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Will a 40W ballast limit the output power to 40W?
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Evangelopoulos Panagiotis:
Greetings,

I want to build a mains select-able power limiter. I have quite a bit of experience when it comes to electronics and mains circuits but I've never played with ballasts before and thus I am a bit confused.

My main question is this: Will a 40W ballast limit the output power to 40W or drop the input power by 40W?

Thank you all in advance,
E.P.
Gyro:
Unfortunately it's not quite that simple. The ballast is a simple fixed value inductor which relies on the difference between the mains voltage and the discharge voltage of the fluorescent tube to provide the right current. During starting, it provides heater current but I think this is of higher magnitude due to the lower voltage.

As a series impedance, it will limit the AC current to a fixed value for a given voltage though.


P.S. The ballast will probably get very close to saturation with full mains across it - they buzz to varying extents during tube staring (heaters) - they're built to a price.  You might find the standard dim lightbulb (aka mains voltage PTC thermistor) a better bet.
Evangelopoulos Panagiotis:
Thank you for your help Gyro,


--- Quote from: Gyro on May 05, 2020, 06:13:36 pm ---Unfortunately it's not quite that simple. The ballast is a simple fixed value inductor which relies on the difference between the mains voltage and the discharge voltage of the fluorescent tube to provide the right current. During starting, it provides heater current but I think this is of higher magnitude due to the lower voltage.

--- End quote ---


I see, so the power rating on the ballast is meant to be associated with the rating on the fluorescent light bulbs not an actual load.


--- Quote from: Gyro on May 05, 2020, 06:13:36 pm ---As a series impedance, it will limit the AC current to a fixed value for a given voltage though.

--- End quote ---

So it is possible but not the best idea. Will a 50W and a 100W ballast in series make a 150W ballast? My intuition says that the result should be 50W...


--- Quote from: Gyro on May 05, 2020, 06:13:36 pm ---P.S. The ballast will probably get very close to saturation with full mains across it - they buzz to varying extents during tube staring (heaters) - they're built to a price.  You might find the standard dim lightbulb (aka mains voltage PTC thermistor) a better bet.

--- End quote ---


So you recommend to simply have a selector switch with PTCs of different values in order to control the output power? Will they respond quickly enough or even survive a mains short circuit?

E.P.
Gyro:
You're welcome,

Yes, exactly. The ballast label means for a 40W tube.

If you were to put two ballasts in series, you would be putting two inductors in series, so the inductance adds. It's difficult to think of in terms of watts (the discharge voltages of different sized tubes is different), but hypothetically you would end up with a lamp wattage of less than 50W because the inductance of the two ballasts in series is higher than the 50W one on its own.

Sorry, I shouldn't have put the bit about the "aka PTCs" it doesn't translate well. I was meaning putting an actual mains filament lamp in series. Because the resistance of the filament increases with temperature (the PTC effect), a lamp in series with the supply can be used as a power [Edit: well, current] limiter when testing mains equipment. If the current consumption is ok, the filament remains cool, so low resistance. If, worst case, the equipment has a short, then the lamp will light to full brightness and limit the current to a safe value (hence the common name of 'dim bulb tester'). Because the thermal mass of the filament is very small, operation is fast.

You can arrange a switched bank of different wattage lamps, or just have a single holder and (safely) swap the bulbs, depending on how much you're going to use it.
james_s:
A ballast acts as an impedance. When trying to understand the effects of placing multiple ballast chokes in series or parallel it's probably easier to think of them as resistors because that's roughly how they behave. A resistor that only works on AC and dumps the excess power back into the line rather than burning it up as heat. Indeed a plain resistor will work to ballast a fluorescent lamp although it will not be very efficient. The old GE "Brite Stik" fluorescent lights used a resistive ballast.
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