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Date:         Tue, 1 Aug 2017 19:51:00 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Time Delay LED Lighting Circuit
Comments: To: Neil N <musomuso@gmail.com>
In-Reply-To:  <CAB2RwfjzDp7DfRspqZXyP++Rg+5n39U_8oiv4EDXXf3-1iudmA@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"

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> >


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