[Laser] LED negative resistance

[n5gui at cox.net]

Tom

Thank you for the suggestion that the light output might be PWM.  I had not considered checking for that.

This evening, I set the 3V bulb on a jig to hold it up and powered it with the AC supply and meters as I did before.  I first tested the light when the system was set at a point that I thought should have had the most obvious pulse character ( if you plot the voltage on a horizontal axis and the current on the vertical, the curve looks like a third order polynomial with a maxima and a minima.  I set it on the maxima ).  I first checked for pulsing with a vibrating mirror ( If you look at a spot of light with a mirror that is vibrating, the light will form a track to your eye.  When I use a hand mirror, the track is usually oval.  This works well for detecting pulsing lights without needing an electronic photo receiver.  I use it to demonstrate an MCW laser pointer and PWM system. ).  I easily say the pulses of the neon power indicator on the AC supply, but not on the bulb being tested.  Then I got the light receiver out.  I heard hum from the overhead lights and the neon indicator, but only hiss, which I expect to be from the receiver.  I tried to listen to the light receiver as I varied the output of the power supply.  There was some "squeal" like the tuning of a poorly stable oscillator on the work bench while monitoring it with a communications receiver with the BFO on.  The squeal was very weak and faded when I stopped adjusting the voltage.  It was so faint that I could easily believe that it was fed down from the power supply circuit rather than being an artifact from the LED.

That seemed to indicate that the light output was not pulsed, or if it was, the pulse rate was too high for me to detect.  I certainly would have been able to detect pulse below 3KHz.  If I had to guess, I would say that the pulse rate was above 10KHz for me to not to have detected some evidence by the two methods.  To the best of my ability to detect, it was continuous.

While I had the gear out, I set a three LED modified lantern head on a demo using a laser pointer for MCW.  The laser pointer draws about 20 ma. in normal operation and I drive it with a constant current pulsed  about 800 Hz using a 555 timer, which is in turn controlled by a telegrapher's key.  The lantern head seemed to work very well even though it is designed for about 150 ma drive in normal operation.

I then set the 3v LED bulb on the MCW rig.  It also glowed, but very dimly.  The light receiver gave the same tone for the LED units as for the laser dot on the wall in when in its demo configuration.  I then put the 6V bulb in the rig and was surprised that it was much brighter than the 3V bulb.  I can only surmise that the 3V bulb's dead current of 20ma that I measured before is what prevented it from performing more like the higher voltage cousins.  The result suggests that the modified LED lanterns and the 6V bulb may work far better than I feared.  Also, I may not be able to use the 3V bulb the way I had intended.  At least not less than 30 ma. drive.


Thank you again for your input into my investigation.


James.
 n5gui



---- Tom Becker <GTBecker at RighTime.com> wrote: 
>  > ...  at about 1.1 volts and was still drawing about 20 milliamps...
> 
> The 3V unit sounds like a driver to me.  Is its light output continuous 
> or PWM'd?
> 
> 
> Tom
> 
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