On Wed, 14 Feb 2007 08:58:53 -0800, "Jack" wrote:


"HK" wrote in message
news
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.



In parallel you are dividing the current. In series you are dividing the
voltage.

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.



There is no threshold for the incandescent lamp, after all its an analog
device and light output is varied from zero to 100% continuous with respect
to varying the voltage. You don't see it because you haven't enough current
to heat the filament up - see if you could see it in the dark. I'm sure if
you have a 6V car or motorcycle battery instead of the C cells, the 12v lamp
would light up.


An incandescent lamp gives off light because the filament is hot. The
frequencies given off depend on the temperature of the filament. To
see light, you need enough current to heat the filament enough that
it's temperature corresponds to frequencies in the visible range.

If you get light from a lower voltage/current, the light will be
redder (or yellower). These colors correspond to the low end of the
visible spectrum. At full voltage you get more blue (high end
frequency).
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy