led's

[[email protected]]

I need help with a way to light up my SWR meter with internal led which would be really close to RF passing thru the SWR meter. That way it would onlylight up the meter when I transmit. if anyone could point me to a circuit that would do this, probably need two leds one on each side of the meter, would be really grateful. I know there are ways using 12V but I want to stay away from batteries or having to plug/unplug cables. My email is [email protected]. Thanks, Cort K4WI

 

[Mr. Man-wai Chang]

A good LED is bright at 1 mA, especially green ones.

Just 1 mA? You talking about surface-mount LEDs?

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[Michael Black]

Just 1 mA? You talking about surface-mount LEDs?

LEDs have gotten more efficient, which is how there are now the
superbright.

Forty years ago, you'd put 15mA into an LED and get a bit of light, good
for display but not for lighting anything up.

Now you can put that current through some "average" LED and get a lot more
light.

He also factored in the sensitivity of the eye, apparently more sensitive
in the green spectrum so you can see it better than another color, thus
lower output is needed to be a bare minimum.

You drop the current through an LED that has great output at 15mA, and you
get a decent indicator at much lower current.

MIchael

 

[Mr. Man-wai Chang]

I have some Avago high-efficiency green LEDs, in T1-3/4 packages. Greens are not
just efficient, they coincide with the eye's sensitivity curve.
These are bright a 1 mA and visible in room light at 1 uA. I tried to determine
the minimum current that would produce discernable light up close, in total
darkness, dark-adapted. It was about 800 picoamps.

Do you have the picture and datasheet of that 1mA LED? Could I read them?

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[Michael Black]

Just get a high efficiency LED and run about 1mA and see! - everyone else are
not going to do all your work for you

I think he's still trying to wrap his mind around the idea that putting
more current through an LED won't automatically increase the light output.


If only, all those Poly-Pak type LEDs forty years ago, that were dim with
15mA going through them, would have been fine.

Michael

 

[Michael Black]

It does, up until the LED dies.

"But does it short out, or open up?"

Michael

 

[Mr. Man-wai Chang]

Just get a high efficiency LED and run about 1mA and see! - everyone
else are not going to do all your work for you

To get to 1mA means I would need to buy another resistor. That's money.

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[Mr. Man-wai Chang]

"But does it short out, or open up?"

1. Would a 3V resistor short itself when connected to a 3V DC input?

2. If answer to (1) was "no", then why do I need a current-limiting
resistor connecting 2 3V LEDs in series to a 5V DC input？

3. If answer to (1) was "yes", then what current-limiting resistor
should I use in (1)?

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[Mr. Man-wai Chang]

It does, up until the LED dies.

Are you two talking about 3mm/5mm LEDs or hi-power LEDs here?

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[Mr. Man-wai Chang]

They were a gift from James Arthur, and I don't know the part number.
Just browse the Avago web site, or Digikey, for high-output green
LEDs.

I was surprsded by the sub-nA light output. At roughly 100 mV/decade
current, I'd have guessed that the voltage drop would be so low that
it wouldn't have enough energy to make photons.

It would be fun to use a PMT and some signal averaging to see how
little current would still make some light.

If it's a military grade LED, then I was NOT supposed to be able to find
anything about it!

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[JW]

1. Would a 3V resistor short itself when connected to a 3V DC input?

WTF is a 3V resistor?

 

[JW]

WTF is a 3V resistor?

For that matter, WTF is a 3V DC input?

 

[Mr. Man-wai Chang]

I repost the questions. Thanks!

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[Mr. Man-wai Chang]

1. Would a 3V LED short itself when connected to a 3V DC input?

2. If answer to (1) was "no", then why do I need a current-limiting
resistor connecting 2 3V LEDs in series to a 5V DC input？

3. If answer to (1) was "yes", then what current-limiting resistor
should I use in (1)?

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[George Herold]

They were a gift from James Arthur, and I don't know the part number.

Just browse the Avago web site, or Digikey, for high-output green

LEDs.



I was surprsded by the sub-nA light output. At roughly 100 mV/decade

current, I'd have guessed that the voltage drop would be so low that

it wouldn't have enough energy to make photons.

Ahh, but it's those electrons on the far end of the exponential 'Boltzmann tail' that have enough extra thermal energy to make it into the depletion region. (I'll have to try I-V curves for LED's at low temperatures someday,dunking into LN2 should be pretty easy.)

George H.

 

[Mr. Man-wai Chang]

You don't make any sense.
Do you have a real name?

Just guessing....

I googled "1mA LED", not

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[Mr. Man-wai Chang]

Just guessing....

I googled "1mA LED", not

.... and found this link:

Avago Technologies
Red non-diffuse round LED,1mA 1.6V
http://uk.rs-online.com/web/p/visible-leds/0826521/

Is it the one you were talking about?

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[Mr. Man-wai Chang]

I don't believe that's true, since some LEDs will drop 3V at their
rated current.

He might be right.... Scroll down for the table in the following link:

http://www.marutsu.co.jp/user/led.php


This link is actually quite useful to people using LEDs. Too bad it's
Japanese...

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[George Herold]

Well, now you're talking about physics and science and stuff like that.

Grin.. well lots of electronics looks like a branch of science to me.

Seems to me that you need X volts to make a photon of X electron-volts energy.

Can thermal energy bootstrap an electron to make a photon that's more EV than

the voltage across the junction?

There was some news 'splash' from (maybe) mit of someone getting more lightout of an led than power in.. at some ridiculously low current.

(The good thing about electrons is there are a whole lot of them, so there are a few with many kT's of kinetic energy.)

There's also this 'common' physics demo where they look at the led wavlength and forward voltage drop for a bunch of different color leds and plot things up to get a measure of Planck's constant. (A bit of a bogus experimentwhen you look at the details.)

Modern LEDs are remarkable gadgets. It would be fun to explore the cornercases,

"someday" as you say.



We're currently experimenting with cheap (like, $12) semiconductor lasersto

find some that accidentally make clean picosecond light pulses when whacked by

one of our laser drivers. The manufacturers certainly don't know if they might.

Most laser data sheets are pitiful.

Just spent over a grand on Thorlabs hardware to hold a laser in place andget

some of its light into a fiber.

Ouch.. the only thing more expensive than optics is high vacuum.

Speaking of modulating diode lasers, Cliff Stoll (who's quite a lovable 'character'.) Was visiting the other day. He does a bunch of educational outreach and uses a modulated diode laser, beam splitter, and cheap corner cube reflector to measure the speed of light. But what he needs is a cheap (fast) photodiode detector. I put him on to Phil's book... but I've been thinking it might be a nice project.

George H.

 

[Mr. Man-wai Chang]

If red is the same color as green, yes.

Have you ever touched a real one?

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