Why capacitor in ancient fluorescent lamp?

Thread Starter

Jack of some trades

Joined Mar 16, 2020
14
Hi,

I was working on a real old school two-tube fluorescent desk lamp. There of course are two choke ballasts, one for each bulb, but one has a 4.7 uF AC capacitor in series with it. I can't find it on the Internet now but seems I remember that being done so that the flicker of one bulb will be out of phase with the flicker of the other, so that the user will perceive less flicker. Is that correct? Thanks.
 

Ian0

Joined Aug 7, 2020
9,808
What mains voltage does it operate on? A fluorescent tube needs about 100V, so the ballast circuits are different for 230V mains and 120V mains.
That sounds like a "lead-lag" ballast where the capacitor shifts the phase to reduce flicker. It's quite similar in principle to the capacitor in a single-phase induction motor.
 

Thread Starter

Jack of some trades

Joined Mar 16, 2020
14
What mains voltage does it operate on? A fluorescent tube needs about 100V, so the ballast circuits are different for 230V mains and 120V mains.
That sounds like a "lead-lag" ballast where the capacitor shifts the phase to reduce flicker. It's quite similar in principle to the capacitor in a single-phase induction motor.
It's a 120 volt lamp with two F15T8 bulbs and simple choke ballasts. There's no starter, just a manual start switch.
 

MisterBill2

Joined Jan 23, 2018
18,477
Hi,

I was working on a real old school two-tube fluorescent desk lamp. There of course are two choke ballasts, one for each bulb, but one has a 4.7 uF AC capacitor in series with it. I can't find it on the Internet now but seems I remember that being done so that the flicker of one bulb will be out of phase with the flicker of the other, so that the user will perceive less flicker. Is that correct? Thanks.
Indeed the series capacitor is to provide the phase shift to reduce the flicker that was present in the older types of tubes, because the phosphors were not as persistent.
And while the supply voltage is higher, the conducting voltage drop, at least in the white four foot tubes was about 65 volts if you used a DC supply. I learned that when trying to make a flash tube arrangement using white florescent tubes in place of the xenon flash tubes. I had a full-wave voltage doubler off the 120 volt mains, which gave me about 270 volts open circuit across the tube, without the heaters powered. The trigger was a bare wire wrapped around the center half of the tube, and the trigger pulse came from a capacitor driving a primary of a flyback transformer. The tube would give a bright flash but then stay lighted, loading the power supply down to about 65 volts.
 

MaxHeadRoom

Joined Jul 18, 2013
28,684
Indeed the series capacitor is to provide the phase shift to reduce the flicker that was present in the older types of tubes, because the phosphors were not as persistent.
I know I have mentioned this before, but I recall in the UK when fluorescents first came on the scene and were installed in industrial machining operations, machinists were loosing fingers due to the stroboscopic effect making the tooling appear stationary.
The answer was to wire them in banks, across 3 phases. ;)
 

Ian0

Joined Aug 7, 2020
9,808
I know I have mentioned this before, but I recall in the UK when fluorescents first came on the scene and were installed in industrial machining operations, machinists were loosing fingers due to the stroboscopic effect making the tooling appear stationary.
The answer was to wire them in banks, across 3 phases. ;)
I think that the lead-lag ballast was for applications where there wasn't a convenient 3-phase lighting circuit.

Wouldn't machinery appear to be rotating very slowly backwards? Usually they use induction motors not synchronous motors.
 

Ian0

Joined Aug 7, 2020
9,808
Maybe they grabbed the tool as it appeared to rotate slowly?
All I remember is, it was a problem. ;)
I heard the same thing. At a guess, people perceived that the machine was slowing down and was running slowly enough to be stopped by hand i.e. to change a drill bit. Filament lamps persisted in machine shops for a long time after fluorescents had taken over in other areas.
 

MaxHeadRoom

Joined Jul 18, 2013
28,684
I heard the same thing. At a guess, people perceived that the machine was slowing down and was running slowly enough to be stopped by hand
Also at that time, speed control of a spindle was often by belt/pulley reduction, so all it had to be was a pulley that happened to match or be a division of the supply freq.
 

Pyrex

Joined Feb 16, 2022
265
There of course are two choke ballasts, one for each bulb, but one has a 4.7 uF AC capacitor in series with it
It's a "lead-lag" ballast. To reduce flicker. In real world, not the best solution, as the choke in L-C branch heats much more, than in L branch. And capacitor's life span is not very long, usually it's lose the capacity gradually till to a moment the lamp won't start. The phenomenon of a non-sinusoidal current in the L-C branch
 
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