Buzzing Fluorescent Light

[eadie]

The issue is a buzzing fluorescent light I just installed in a closet ceiling. The light is a Lithonia Lighting 14 In. 2-Light Low Profile Round (contains its own electric ballast--two circline bulbs). Here's the (possibly) relevant details:

We have knob and tube (K&T) wiring in much of the house. I switched to nm from the line running into the closet (everything done in metal boxes except the ceiling box, which is plastic). This splits to an incandescent sconce (outside the closet) and a line controlled by a motion activated switch on the load side. The switch controls the ceiling-mounted fluorescent light and an outlet (from which I’m going to run a couple of under-shelf lights).

When the motion-activated switch is set to manual off or on, no problem. But when the motion sensor is active, the light buzzes when on, then flickers dimly when it shuts off. I also noticed that if I switch the light to the on position, the light gets the slightest bit brighter and stops buzzing (which made me think that the switch is acting a bit like a dimmer, which I think fluorescent lights don’t like—at least this one’s manual said dimmers are a no-no). Also, since I’m going from K&T, nothing is grounded. I thought that might be an issue, but then I noticed something strange.

When I plug in an incandescent light bulb to the outlet controlled by the same switch (through a pendant lamp cord with a switch), the buzzing and flickering stops. If I switch the incandescent bulb off, the buzzing is back. But, since this is the closet, I can’t have an incandescent light in there.

Ideas? I’d hate to have done all this improvement only to have a noisy light that turns into a dance-party strobe light!

Thanks!

PS, I realize you don't want to add too much to K&T circuits, but rest assured that both the sconce and the internal light were already on the circuit; I just added the switch and changed the unsafe incandescent fixture in the closet to the fluorescent and the outlet (which will only run fluorescent)--overall, should be a lower load.

[HayZee518]

If your motion sensor's output is via a small ice cube relay then I'd have to rule out the buzzing. if the output is through a triac then there's a possibility the output is chopped dc which would account for the buzz. somthing else to look into is the circline ballast. the real older ones have iron punchings that make up the transformer inside, sealed with pitch. newer ballsts are electronic. the buzzing is something called an eddy current. when you feed 120 volts into a ballast with iron punchings, normally the laminations are insulated from each other but when the insulation breals down, currents are generated between the laminations and this is what causes the buzzing - the laminations are moving against each other.

[HayZee518]

According to the electrical code a light fixture either incandescent or flourescent can't be within 18 inches from combustables. this would mean anything on a shelf within the closet. when I wired houses I put the flourescent up above the doorway inside the closet, not on the ceiling.

[eadie]

The switch is a new, standard size from HD, with a black and white output--not sure what the ice cube relay means--can you clarify, so I can check? The light is dDefinitely a new, electronic ballast--a brand new light, too, so no issue on the age of the connex. Although, if it were loose, would that be an issue? I could re-seat the connections. Any idea why the incandecent being on the circuit would stop the buzzing?

Also, about the code, I thought fluorescents weren't treated the same as incandecents because they're not hot. Can you direct me to a section so I can read up?

Thanks.

[HayZee518]

unless they've changed the switching colors recently, motion sensors and photocells have the switched component RED. The 120 feed is a constant supply [not switched] to the device. The output red wire can be switched. I can't figure out why the buzzing would stop when you turn on a light downstream of the motion sensor. a regular light bulb wired in parallel with a white-black cable wouldn't effectively increase the amp draw considerably, but the only thing that comes to mind is that the lamp for some odd reason is in series with the cable. in other words the black is connected to the fixture and out of the fixture to the next device. removing the lamp would make the downstream device in operable. a bulb of a large enough wattage would complete the circuit and glow. the downstream load would increase or decrease the lamp brighhtness upon how much amps it draws.

[eadie]

Yes, sorry, you're right, it is red from the switch to the light. Note that the red from the switch controls both the light and the outlet in which I plugged the incandescent light--I pigtailed the red to the black for both the light and the outlet.

Mystery!

[HayZee518]

then I'm baffled too! it may be the electronics inside the motion sensor itself. there's probably an on board step down transformer that goes to a full wave or bridge rectifier for dc control of the motion electronics. if could be of inferior design [chinese parts] all I can say is return the part and get a new one.

[596]

Fluorescent lights do not like things such as motion sensors. To work properly, the motion sensor itself has to be rated for ballasted loads, and even more specifically has to be rated for the TYPE of ballast you have...magnetic or electronic. Some motion sensors work on one type or the other , but not both. Some work on NEITHER. If you paid less than $40 for the motion sensor, it is likely NOT rated for ballast loads, and that is your problem.