[Laser] LED negative resistance

[n5gui at cox.net]

I ran a test on an LED replacement bulb for a two cell flashlight and got results that I was not expecting.  Does anyone reading this have an explanation?

I am working on a project that I dub "Talking Flashlight" which is a light communication system intended to be built and used by youth, like Boy Scout Venturers. The idea for the transmitter is to voice modulate a low cost LED flashlight, by AM or possibly PWM.  I have tested several 6v lantern style that have recently become available, finding that for beam "tightness" and brightness to my un-calibrated eye, replacing a filament bulb with the replacement LED unit in a traditional lantern is better than the three or 10 LED cluster types.  

At the same time, I was working on a circuit to control the current through the lamp(s) from an audio amplifier.  Since the design of the lantern is 6V, it is worthwhile to  modify the cluster type LED lanterns by paralleling the current limiting resistors with lower values. ( Lowers the supply voltage needed to modulate close to 100 percent when the lamp bias is near the rated current.  The bulb replacement LED unit is a sealed device and therefore I cannot access the bias resistor.

I have found that Ray-O-Vac has a 6 volt and 3 volt replacement unit locally available, so I decided to try each.  I started by recording the voltage versus current.  The 6 volt unit was no surprise, it behaved as I suspected - an LED with a 22 ohm current resistor.  The 3 volt unit behaved like an LED with a 10 ohm resistor when I started the measurements.  However, when the driving voltage dropped below 2.25 volts, the current increased until the voltage was decreased to about 1.75 volts, then the current dropped as expected.  I was not paying close attention, but it seems that the brightness of the lamp dropped steadily during the test.  It went out as far as I could tell at about 1.1 volts and was still drawing about 20 milliamps.  I could still see light from the 6 volt bulb with less than 0.1 milliamp of drive.

This seems like the behavior of a tunnel diode, but I would not expect an LED to function at this low of a voltage.  Certainly not a "white" LED.  It is possible for there to be a booster circuit built into the base of the bulb, but I would think that it would cost more than the simple resistor of the 6 volt bulb replacement, but they retail for the same.

Does anyone know more about this?

I will probably try one of the bulbs using the circuit that I am working on for the cluster type lanterns and see if it seems to affect the audio at the receive end, but until I get to that stage of the project, I am curious.



James
 n5gui