Rfi??

[Neutron Bob]

I have had a burning thought on my mind over the last day, concerning the venerable light emitting diode.

Back in 1977, I modified an AM table radio with a new device, a nifty new green LED, as a convenient power indicator light. The idea was to provide a reminder that the radio was energized, so that I would remember to turn it off...and save 9V batteries.

Whenever the LED was active, I noted a significant amount of interference on that radio. Does anyone know if an actively lasing wee chip of GaAsP emits any RF or HF energy? In most circuits, this is not a major issue, but that old table radio wasn't very happy indeed...

Well, a question just took 26 years to be asked!

hehe

Bob

[Sch3mat1c]

LEDs aren't lasing. They are simply a glowing chip. An LED laser has specially-aligned mirrors on the chip itself which causes the laser action.
(AFAIK )

Now, seems highly unlikely that an LED would cause RFI. They don't even have the negative-resistance action (see neon lights) which could cause a relaxation oscillator type effect.
More likely the LED's current draw was affecting the circuits.

Tim

[Jeff R]

Some special LED drivers modulate the output to save power. Running an LED off of a battery 100% of the time can eat up power, wheras if it is modulated you can save on average power, though the brightness will be down a bit. If you drove the LED off of a resistor from the battery, then this shouldn't cause an RFI problem, but if you used a special driver that switches at, say, 100 KHz, it might be.

Just guessing.....

[jackinnj]

a lot (perhaps most) RFI problems are due to poor connections and poor grounding. It could be that you have a poor solder connection for the LED. If you think of it, if you have 10 or 20 ma going into the LED for a few tens of microseconds intermittently you have the potential for a lot of RFI.

[Neutron Bob]

Interesting thoughts here...thanks for your responses. This observation was made almost 30 years ago of course. The amount of interference was significant in that particular radio (AM).

My curiosity follows from studying the theory behind the light source. Electron flow across the junction causes an oscillation at the desired light wavelength; different materials yeild different wavelengths of light, and the familiar LED light colors are the result.

Though the emissions are not large, I am curious about the nature if them, and the noise floor. Thankfully, this does not involve a problem case- it's more of a technical curiosity.

Perhaps if I play with a transmitter-receiver pair (LED and photodiode), I can have a wee bit of fun.

[JBL]

The oscilliations that emit light are not a direct effect of the electron flow.

When the electron pass throught the material atoms get into their "excited" states. When the electron in excited states come back to their normal energy level they emit an energy packet(called quanta in english i think). This packet of energy packet is (more or less) in the form of a periodic wave. Wich is light in the case of led but could be other type also.

[Sch3mat1c]

And also, light and AM broadcast band are about 18 orders of magnitude apart. The tank in the reciever should attenuate something so out-of-band by oh, 1,000,000dB?

Tim