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You can use the inherent capacitanceof an LED to make a seriesresonant boost circuit that can create avoltage large enough to light the LED.Depending on the color of the LED, youneed a voltage higher than 1.6V to turnit on. The threshold, or knee, voltage riseshigher as the LED wavelength becomesshorter. All PN-junction diodes, includingLEDs, have capacitance due to depletionand diffusion profiles.
You can light an LED using its capacitancein a series LRC (inductance/resistance/capacitance) resonant circuit. Insuch a circuit, the Q factor determinesthe multiple of the generator voltage thatappears across LC. If you fashion a circuitwith a high enough Q factor, you boostthe generator voltage enough to light theLED. The Q factor of the resonant circuitis a function of the resistance, inductance,and capacitance, as the following equation shows: 
You can verify this calculation with asimple circuit using a blue LED in serieswith an inductor (Figure 1 ). The kneevoltage of the LED is 2.45V, and the signalgenerator has an internal resistance of50Ω. An inductance of 100 μH and the50-pF capacitance of a typical LED yielda Q of 28. The amplitude of the sinusoidalsignal generator is set at 650 mV p-p.You can then vary the generator’s outputfrequency until you see the circuit’s resonantpoint. As the circuit approaches the resonant frequency, the voltage across theLED starts to increase. The resonant pointmanifests itself as a small jump in voltage,rather than a smooth progression, dueto a positive feedback at resonance. Thepositive feedback happens because thecapacitance of any PN-junction deviceis not linear (Figure 2 ). As the circuitapproaches the resonant frequency, theLED voltage increases, which also increasesthe LED capacitance, resulting in lowerresonant frequency.Advertisement
For a blue LED, the voltage waveformas the circuit approaches resonance is1.55 MHz. The circuit settles at 1.69 MHz(Figure 3 ). The forward-biased LED isthus emitting light, clipping the positiveparts of the boosted waveform. Using thesame 650-mV-p-p generator amplitudeon other colors of LEDs produces differentresonant frequencies. You can see asimilar effect with a square-wave generatorbecause it also contains the fundamentalcomponents of the resonantfrequency.
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