On Feb 14, 10:21 am, "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.

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.

Lots of factors are at play here. Changing the supply voltage will
change the filament temperature, and hence the filament resistance.
So cutting the voltage in half will not cut the current in half.

I made a spreadsheet which accounts for the changes in temperature and
resistance, and here's what I found:

A 100W, 12V lamp, when run at 6V, will require 5 or 6 Amps and run at
30 to 35 Watts. The filament temperature will be in the range
1800-2100 C, and will definately be visible. BUT you're 6V supply
must be capable of 6 amps, which your battery of "C" cells is NOT.

A 10W, 12V lamp will need almost 0.5 to 0.6 A at 6V, and run at 3 to
3.5 Watts. I'm not sure if C cells are capable of this or not.

A 1W, 12V lamp will need only about 50-60 mA, and your C cells will be
sufficient to power it.

Hope this helps.

Mark