interesting electrical question

[butchblack]

Interesting thing happened a couple of days ago and I thought someone on TB could explain why. I was listening to the radio and turned on the desk lamp with a CFL bulb in it and got static/60 cycle hum. That's normal enough. I then turned on a second lamp with a CFL and the hum stopped. The radio and the first lamp were plugged into the same outlet, the second a different outlet but both were on the same fuse circuit. Any ideas?

[VincentSalizeri]

Probably phase cancellation. I'd assume the lamps were out of phase with each other.

[ggunn]

Originally Posted by VincentSalizeri

Probably phase cancellation. I'd assume the lamps were out of phase with each other.

That'd be my guess. House wiring in the US usually starts with 240V split phase plus a neutral (3 wire), and the two hot legs are 120V to ground/neutral and 180 degrees out of phase with each other. 120V circuits are made from one or the other hot legs and the neutral. To try to balance the load, circuits in a house are usually split about 50/50 between the phases, so in your case one CFL is powered by one of the hot legs and the other, the other. When they are both on, the EM broadcast from them cancels out. They wouldn't be on the same circuit, though, but lots of house wiring has a circuit layout that doesn't follow room boundaries.

If they are on the same circuit, the next thing I would suspect is that the CFL's are internally wired oppositely. Eliminating that, the NEXT thing I would suspect is that somehow the difference in distance from the radio to the CFL's is exactly right for the waveforms to cancel at the location of the radio, but that is "out there". Phase cancellation of some sort, for sure, though.

EDIT: Another possibility is that either the wall socket or light fixture of one of the CFL's has hot and neutral reversed, which is a dangerous condition. Does the hum happen no matter which CFL is on alone?

[butchblack]

Originally Posted by ggunn

That'd be my guess. House wiring in the US usually starts with 240V split phase plus a neutral (3 wire), and the two hot legs are 120V to ground/neutral and 180 degrees out of phase with each other. 120V circuits are made from one or the other hot legs and the neutral. To try to balance the load, circuits in a house are usually split about 50/50 between the phases, so in your case one CFL is powered by one of the hot legs and the other, the other. When they are both on, the EM broadcast from them cancels out. They wouldn't be on the same circuit, though, but lots of house wiring has a circuit layout that doesn't follow room boundaries.

If they are on the same circuit, the next thing I would suspect is that the CFL's are internally wired oppositely. Eliminating that, the NEXT thing I would suspect is that somehow the difference in distance from the radio to the CFL's is exactly right for the waveforms to cancel at the location of the radio, but that is "out there". Phase cancellation of some sort, for sure, though.

EDIT: Another possibility is that either the wall socket or light fixture of one of the CFL's has hot and neutral reversed, which is a dangerous condition. Does the hum happen no matter which CFL is on alone?

No I only get hum from the cfl on the same plug as the radio.