# Constant current source and halogen light bulb

What happens to a halogen light bulb specified at 10V DC and 1A (10W) when you connect it to a constant current source that runs on 24V DC and can be regulated up to 0,8A.

You would need to set the current to a maximum of 10W/24V=0.416A to not destroy the halogen light bulb or would there only be a power drawn up to 10V of the halogen bulb maximum voltage rating and a maximum current of 0.8A resulting in only 8W?

Since the maximum current you can get from your current source is 0.8A, this is the most current you can drive through the bulb and is below the rated current.
For simplicity, assume the bulb acts like a resistor (it doesn't, it has a non linear I/V characteristic as the filament resistance increases with temperature) so it has a resistance of 10 ohms (R = 10V/1A), at 0.8A there will be 8V (V=IR) across it from the constant current source and it will be dissipating 6.4W.
Don't forget that, just because the constant current source can provide up to 24V it doesn't mean that it will always do so.

Tungsten filaments have a +ve temp. coefficient that results in the final resistance of 10V/1A= 10 Ohms around 3000'K and the resistance is 5% of this value = 500 mOhm at room temp or 300 'K.

Thus the bulb will never get very hot with < 0.5A and $$\I^2R=0.5^2*0.5=0.125 W\$$ initially but then warming up to over 1 watt and still not shed any light.

With 0.84 A you still cannot draw 10W and it will be limited with a lower resistance any may only shed 6 or 7 watts regardless of the DC voltage > 10V

It actually won't even light up because any incandescent bulb requires a startup current (inrush current) nearly 10x the continuous current to heat the filament to glowing temperature.

By limiting the current to 0.8 A (or even 1 A), you will not allow the filament to heat up.

If it was a resistor (=10 V, 1 A ==> 10 Ω), your 0.8 A current would generate 0.810 = 8 V across it. You don't need to set the current to 24W/10W because you won't have 0.416 A24 V across it.

• I'm sorry, I beg to differ. A bulb doesn't need an inrush current to start up. it's perfectly possible to wind the current up slowly from zero to the full rating. In my A-Level physics practical exam I did just this experiment. Dimmer switches also can do this and before their availability theatre stage lighting was dimmed by high power variable resistors or autotransformers. I agree that if you connect a bulb to a voltage source there will be an inrush current, but it's not necessary to get the bulb to light. Feb 21, 2022 at 2:16