What resistor is required to dim a 150 watt light bulb to half power of 12 volt ciruit?

Resistance of bulb is

P = E²/R

150 = 12²/R

R = 0.96 ohms

assuming the resistance is linear with temperature, which it is not,

half power is 75w

75 = E²/0.96

E = 8.49 volts (note that this is not half the voltage)

I = E/R = 8.49/0.96 = 8.84 amps

resistor has to drop 12–8.49 = 3.51 volts

R = E/I = 3.51/8.84 = 0.40 ohms

rating of resistor

P = EI = 3.51 x 8.84 = 31 watts, use a 50 watt resistor.

edit, fixed dumb arith mistake.

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newly-wed's calculation gives you the effective resistance of the 150 watt bulb, but not the resistance required to give half the power. If the power is reduced to 75 watts, then the current flowing is 75/12 = 6.25 amps and the total resistance is 12/6.25 = 1.9 ohms. In theory you subtract 0.96 ohms for the bulb to get the value for the external resistance, but in practice the resistance of the bulb at half power is slightly less than at full power, so the figure is only approximate. The external resistor will still be dissipating 30-40 watts and needs to be appropriately specified (e.g. 60 W). This is a very inefficient way of reducing the lamp power. It is better to use some electronics to drop the voltage.

Other posters gave great answers. The one that thought you had a two strip light must have thought you had a 3 foot tank, which would have equated to that wattage. The coral referenced by others is very valid, and still applicable. You can add things like branching montipora to the SPS. Your leathers are moderate light leathers, and should do fine. Basically, stick with any coral that is low, low-moderate, and moderate light requirement classification and you will be golden! On the leathers you have and the feather dusters, be sure to feed a filter feeder food to keep them healthy. The other posters are correct, the yellow tail damsel is a small monster and the domino grows into a large monster! The blue damsel would be expected to be the lesser aggressor, but still not exactly peaceful. If doing the anemone, know they move! They can be difficult with coral, especially in a small tank, as they tend to sting or outright move on top of the coral!!! So, if you chose to attempt a bubble, wait on coral, if you want coral, ditch the anemone plan.

P = IE

IE = P

I = P/E = 150/12 =12.5 Amps

R = E / I = 12/12.5 = 0.96 ohms

BULB RESISTANCE 0.96 ohms

check: P = V^2 /R = 144 / .96 = 150 W

Current required for 75 watts thru .96 ohm bulb.

(assumes bulb of constant resistance)

P=I^2 R

I^2 = P / R

I^2 = 75/.96 = 78.125

I = 8.8388 Amps

Total resistance to limit current to 8.8388 amps when 12 V supplied:

Rtotal = 12V / 8.8388 A = 1.3577 ohms

Rneeded = Rtotal - Rbulb = 1.3577 - .96 =

0.3977 ohms

check: 12v / Rtotal = 12/1.3577 = 8.838 amps

I^2 R = 8.838^2 times .96 = 74.99 W

power is P=V.I you don't have I but you want to solve the problem using resistors, so using V=I.R, replace I by V/R

Now you have P=V.(V/R)=V^2/R

rearrange this equation so you have R on the left side, R=V^2/P

now put in the numbers, R=(12)^2/150=0.96 ohm

You can use a light dimmer regulator(electronic) for reducing the light discharge from the bulb for practical purposes,and (or)use I=E/R.formula.

1 ohm