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CFL frequency revealed

CFL frequency revealed

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Using an oscilloscope I got for free from school, I was able to determine the frequency at which CFL ballasts run the lamp. CFLs produce enough electromagnetic interference that I was able to hold the probe near the lamp like an antenna, and see the high-frequency waveform on the display. I tested the five CFLs I have on hand, and the frequency ranged from 40 to 55 KHz.

I was also able to use the same method to see 60 Hz noise being generated. This can be confirmed by viewing the lamp through a CMOS camera.

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Date added:Sep 18, 2015
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dor123
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Sep 19, 2015 at 11:54 AM Author: dor123
Most of the HF fluorescents and CFLs I've seen, shows 100hz beating effect through my camera.

I"m don't speak English well, and rely on online translating to write in this site.
Please forgive me if my choice of my words looks like offensive, while that isn't my intention.

I only working with the European date format (dd.mm.yyyy).

I lives in Israel, which is a 230-240V, 50hz country.

Medved
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Sep 19, 2015 at 12:00 PM Author: Medved
The ballast does suppress the 100 (120)Hz ripple coming from the rectifier, but only partially - as the ripple tend to be really large (the voltage fluctuate between 200-325V on the 230V AC supply). You see, how the ballast varies the operating frequency to achieve that.

No more selfballasted c***

dor123
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Sep 19, 2015 at 12:15 PM Author: dor123
So why HID lamps operating on LF sqware wave ballasts, don't show any 100hz beating effects on the camera?

I"m don't speak English well, and rely on online translating to write in this site.
Please forgive me if my choice of my words looks like offensive, while that isn't my intention.

I only working with the European date format (dd.mm.yyyy).

I lives in Israel, which is a 230-240V, 50hz country.

Medved
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Sep 19, 2015 at 12:41 PM Author: Medved
The LF square wave ballasts regulate the current as a plain DC, there it is just enough to have the DC rail voltage at least a bit above the lamp voltage to allow really 100% supression of any current variation. There the supression has to be complete, as the mains frequency would form beat components with the chopping frequency of the ballast, which would result to slower flicker or even a DC current component.
But regulating DC is quite easy task (in the simplest form a hysteretic controller would be extremely simple, yet already supress all the AC component), first the regulated signal is already DC (so needs no rectification), plus the response of the filter (the output current vs the duty ratio) is rather straight forward (plain integration on the coil), what makes even a fast response loop accurate and stable.
The fluorescents operate way better on HF and that has to be generated via a resonator circuit (used for the start). And that has way wilder response to the input (inverter frequency), plus the output signal needs a recrtification and filtering to provide the required feedback, further complicating the overall response to the regulation changes. That means the loop could be either accurate or fast, but not both at the same time. When it is fast (the saturation of the feedback transformer core), it is not that accurate to completely suppress the 100/120Hz ripple. And when it is made accurate (for dimmable circuits), it's so slow, it can not respond that well on the 100/120Hz ripple either.
The only way to make really ripple free fluorescent would be to make the DC bus filtered and stabilized first, but that would need an extra converter between the electrolytic tank capacitor and the main inverter (so the inverter would get nice plain DC, so nothing to suppress anymore). That would mean a 3'rd stage into the ballast...

Plus the higher power of HID's makes the use of an active power factor correction circuit really unavoidable, while the PFC results to voltage fluctuation of not worse than 300..450V range (that translates into 150..225V available for the lamp), so way more margin above the 70..100V lamp with the DC regulation.
The low power fluorescents allow the use of just low value of tank capacitor to bring the power factor above the 0.6 requirement, but that means way higher amplitude of the DC ripple.

No more selfballasted c***

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Sep 19, 2015 at 01:24 PM Author: Silverliner
Very interesting, I do want an oscilloscope someday.

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Sep 20, 2015 at 06:49 AM Author: Medved
@Silverliner: I have bought a DS-203, it is not that expensive, it is 2 channel and mainly it is plastic case and battery powered, so possible to measure things connected to a mains.

Downside is, many of the controls are not that straight forward and even the operation has some glitches, but it is fine for the basic measurements...

No more selfballasted c***

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Sep 22, 2015 at 12:03 AM Author: Kappa7
The active PFC (when present) doesn't provide a regulated DC rail voltage?
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Sep 22, 2015 at 12:40 AM Author: Medved
Regulated only with it's DC value (so may ensure constant brightness when the AC mains slowly varies due to e.g. grid balancing), but still with a significant 100/120Hz ripple present (so still some flicker will be there).
To supress that ripple, the downstream power conversion has to have quite some open loop gain headroom at the 100/120Hz. Quite easy for DC regulation by a DCDC operating above 20kHz, but for an AC inverter it would need operating frequency above 200kHz (that allows an open loop gain of more than 10 or so, supressing the ripple in about that ratio). And that is the domain for LLC DCDC converters, but the 40..50kHz used for supplying the fluorescents is too low to allow such ripple suppression (and obviously remain stable by itself; so at the 100Hz would be barely the unity gain cross-over frequency, so no supression from the loop gain at all).

No more selfballasted c***

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