# Operating a 60W incandescent bulb

I want to dim a 60W incandescent bulb using PWM on a MCU. The bulb is rated at 220-240V 50-60Hz at 60W. I am planning on using a optoisolater connected to the PWM pin. I want to run it on DC, so since the bulb is 60W and P = VI then am I correct in saying that I can get full brightness at say a voltage of 30V and current of 2A ?

No, the bulb will need 220 to 240 V to fully light, whether AC or DC.

A 220 V bulb isn't going to light with 30 V applied. If that's what you want, get a 30 V bulb. Better yet, get a bunch of LEDs and run them from the 30 V at much less current.

I want to run it on DC, so since the bulb is 60 W and P = VI then am I correct in saying that I can get full brightness at say a voltage of 30 V and current of 2 A?

If you were designing a bulb then you could choose any voltage and work out the current required as in your question. Unfortunately someone has already made that decision for you so two of your three variables were specified:

• Power: 60 W.
• Operating voltage: 240 V.

That only leaves the current as a dependent and will be given by $I = \frac {P}{V} = \frac {60}{240} = 0.25~A$.

30 V is an odd choice for a lamp. If you were prepared to go for 24 V then you would find a wide selection of truck lamps to suit. A headlamp is typically 55 W. You know enough to work out the current!

Watch the cold resistance - as mentioned by pericynthion - as the current will be much higher when the lamp is cold. Measure the resistance so you know what to expect.

No, the bulb will not draw 2A at 30V. If you were to model it as an ideal resistor, it would have a resistance of (230V)^2 / (60W) = 880 ohms, thus drawing 35 mA. In reality lightbulbs have rather lower resistance when cold so it would draw several times that current, but it will still put out much less than 60W. You would need 220-240V (AC or DC) to get 60W from it. Use a lightbulb rater for lower voltage, or use 220VAC and a triac - look up how a household dimmer switch works.