Will a 40W ballast limit the output power to 40W? - Page 1
Forum section: electronics / projects

author: Evangelopoulos Panagiotis
author_rank: Contributor
timestamp: May 05, 2020, 05:15:01 pm
content: 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 powerto40W or drop the input powerby40W?Thank you all in advance,E.P.

author: Gyro
author_rank: Super Contributor
timestamp: May 05, 2020, 06:13:36 pm
content: 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.

author: Evangelopoulos Panagiotis
author_rank: Contributor
timestamp: May 05, 2020, 07:50:39 pm
content: Thank you for your help Gyro,Quote from: Gyro on May 05, 2020, 06:13:36 pmI 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 pmSo 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 pmSo 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.

author: Gyro
author_rank: Super Contributor
timestamp: May 05, 2020, 09:11:30 pm
content: You're welcome,Yes, exactly. The ballast label meansfora 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 oflessthan 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 anactualmains 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.

author: james_s
author_rank: Super Contributor
timestamp: May 05, 2020, 09:21:14 pm
content: 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.

author: Evangelopoulos Panagiotis
author_rank: Contributor
timestamp: May 06, 2020, 09:31:25 am
content: Quote from: Gyro on May 05, 2020, 09:11:30 pmI'm not a big fan of the light bulb designs but it seems I can't avoid them. I don't like them since they make the design bulky and fragile. But I guess on the plus side the device will be lighter and cheaper.I just wish you could get high wattage lamps in small form factors.Quote from: james_s on May 05, 2020, 09:21:14 pmI'm gonna have to get a few light bulbs and ballasts to experiment with and see how they behave.Thank you all,E.P.

author: Gyro
author_rank: Super Contributor
timestamp: May 06, 2020, 01:15:53 pm
content: Don't forget you can get candle bulbs, pigmy lamps, oven lamps etc. which are tougher, they don't have to be full sized ones. They will also last forever, so you can partially enclose then (you need to be able to see if they light up of course).Have fun, and let us know what sort of setup you end up with, it's a popular requirement (SMPS repair etc.)

author: Siwastaja
author_rank: Super Contributor
timestamp: May 06, 2020, 01:37:59 pm
content: Just drop your intuition of "watts".It's just an inductor, nothing less, nothing more. Apply standard inductor knowledge and formulas to it, then you can go on and use it for whatever purpose. It may even have inductance (unit: Henry) labeled, if you are lucky! It doesn't likely list saturation current, though.So it "limits current" by producing a specific impedance at a specific frequency, |Z| = 2*pi*f*L, where f is obviously 50 or 60 Hz depending on where you live. Now current is limited by the standard ohm's law I = U/Z; where U needs to be the voltage difference left after the drop in the fluorescent bulb itself.The watts are only relevant in the intended purpose, using the ballast inductor with a fluorescent bulb. It's easier to label "40W" ballast for "40W" bulb, than to specify in the bulb that it requires an x Henry inductor, and then shop for an x Henry ballast.

author: Evangelopoulos Panagiotis
author_rank: Contributor
timestamp: May 06, 2020, 02:02:56 pm
content: Quote from: Gyro on May 06, 2020, 01:15:53 pmI totally forgot that those kind of bulbs existed! I'll post the final project here when I decide to build it.Quote from: Siwastaja on May 06, 2020, 01:37:59 pmMath is always the answer and I plan to follow these equations if I decide to build an inductive ballast based power limiter.