Electronics > Beginners

I'm confused...(musings on LEDs and capacitors)

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rrinker:
 With 6F, that LED is going to glow for a long time.
One thing the smaller physical size supercapacitors has enabled is a "keep alive" circuit for model trains. I only have one locomotive with a keep alive in it, but with (I believe, I haven't taken it apart to see what value the caps are) a total of .1F available, this loco will keep rolling for nearly a minute if I take it off the rails and set it on my bench - like a windup toy. Motor turning, driving the thing, and the LED headlight is on.

RJSV:
   Wow, what a great thread,  and all responses are helpful, there.    I have always called the VI curve for an LED type as 'spongy', which is exactly wrong.  How about this explanation:  You can press on a glass window all you want,  but its the movement that indicates breakage is emminant.
Uh, maybe lame,  but seriously many of (us) have a difficult time on the LED drive theory.

   One way to view it is only see the voltage as a 'reaction',  and calculate using a 'nominal' voltage (crudely viewed as not changing).
That means calculate the current using your applied voltage, minus the 'characteristic voltage'.  You can measure that,  likely it will be 1.8 volts, for a red LED.  So use 9 volts (measure the battery) minus the 1.8 volts,  thats what is driving your series connected resistor and LED.
Adding a second LED just right directly in parallel with the first LED will cause the 'Characteristic voltage to only look a bit lower, like 1.7 volts (keeping in mind the LED pair should be very similiar or from the same manufacture batch, etc.).

   Now, I would argue:  "You said no voltage,  and now you are saying apply 9 volts, etc.".  Well remember, placing that series resister creates a
more linear look,  from the source capacitor viewpoint,  the LED is buried somewhat, or isolated from a pure applied 'voltage'.
Now I'm confused!   (kidding)  RJ

GadgetBoy:
If anyone is curious, the 6F cap lasted a week and a half.

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