I perhaps should add that LEDs by their nature require a current-limited
supply, either by the internal resistance of the battery/power supply, an
in-line resistor, or active electronic limiting.  Like any diode, once they
reach the voltage where they begin to conduct (~1.8V to 4+V for various
types) they will pass drastically increasing current for small increases in
drive voltage.

I just tested three random 5 mm LEDs from a Chinese bulk supplier -- one
red, one blue, and one white (which is actually a blue LED with  a yellow
phosphor covering the chip).  LEDs like these are typically rated for 20
mA, and after my test these had permanently changed characteristics from
overdriving.  I tested at current levels of .06, 0.6, 6, 20, 60, and 120
mA.  Voltages were as follows:

Red: 1.6617, 1.7765, 1.9425, 2.0769, 2.1300, 2.0100.  These values were
higher than I expected, typically red LEDs run around 1.8v at a few mA.
Note the falloff at the highest current, as the LED was being damaged.

Blue: 2.4917, 2.6055, 2.9003, 3.2820, 3.6830, 3.9600.  A second sample from
the same bag was closer to 2.1V at .06 mA (not tested further).

White: 2.5639, 2.6904, 3.0303, 3.3745, 3.6960, 3.9125.

I would expect higher-powered LEDs to have a more rapid current increase
per voltage change, but I haven't tested any as all the ones I have are
installed with driver circuits.

My test rig was a variable 0-15V supply in series with a 2K ten-turn
wirewound potentiometer.  I varied both voltage and resistance as
convenient to get the desired currents.

Yrs,
d

On Mon, Jul 31, 2017 at 11:17 PM, Neil N <musomuso@gmail.com> wrote:

> Ironically I was outside taking apart, putting together, taking apart
> a fog light switch. Who knew a later style dual bulb fog switch would
> be in my '81 air cooled? Long story. Anyhow....
>
> Great to see the replies. I will read them!
>
> On 7/31/17, David Beierl <dbeierl@attglobal.net> wrote:
>
> > With the given values and a two-watt festoon bulb, the cap has enough
> > energy to supply the lamp for a third of a second at full brightness.  In
> > practice it would fade down over a longer time, maybe a second?
> >
> > Yrs,
> > d
> >
>
> --
> Neil n
>
> Blog: Vanagons, Westfalia, general <http://tubaneil.blogspot.ca>
>
> 1988 Westy Images <https://picasaweb.google.com/musomuso/New1988Westy>
>
> 1981 Westfalia "Jaco" Images, technical <http://tubaneil.googlepages.com/>
>
> Vanagon-Bus VAG Gas Engine Swap Group <http://tinyurl.com/khalbay>
>