Natwerk Designs

Project Design: Relays Controlling Light Bulbs?

Hi all, I am designing a project for school. One of the systems uses a light bulb controlled by the relay. I chose a 13W CFL bulb which claims to be as brights as 60W. Using P=VI, I calculated the current to be 110 mA. I = P/V = 13W/120VRMS Power is always expressed in RMS value = 108.3 mA However, printed on the base of the light bulb it says El/mdT 13W 120V 220mA. Why 220 mA? I chose a 150 mA relay and a 200 mA fuse. Are these specs too small now? I doubt I would be able to find a light bulb less than 150 mA. texasmav, Are regular light bulbs purely resistive? If so, I can find a low power light bulb. Thanks

Public Comments

1. The answer is those bulbs have reactance as well as resistance. The total current is greater than that for the watts alone. your power factor is 108.3 / 220 = 0.49, which is a low power factor, but about right. It appears your equipment have the wrong ratings. Edit Yes, incandescent lamps are pure resistance. However, they have tungsten filaments which draw up to 1700% of their full load current when starting. That's why switches are T-rated (Tungsten rated) to prevent the contacts from welding together. Second edit: Why don't you try a 6 volt flash light bulb with a 1.5 volt battery. This will cause the filament to glow rather than burn brightly, but the demonstration will be achieved. Or pilot another relay having a higher contact rating to actually light a lamp.