Figuring out LED specs

[Robert Monsen]

Anthony Fremont wrote:

"Jonathan Kirwan" <jkirwan@easystreet.com> wrote in message
news:hn20v0hj9sr97bnmu2vjffa3da69uinff9@4ax.com...

On Thu, 20 Jan 2005 12:31:14 GMT, "Anthony Fremont"

spam@anywhere.com> wrote:

First you have to sort by color. Put the leads directly across a

3V

lithium watch battery (no resistor) which will safely light red

LEDs,

You've been lucky. Without current limitation (perhaps your battery

was

weak), pretty much any LED will burn out soon after you exceed the Vf
and get it turned on.

Have you looked at the battery specs for a CR2016 or CR2025 or CR2032?


Honestly I haven't. It makes sense that being able to deliver high
current supply wasn't a primary design consideration, long shelf life
was.


I don't think it would be possible to burn out a red LED with one.


That may be, they're pretty tough (mostly).


Alkaline? Yup -- that would be bad. But these lithium button

batteries used in

watches just aren't designed to deliver appreciable currents.


I'm actually kinda surprised that they won't supply much current because
I have an SLR 35mm camera that uses a lithium battery (2cr5). Besides
physical size, does anyone know what the big difference is that allows
it to output, what must amount to, several amps of current when it
recharges the flash?

That is a 6V lithium battery. It can deliver at least an amp for short
periods of time, according to the datasheet:

http://www.duracell.com/oem/primary/lithium/245.asp

Watch batteries are a different thing altogether.

http://www.duracell.com/oem/Pdf/silver.pdf

They are rated to deliver at most 1mA.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[Jonathan Kirwan]

On Thu, 20 Jan 2005 16:40:23 -0800, Robert Monsen <rcsurname@comcast.net> wrote:

Heck, with that JFET, connecting the gate to the source would
limit current to max 6.5mA, since that's the Idss.

Now, do all that with the JFET using only a 1.5V AA battery, two resistors, a
capacitor, an 1N4148 diode and a simple, hand-wound toroidal.

V+ V+ V+
| | |
| | |
| )| .|( (about 50" of
| )| T1 |( magnet wire
| )| |( for both
| + )|. |( windings)
--- ,' |
- B1 | | D2 R2
--- \ +---|>|---+---/\/\---,
- / R1 | 1N4148 | 220 |
| \ 2200 | | |
| / | | |
| | |/c Q1 | --- ~
| '--------| 2N3904 --- C1 \ / ~
| |>e --- 100uF --- LED
| (B1 > 1V) | | |
| | | |
gnd gnd gnd gnd


Jon

 

[Don Klipstein]

In article <2005012000261416807%@news.giganews.com>, Mike Chambers wrote:

I have a bunch of loose LEDs. Some of them are completely clear. Is
there anyway that I can figure out the specs for the LEDs? I no longer
have the packaging?

i.e. how many volts they use, and their current?

I am afraid if I hook them up to check them out, I will burn them out.

Every LED I have ever seen does fine at 10 mA. All of "T1" (3 mm) size
or larger in my experience do OK at any current 10-20 mA. Some are bright
enough at lower currents.

If the LEDs have leads with unequal length, in my experience about 99%
of the time the longer lead is positive.

In my experience about 99% of the ones with "bullet" packages (T1/3
AKA mm or T1-3/4 AKA 5 mm) with a flange and a flat spot in the flange
have that flat spot being being negative.

Determining polarity by seeing which lead the chip is mounted on - I
have seen enough red LEDs go either way, most other visible ones have the
chip mounted on the negative lead.

I have yet to damage an LED with reverse polarity at 5 volts or less.

Voltage drop at 20 mA is usually close enough to 1.85 volts for red, 2
volts for orange and yellow, and 2.1 volts for yellowish green, and 3.5
volts for non-yellowish green, blue-green, blue, violet, UV, purple, pink
and white. Give or take a few tenths of a volt.

Use 4.5 or 5 volts and a 100 to 220 ohm resistor, see what happens...

- Don Klipstein (don@misty.com)