don't
even think amperage is the problem, and I don't think a 12 volt large
size car battery could light a 12 volt bulb when it is so discharged
it puts out 6 volts, though I don't remember ever having a 12 volt
battery that was so low.** Or if output ampterage is still linited,
assume 100 such batteries in parallel, but still with only 6 volts.

Even a brand new 6 volt battery, even a big one like a 6 volt car
battery (still used for golf-carts??) I don't think could light a 12
volt bulb.


**Well actually now I do. It's a gelcell I bouught for testing
indoors, and it is down to about 5.5 volts, maybe partly because when
it was 7, I connected a battery charger backwards! and drove it even
lower. So in the next day or two I'll try to try it with a 6 volt
bulb.