Strange Signal from Fluorescent Lamp

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Greets -

 

Just a heads up. 

 

Several months ago, I changed the ceiling fluorescent fixture in the room where I do my electronic work. For the first time since then, I fired up my scope. When I attached the probe, I noticed an unusual 40KHz signal when the probe was not connected to anything. By moving the probe around, I found the signal was coming from the fluorescent fixture. Then, I remembered - this fixture has a new "electronic ballast" using the same principles as CFLs. 

 

So, if you run into such a strange signal after changing a light fixture, be prepared aware. A meter from the fixture, the signal is small enough to barely detectable. When the probe is connected to something or the ground lead is connected to the probe nose, it is gone.

 

Again, just a heads up. I'll bet this will become more common as these fixtures are installed more and more.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Sat. Nov 15, 2014 - 05:35 AM
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So what do you think are my chances of remembering this if I ever need, or am bothered, by an extraneous 40KHz signal?

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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The same thing will happen with LED bulbs,. though not necessarily at the same freq. Then there are those pesky harmonics that your newly designed hf receiver will pick up on all bands.

 

 

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Confirmed.  Same test, same result (scope probe at 5" from CFL bulb):

screenshot

 

Ballasts' pulse transformers are seldom shielded (co$t). Cheaper lamps even use "E" or "U" shaped cores instead of toroids, which are better in blocking EMI.

The FET heatsinks are not always grounded, so the whole lamp is a perfect AM transmitter.
 

while(!solution) {patience--;}

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And if WE made something that radiated like that...

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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Actually, an AVR inside those ballasts would be fun.  A GPIO pin to the FET gate through an optocoupler, some code to run a FM/FSK modulation, and the lamp becomes an FM bug.

The tracks between FET and coils could be re-drawn on PCB to form an antenna around the board,  or solder a wire.

while(!solution) {patience--;}

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G0NZ4L0 wrote:
Actually, an AVR inside those ballasts would be fun.  A GPIO pin to the FET gate through an optocoupler, some code to run a FM/FSK modulation, and the lamp becomes an FM bug.

The tracks between FET and coils could be re-drawn on PCB to form an antenna around the board,  or solder a wire.

I dig! ;)

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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xD

while(!solution) {patience--;}

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CFLs tend to use a 'royer' converter with transistors. It's an interesting circuit. 

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Kartman wrote:
CFLs tend to use a 'royer' converter with transistors. It's an interesting circuit.
Aren't those CCFLs?

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Been there, done that, got the burnt t shirt. :)

we put Zigbee on the lamps to do dimming and failure management, high frequency or 60hz both suck in their own ways.

Keith Vasilakes

Firmware engineer

Minnesota

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I think @G0NZ4L0 was referring to a surreptitious eavesdropping device.  No radio per se, but an AVR to modulate the inherent AM emissions from the ballast into an FM signal by convolving the FET drive with, say, a mic input.

 

But Zigbee-controlled LX is groovey too :)

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Joey, i meant the compact fluorescents - the spirally things you light your room with. On my side of the pacific, they have a royer converter. The more expensive 'dimmable' ones usually have a ST chip in them.

I reverse engineered a Phillips led lamp recently. They have a buck converter in them. The electronics are enclosed in a aluminium can that works as a heat sink.

 

I get the gist of the listening device - would've been a boon in the 50s/60s! No microwaves needed!

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Kartman wrote:
Joey, i meant the compact fluorescents - the spirally things you light your room with. On my side of the pacific, they have a royer converter. The more expensive 'dimmable' ones usually have a ST chip in them.

I reverse engineered a Phillips led lamp recently. They have a buck converter in them. The electronics are enclosed in a aluminium can that works as a heat sink.

My knowledge of these things is spotty to be sure.  When CFLs first began to appear on the market here in Canada there was a great variety of types available.  Most were a pretty putrid and harsh blue glare.  Some were warmer.  Some shed light which was indistinguishable from incandescents (once warm, a few minutes).

 

Some exhibited the familiar delayed flashity-flash-on when struck.  Some were 'instant' on.

 

My understanding is that the 'instant' on variety were CCFL, but you can't believe everything you read on the Internet ;)

 

But then again, I've never even cracked one of these open, let alone reverse engineered one!

 

Quote:
would've been a boon in the 50s/60s!
Might still be fun to play with today.  One of these would be handy.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Fri. Nov 21, 2014 - 10:47 PM
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I think we're on the same page. As to what the specific difference between a ccfl and cfl is, i'm not sure. The ones with an electronic ballast strike fast as they use the resonance of a tuned circuit to strike the tube rather than the bimetalic starters that go blink blink blink. The long tubes had heated cathodes, so they're not ccfls. As for the cfls, i'll have to look if they have a heated cathode.

 

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Seems like a switching ballast is not the best wingman for an RF module: EMI, temperature, high voltages inside, etc.

 

 

 

 

while(!solution) {patience--;}