On 14 Feb 2007 08:42:08 -0800, "dpb" wrote:

On Feb 14, 10:23 am, "Bob" wrote:
On Feb 14, 10:03 am, "Toller" 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.

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.

It is an interesting question actually.
A 15w 12v bulb, when wired to a 6v battery draws .6a. Apparently your
batteries can't produce .6a, so the bulb doesn't light up. Wouldn't be very
bright anyhow.

You're on the right track. The bulb filament glows from the amount of
current through it. Too much current and the bulb is overbright or
blows. Not enough current and the bulb glows weak or not at all.
Current is a function of voltage divided by resistance. Higher
voltage bulbs have higher resistance filaments. Therefore 6v divided
by higher resistance equals less current and less glow.

And, to follow up, OP could take the bulb and his VOM and measure the
resistance across the filament and see what it is to see what the
current draw should be...or maybe the bulb isn't good, even?

Or, of course, take enough batteries in series to make 12V ...

That won't increase the current capability. It may still be
insufficient for that bulb.

What is the current capacity for new C cells?
--
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