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Fluorescent lights canoot pass EMC?

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Jun 22, 2008
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Here is a schematic which gives identical drive to a fluorescent ballast as seen in a fluorescent Lamp

Schematic of fluorescent lamp driver.


The fluorescent lamp is just represented here as a 50 Ohm resistor at the right hand side.

Current in fluorescent lamp.


-please observe the way that the current is not “flat” but is high frequency AC at 61KHz.

-It’s not sinusoidal so it will have harmonics extending well above 61KHz.

-I’m no Microwave Engineer, but remembering that many fluorescent lamps are tubes of length around one metre, and thinking of the high frequency AC in them……., ……..

-put these two together and you can see that we are on the way to making a decent antenna out of our fluorescent lamp.

It’s going to have radiated emissions that any reader can surely see will never ever pass any EMC Radiated Emissions Test. ?

So my question is,
Why is it that fluorescent strip (or ‘tube) lights are allowed, when they obviously wouldn’t pass an EMC test. ?

That's an excellent question. Try putting an AM radio anywhere near a flourescent tube and the radiated interference is more than obvious, even on choke ballasted AC mains tubes.

I think the answer is more political than technical, yes they would fail an EMC test, but no, nobody is going to demand millions of them are taken out of service.

The problem is actually slightly worse than your waveforms show. You are demonstrating the voltage across a resistor, in a real tube, the plasma extinguishes at zero volts and 're-ignites' at some voltage higher than zero so there is an additional step in the current waveform.

I did contact a company called 'Osram' who make CFLs because they wreaked havoc with my Ham radio equipment. Their reply was "try putting a plant in front of the light" :| !!! It might work if I plant a whole forest but then the light wouldn't shine through either :D

Yes, turning-off and re-igniting the gas discharge at every zero crossing causes the strongest interferences of a mains operated CFL.
But it's still a smooth current rise and most likely not having strong impact in the "radiated" EMI band > 30 MHz. I can imagine however,
that's it's annoying when operating a sensitive AM receiver.

But electronic ballasts are actually underlying the EMI regulations, and I know of a laboratory that has a stock of calibrated lamps
just for respective measurements. So they can and they do pass it.

thankyou FvM and betwixt,

I still find it hard to believe that a long fluorescent tube passing a current like the one displayed in my first post could pass an EMC test.

I've worked in SMPS companies where, for the sake of EMC compliance, the current from the SMPS output filter to the load was massively filtered to ensure that it was pure, "flat" DC, so it amazes me that the above current could pass EMC.

Most places i worked are paranoid about even a slight excursion from pure DC load current.

I worked in a LED car headlight SMPS place, and tiny 10W SMPS's were said to have failed EMC , even though they were pretty heavily EMC filtered and gave what looked like more or less pure DC LED current.

Knowing how much billions of Government money has been tipped into fluorescent lighting technology, i can't help but suspect that fluorescent tubes are "secretly" getting a "helping-hand" through the EMC test.......a blind eye is surely being turned to the EMI test chamber's spectrum analyser ?

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