What's the start current for fluoroscent tubes? - diyAudio
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Old 4th April 2004, 08:25 PM   #1
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Default What's the start current for fluoroscent tubes?

Does anyone know a rule-of-thumb or a formula for estimating
the max. start current for fluoroscent tubes? In my specific case
I have two 230 VAC 36 W tubes in parallel, each with its own
choke and then there is an 8 uF cap for compensation. The
chokes are spec'd as 0.43 A with cos Phi = 0.5.

My problem is that I previously had no switch on these,
but just plugged/unplugged it from the wall outlet. Now when I
bought a switch to mount on the cord, they only had 4 A rated
ones so I wondered if that would really be sufficient, asking
especially if it would handle the start current. They said it
would, but I soon found out that it didn't. It worked 4 or 5 times
then welded in the on position. Opening the switch, I could
shut it off with a screwdriver and then it worked 4 or 5 times
again and then welded. I had bought one more switch (intended
for other use) and tried that one instead, and exactly the same
thing happened. Since they don't get stuck if exercising them
with no power connected I assume it is indeed a welding
problem and not a mechanical problem. Hence, I assume the
start current is more than 4 A for two such tubes, but what
rating is required?
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Old 5th April 2004, 10:05 AM   #2
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I think u can try putting a cap across the switches to minimise the arcs...so it wun weld itself shut again...did u out the PFC caps before or after the switch??
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Old 5th April 2004, 12:20 PM   #3
ClassD is offline ClassD  Australia
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8uf? That is a pretty big cap for a pair of fluro lights. When the swtich closes, the inrush current would be huge!
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Old 5th April 2004, 12:46 PM   #4
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Quote:
Originally posted by li_gangyi
I think u can try putting a cap across the switches to minimise the arcs...so it wun weld itself shut again...did u out the PFC caps before or after the switch??
I thought about that, but ruled it out for safety reasons. I am
not sure if it would work either.


Quote:
Originally posted by ClassD
8uf? That is a pretty big cap for a pair of fluro lights. When the swtich closes, the inrush current would be huge!
That's the value it should be, 4uF per tube. We are trying to
compensate for a phase angle of 60 degrees or more.

However, maybe you said something important here.
Maybe the cap is really the problem rather than the tubes and
chokes? I didn't even think of that, but since the cap is in
parallell with tubes and chokes, then if we happen to switch on
when we have peak voltage.....!!!!!!! There is nothing to
limit the current but the cap ESR and the R and L of the wiring,
but then it seems no ordinary switch would handle it unless
rated for extremely high peak currents, yet ordinary wall-mount
switches rated at 10 or 16 A seem to have no problem with it.
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Old 9th April 2004, 09:37 PM   #5
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Quote:
8uf? That is a pretty big cap for a pair of fluro lights. When the swtich closes, the inrush current would be huge!
Not realy, as stated above thats what it should be, ive seen even 16uf on fluro's before and thats on a double tube, that is high.

I dont see a major drama here, flouro lights normally dont draw more then 3amps inrush when turned on, (only 36w in this case) when running. For mains they arent a problem, for 12v they will obviously draw more so they can be a prob.

The mains switches handle 10-15amps in the house,(USA) if you use the apropiate swith you wont have a problem, no way will a fluro light draw that much.

Trev
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