Capacitor in series with fluorescent ballast

[Andy Cuffe]

I had my fluorescent work bench lamp apart today to try to stop an
annoying intermittent buzz from the ballast. It's a typical two 15W
tube with separate series inductor ballasts for each tube.

I was surprised to see a 4.7uF cap in series with one of the ballasts,
but not the other. I'm wondering why they would have put a cap in
series with only one ballast? The only thing I can think of is that
it might cause one tube to operate 180 deg out of phase from the other
to reduce flicker. The cap was shorted and I'm wondering whether or
not to bother replacing it.
Andy Cuffe
[email protected]

 

[NSM]

| I had my fluorescent work bench lamp apart today to try to stop an
| annoying intermittent buzz from the ballast. It's a typical two 15W
| tube with separate series inductor ballasts for each tube.
|
| I was surprised to see a 4.7uF cap in series with one of the ballasts,
| but not the other. I'm wondering why they would have put a cap in
| series with only one ballast? The only thing I can think of is that
| it might cause one tube to operate 180 deg out of phase from the other
| to reduce flicker.

Might also improve the power factor of the whole assembly. They don't
usually bother with small lamps however.

N

 

[James Sweet]

I had my fluorescent work bench lamp apart today to try to stop an
annoying intermittent buzz from the ballast. It's a typical two 15W
tube with separate series inductor ballasts for each tube.

I was surprised to see a 4.7uF cap in series with one of the ballasts,
but not the other. I'm wondering why they would have put a cap in
series with only one ballast? The only thing I can think of is that
it might cause one tube to operate 180 deg out of phase from the other
to reduce flicker.

That's exactly what it does, this is a lead-lag configuration, the tubes
flicker alternatly to reduce overall flicker. The same basic arrangement is
used in preheat F40 twin tube ballasts.