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Author Topic: LED's  (Read 5765 times)

pnr/p

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LED's
« on: October 09, 2008, 09:46:25 PM »

I feel this may be a dumb question but.....can someone explain Forward and Reverse voltage as seen in LED specs please
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funtimefrankie

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Re: LED's
« Reply #1 on: October 09, 2008, 10:11:19 PM »

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Martin [Admin]

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Re: LED's
« Reply #2 on: October 09, 2008, 10:14:27 PM »

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andrewh

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Re: LED's
« Reply #3 on: October 10, 2008, 06:26:27 AM »

PNR/p

The key bit about forward and reverse voltage is the last letter D for Diode
A diode conducts in one direction and blocks in the other so forward voltage is connecting it the way it lights up, and reverse is , er, the reverse (no light, no current flow)

No damage is done connecting either way, so  "trying it" is, to me, easier than learning which lead is which :-)

Best diagnostic tool for leds is one of the lithium 3V button cells - just hold the leads to the faces of the cell, if it stays dark - try it the other way
Hope this helps -

andrew
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FullLeatherJacket

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Re: LED's
« Reply #4 on: October 10, 2008, 08:24:33 AM »

No damage is done connecting either way, so  "trying it" is, to me, easier than learning which lead is which :-)

Stone me, Andrew! "Short leg goes to neg". I have it written on my workshop wall, next to Wombat's First Rule "Red to Positive, Black to Negative, cut off anything left"*.
BTW if you're buying any stuff from Component Shop then ask Iain for a copy of his guide to using LEDs - it's excellent and very easy to understand....and FREE!!
FLJ

*Don't try this at home, kids.
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andrewh

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Re: LED's
« Reply #5 on: October 10, 2008, 10:07:56 AM »

FLJ,

I'm an old mechanical engineer and try not to remember anything!
If I understand it, I don't need to remember it
If it doesn't matter I don't need to remember it

What I have written on my workshop wall are:
Pobody's nerfect!
Engineers are (approximately) perfect
Don't think what you should think till you know what you should know!

But I cant remember why

Short leg
goes to neg
Hey, I love that
andrew
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malcolmfrary

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Re: LED's
« Reply #6 on: October 10, 2008, 01:07:42 PM »

The "peak inverse voltage", or reverse voltage, is what is will stand up to before dying.  It will normally die quietly in this mode, just turning itself into a plastic blob totally indistinguishable from what it was before.
The "forward voltage" is the voltage that you can measure across it when it is in circuit and conducting.  The available voltage will normally be greater than this, but there will be something in there to limit current.  Sometimes this will be a resistor or other current limiting circuit, sometimes reliance will be placed on the internal resistance to limit current.  If an LED tries to pass too much forward current, it will respond by dying, in this case usually with a very bright flash.
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nick_75au

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Re: LED's
« Reply #7 on: October 21, 2008, 11:24:08 AM »

Not on topic of the original post but....
Is it possible to get leds that have a broad beam or visible from the sides rather than a focused beam ie for bouy lights?
Nick
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Reade Models

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Re: LED's
« Reply #8 on: October 21, 2008, 03:02:28 PM »

In short, yes.

The light output from an LED is mostly directed through the domed end, and different beam angles are often available.  The beam angles are usually specified on the LED data sheets.

Domed LED's are not really designed to be viewed 'side on', but the ones with the broadest beam angles produce more side scatter which makes them appear brighter than the narrow beam ones when viewed from the side.

Malc




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tigertiger

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Re: LED's
« Reply #9 on: October 21, 2008, 03:42:14 PM »

Not on topic of the original post but....
Is it possible to get leds that have a broad beam or visible from the sides rather than a focused beam ie for bouy lights?
Nick

Yes
Paint them white for internal reflection.
Paint them black on top of this, for reducing light leakage.
The file them flat across the top. You will get a flood light with approx 45 degress of light.

Or buy frosted LEDs of frost them yourself with sand paper. Then you get 360 degrees of light in all planes.
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cos918

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Re: LED's
« Reply #10 on: December 16, 2008, 11:11:08 PM »

hi all. Am i going mad. Wright i have been wiring up a loco for dcc and it has LED lights 2 red 2 yellow. I have found that if you look at an led the bit with the cup on it is - and the other bit is + or vic versa. Any way it is all the same for all LED i thought? On this loco it has the red LED cup + and the yellow LED cup -. I have checked this 6 times same every time can this be. I all ways thought that ALL LED ran one way regardless of the colour . I have checked with my stock of LED and they ALL run one way. So is this some sort of new LED Please help as my head is spining %% %% %%

john
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Reade Models

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Re: LED's
« Reply #11 on: December 17, 2008, 06:20:14 AM »

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cos918

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Re: LED's
« Reply #12 on: December 17, 2008, 10:43:09 PM »

hi there that link says what i thought the all LED operate the same way ie + for anode and  - for cathode. But The red LED were the other way round ie - for anode and + for cathode. I was wondering are these some sort of new leds.

john
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Rex Hunt

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Re: LED's
« Reply #13 on: December 17, 2008, 11:18:56 PM »

Answer is simple really.
What you have in effect is 2 leds mounted in parallel inside the same package.
one is reversed so you have in effect


+...............-
-................+

so no matter which way round you connect the supply, it will be right for one of them to work.....the other remains 'off' until the polarity of the supply is reversed.

The one that was off comes on....and the one that was on goes off!

Rex
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roycv

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Re: LED's
« Reply #14 on: December 18, 2008, 12:59:46 AM »

Hi all, a couple of things.

1 I have had some blue LEDs for some years and have never got them to glow.  Any ideas?

2. Has anyone experience of the new bright white LEDs?  And where is best place to obtain them?

Regards to all, Roy
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tigertiger

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Re: LED's
« Reply #15 on: December 18, 2008, 03:24:53 AM »

Hi all, a couple of things.

1 I have had some blue LEDs for some years and have never got them to glow.  Any ideas?

2. Has anyone experience of the new bright white LEDs?  And where is best place to obtain them?

Regards to all, Roy

Have you tried increasing the voltage?

I think blue/white are 3v, and green/red/yellow/orange are 2v.
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Reade Models

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Re: LED's
« Reply #16 on: December 18, 2008, 05:23:23 AM »

Hi all, a couple of things.

2. Has anyone experience of the new bright white LEDs?  And where is best place to obtain them?

Regards to all, Roy

I hate to have to ask, but have you looked in the Trader's Directory?

Malc

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Guy Bagley

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Re: LED's
« Reply #17 on: December 18, 2008, 11:46:08 AM »

Hi all, a couple of things.

1 I have had some blue LEDs for some years and have never got them to glow.  Any ideas?

2. Has anyone experience of the new bright white LEDs?  And where is best place to obtain them?

Regards to all, Roy

 we have used hundreds of them.... for work as well as for models boats too....
 we get them via  www.ledlight.co.uk - they are agents i tihnk for a multitude of LED based items
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cos918

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Re: LED's
« Reply #18 on: December 18, 2008, 09:13:43 PM »

Hi all, a couple of things.

1 I have had some blue LEDs for some years and have never got them to glow.  Any ideas?

2. Has anyone experience of the new bright white LEDs?  And where is best place to obtain them?

Regards to all, Roy

ho there . the cheapest way to get white led is go to a shop and buy Christmass  lights there very cheep now as we get close to the big day. eg 50 led for a 5 at one shop thats 10p an led can go wrong

john
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roycv

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Re: LED's
« Reply #19 on: December 19, 2008, 08:22:44 PM »

Thanks very much to all,
regards Roy
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ministeve

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Re: LED's
« Reply #20 on: December 19, 2008, 09:18:38 PM »

i always use a firm called goodwill sales on ebay very good stuff at great price never been let down by them and i buy quite a lot.

no ties with them either just happy with them sorry couldn't make a link can't remember how :embarrassed:
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tigertiger

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Re: LED's
« Reply #21 on: December 20, 2008, 12:45:19 AM »

i always use a firm called goodwill sales on ebay very good stuff at great price never been let down by them and i buy quite a lot.

no ties with them either just happy with them sorry couldn't make a link can't remember how :embarrassed:

Do they have a website?
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tonyH

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Re: LED's
« Reply #22 on: January 04, 2009, 10:54:36 PM »

Evenin' All,

Going back to the question from Nick regarding light scatter, do the rectangular LEDs emit to the side or is it still from the end?

I want to put a couple of 'trough' type deck lights on the current project and can include foil reflectors so the frosting option would work as a backup.

Tony
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Reade Models

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Re: LED's
« Reply #23 on: January 05, 2009, 12:02:38 AM »

From the end.

Malc


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wombat

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Re: LED's
« Reply #24 on: January 05, 2009, 02:50:59 PM »

Evenin' All,

Going back to the question from Nick regarding light scatter, do the rectangular LEDs emit to the side or is it still from the end?

I want to put a couple of 'trough' type deck lights on the current project and can include foil reflectors so the frosting option would work as a backup.

Tony

Depends on the type of the LED - some are side emitting, some are top emitting.

Wom
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