Why does my ballast hum?

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iwirehouses

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I have a paricular t8 electronic 4 foot fixture that I will usually install. I mainly use electronic ballasts because they don't make any noise. I just reecently bought a different electronic fixture and wasn't pleased to find that it hums. Upon inspection I noticed that this fixture only takes t12's. In my mind this makes it a t12 electronic ballast. So I figured only t8 electronic ballasts are nice and quiet. I went home and turned on the fixture in my garage and noticed a tiny hum. Now this fixture is a t8 electronic ballast that is also listed for t12's. What do I have to look for specifically to get a ballast that dosn't hum? And what does "Sound Class A" mean?
 
"Sound Class A" is the noise rating of the equipment. Noise ratings range from A - the quietest, to F - the loudest. "A" has a decibel rating less than 26, whereas "F" can have a decibel rating greater than 36.

Bad voltage, ballast end of life, manufactured defect, and improper lamp use can cause ballast hum.
 
OK, somebody has to say it.

It hums because it doesn't know the words. :rolleyes:

Fine. Got that out of my system.
 
Hmmm...well the original ballast that hummed like hell, which apparently is a T12 electronic ballast, says Sound Class A. It sounded about as bad as a magnetic ballast though. The light is on the second floor so it might have just under 120 volts. The ballast was brand new, and we installed two of them side by side. One definitly hummed more then the other but they where both noticable.
 
charlie b said:
OK, somebody has to say it.

It hums because it doesn't know the words. :rolleyes:

Fine. Got that out of my system.
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Why are you addressing me? :D


(And I restrained myself this morning for nothing!)
 
I replaced the ballast with a different t8 electronic and it still hums, a little less though. While I was trying to figure out what was wrong, I found that (tested with two ballasts in two locations) a hardwired ballast with a switch loop hums and a plug connected ballast dosn't hum.

When wiring the lights, I fed the light, terminating the neutral right there, then maybe 10 ft of 12/2 looping the hot through a switch.

For troubleshooting purposes only, I bypassed the switch by hooking the ballast right to the hot line in the light so it was always on. The hum was vanished. How does this work?
 
iwirehouses said:
For troubleshooting purposes only, I bypassed the switch by hooking the ballast right to the hot line in the light so it was always on. The hum was vanished. How does this work?
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Dimmer, maybe?
 
no, its not a dimmer. Its wierd, it happened on two different houses with two completly different ballasts. With a switch loop, it hums, when the hot and neutral terminate right there in the same box it dosn't hum?
 
The only other thing I can think of is that the feed and the leg, or the feed and neutral, are not on the same circuit.
 
Humm?

Humm?

Maybe it hums because it forgot the words. (Very old joke.)

It may hum because it is cold.

But I'm wondering about T12 vs T8. I'm not sure I understand your situation. Did you say the electronics are for T12 only and it hums when you try to use it with a T8? That's not too surprising since T12 and T8 lamps show a very different electrical load (R, L, and C) to the ballast.

It might hum because it is trying to work at a different frequency that it was designed for. I'm guessing you are not hearing 60 hz low pitch hum, but something like 3K hz high pitched hum. The ballast doesn't like your tubes.
 
Well the first was electronic but said to only use T12 bulbs only. I thought all electronic ballasts take t8's. So right away, I thought this ballast was just a cheapie or something. I bought the ballast I usually use which is a T8 electronic ballast but specifies it can also take T12's. Now I kind of feel stupid for not doing this, but I havn't had any T-8's in there yet. Reason being, I figured if the bulb has any factor on how much it will hum, then if you take the bulbs out, it shouldn't hum at all. (kind of a bad logic now that I think of it) Anyways...what I'm trying to say is with t12's in it (which the label says I can do) it hums, with no bulbs in it, it hums a little less, and if I cut out the switch loop and wire it so its hot all the time (temporarily of course) it dosn't hum at all.

I was thinking that it may either be that a standard switch is causing interference and I could try some type of specialty "super sensitive" switch that I never heard of. The other thing I was thinking is maybe the hot being longer then the neutral is causing the problem. This makes absolutly no sense to me, but I know ballasts are very sensitive and there are a few things that I still don't know about.
 
I have tried to follow this thread, but I have not heard the mention of you checking 120v to ground or is the supply 277v? What was or is the reading? Just curious.
 
I didn't have a volt tester on me. I thought I was just going to be installing the ballasts. I will check that next. It is 120v.
 
the other possiblty you got the "hybird" ballast which it mean part electronic and part magnatic so it been around for short while before it ran full electrionic ballast.

I ran into few of that but not too many now as i am speaking at this time most cheap to med priced ballast will have hybird set up but sometime can get bad batch it can hum like sonof gunner as well

Merci, Marc
 
You hear a hum because the transistors are firing in the audible range, which is 16khz or less. V=Ldi/dt and the di/dt is caused by the firing of the transistors. Change of load or voltage will affect the transistor switching speed and thus the noise frequency. A universal ballast wired at 277 will sound different than at 120V because of the change in switching frequency required to regulate the voltage. Most audible specifications are based on a certain distance from the device, which for electronics is typically 3 ft away. Many times the specification dB of audible noise doesn't specify a frequency which mean it could be at 100hz or 24khz. While most people will agree that low frequency audible noise is more noticable at a given dB, others will tell you high frequencies can drive you crazy. So the frequency of the noise might bother you and be in spec, but not bother others. If you mount a ballast on a 15' ceiling it will be far away from the 3ft spec and should come accross as unaudible. However, if you mount it on an 8ft. ceiling you will probably hear a small hum which should still be under the specification.

I agree that the manufacture's quality makes a difference as the audible spec is way to vague to make any conclusion on what you will hear when installed.
 
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