12 Volt lighting?

Question

If am using a 12 volt, 20 watt,, mr16 bulb. it is recommended you must feed 12 volts for the lamp to burn (light up) properly. Now, what if I feed it only feed it 10 volts... is the lamp now consuming less than 20 watts or does the lamp still consumes 20 watts except now is dimmer.. please provide resources if applicable. Thank you

I want to thank all of you for answering. All of you gave me A LOT more than what I was looking for..Thanks for sharing...

Answer 1

Facts:

W = IV

W / V = I

20/12 = 1.667amps (original current)

If you energize it at 10 volts, then the voltage will go down

V = IR (current will go down)

W = IV (now current and voltage both went down, so wattage will go down by a greater factor than voltage went down)


Opions based on experience:

Final wattage will be ~ 14 watts, making the bulb about 2/3 as bright)

Answer 2

The only thing that is set in the light bulb is the resistance across the coils. So you feeding the bulb only 10 volts will
decrease the power output from the bulb and it will burn dimmer than it would with 10 volts.

Using P = I V for the 12 volt setup you get a current of 5/3 amps

Then using V = I R you find that the resistance is 36/5 ohm. This will be a constant resistance.

Then using V = I R for the 10 volt setup you find that your new current is 50/36 amps, slightly less than before.

And plugging that current into P= I V you get a power output of about 14 watts.

Running a light bulb lower than it's rated voltage will also increase the life of the bulb

Answer 3

I wasn't going to respond but rather give lare a "thumbs up" for his accurate and thorough answer, all the way up to that last paragraph - running a tungsten filament bulb at lower than it's design voltage most definitely increase the bulb's life. There is no increased contamination, the filament is heated in a vacuum. And a 10% decrease in voltage will about double the bulb's life. Granted, you don't get as many lumens per watt, but still, there definitely is advantage.

Oh, yes Hecc(?), as above, at the lower voltage, you will not be dissipating 20 watts, more like 25/36 of that or a bit more, as in 17 or 18, depending on the filament.

Answer 4

ok the previous answers are ok for "resistors".

a tungsten type lightbulb does not have constant resistance, but varies marketly with temperature. at room temperature, a typical non-operating lightbulb has a resistance that is about 1/10 the resistance that it has at normal operating conditions, check it out with an ohm meter.

if you take a lit bulb and start reducing the voltage, it starts to cool, lowering its resistance. the upshot of this is the change tends to keep the current drawn somewhat steady over a range of applied voltage. infact in the 20's and 30's a filament bulb called a ballast tube was used as a constant current regulator in some radio equipment.

in your case, the operating current at 12 volts is about.1.66 amps. if it were a "resistor" the current at 10 volts would be 1.38, ie it would reduce in proportion to the voltage. However the actually current would be closer to 1.55 amps due to the lower filament temperature.

The wattage consumption does not stay constant with changing voltage, but rather it is nearly proportional to the voltage over a range near the nominal voltage. In this case it would be about 15 watts. In a true resistor, the watts will vary as the square of the voltage.

FYI, the reduced voltage will not increase the operating life of the bulb because at the lower temperature, the filament will quickly get contaminated and fail. Increasing the voltage will also lead to premature failure as the temperature gets hot enough to melt the filament.

Answer 5

No. It is consuming less power. The voltage is down and the resulting current is down so the power (voltage x current) is less. That comes from the old and popular Ohm's Law.

The problem with cutting the voltage is that the lamp will not give out most of its light till you get close to full voltage. If you cut the voltage 10% you could lose 25% of the lumens the bulb could put out - a major loss of efficiency.

Even worse, the MR16 is a halogen bulb and their efficiency drops sharply at lower voltage and their life expectancy (which depends on their being hot enough to keep the filament strong) goes way down.

Answer 6

No it has to be 12 volt

Answer 7

V = IR
I = V/R

so that means you get less current.

Now, P = IV
Your voltage decreased, your current decreased, so that means your power decreased.

Now let's look at numbers:
P = VI
20 = 12I
I = 1.67A
R = V/I
R = 12/1.67 = 7.19 ohms
if you were to use 10 volts:
10 = I(7.19)
I = 1.39
P = IV = (1.39)(10) = 13.9 W

So you have a difference of almost 6 watts

Answer 8

The lamp is using less power when you dim the bulb. Power is directly proportional to the current or voltage. If you reduce the current or voltage, you will directly reduce power absorbed by the bulb.