Would a LED short itself?

On Friday, June 28, 2013 11:48:09 AM UTC-7, John Fields wrote:

On Fri, 28 Jun 2013 00:33:59 +0800, "Mr. Man-wai Chang"

toylet.toylet@gmail.com> wrote:



On 28/06/2013 12:33 AM, Jim Thompson wrote:

Any professor/PhD level explanations?



It wouldn't. Someone would have to do something to it.



It depends on the definition of "LED" >:-}





Tell me something I dunno!



---

So far, that part's been easy.



The difficult part seems to be getting you to understand some simple

concepts.



--

JF

Rimshot!
http://www.youtube.com/watch?v=oShTJ90fC34

 

[John Fields]

On Fri, 28 Jun 2013 13:09:45 -0700 (PDT), mrdarrett@gmail.com wrote:

On Friday, June 28, 2013 11:48:09 AM UTC-7, John Fields wrote:
On Fri, 28 Jun 2013 00:33:59 +0800, "Mr. Man-wai Chang"

toylet.toylet@gmail.com> wrote:



On 28/06/2013 12:33 AM, Jim Thompson wrote:

Any professor/PhD level explanations?



It wouldn't. Someone would have to do something to it.



It depends on the definition of "LED" >:-}





Tell me something I dunno!



---

So far, that part's been easy.



The difficult part seems to be getting you to understand some simple

concepts.



--

JF


Rimshot!
http://www.youtube.com/watch?v=oShTJ90fC34

---


--
JF

 

[Mr. Man-wai Chang]

On 29/06/2013 4:09 AM, mrdarrett@gmail.com wrote:

Rimshot!
http://www.youtube.com/watch?v=oShTJ90fC34

Waste of my time...

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[Mr. Man-wai Chang]

On 29/06/2013 2:48 AM, John Fields wrote:

So far, that part's been easy.

The difficult part seems to be getting you to understand some simple
concepts.

SO an LED could never be used as a light-emitting fuse?

--
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[Mr. Man-wai Chang]

On 29/06/2013 5:02 PM, John Fields wrote:

Rimshot!
http://www.youtube.com/watch?v=oShTJ90fC34

---

FYI, the video could also waste your time.

--
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[John Fields]

On Sat, 29 Jun 2013 20:21:05 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 29/06/2013 4:09 AM, mrdarrett@gmail.com wrote:
Rimshot!
http://www.youtube.com/watch?v=oShTJ90fC34


Waste of my time...

---
I suspect if you took your own advice and paid more attention to the
first two words of your sig we'd all be a bit better off.

--
JF

 

[John Fields]

On Sat, 29 Jun 2013 20:22:40 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 29/06/2013 2:48 AM, John Fields wrote:
So far, that part's been easy.

The difficult part seems to be getting you to understand some simple
concepts.

SO an LED could never be used as a light-emitting fuse?

---
How would you rate its fusing current?

--
JF

 

[Mr. Man-wai Chang]

On 29/06/2013 10:43 PM, John Fields wrote:

How would you rate its fusing current?

By just applying 3V over the LED, it would draw whatever current needed?

Summarizing the replies from the answers, most said that the LED might
**NOT** always just blow like a fuse. And that's the main reason of
using a current-limiting resistor.

If LED would always die like a fuse without shorting itself, then there
should not be a need for a resistor.

--
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[Tom Biasi]

On 6/30/2013 7:43 AM, Mr. Man-wai Chang wrote:

On 29/06/2013 10:43 PM, John Fields wrote:
How would you rate its fusing current?

By just applying 3V over the LED, it would draw whatever current needed?

Summarizing the replies from the answers, most said that the LED might
**NOT** always just blow like a fuse. And that's the main reason of
using a current-limiting resistor.

If LED would always die like a fuse without shorting itself, then there
should not be a need for a resistor.

With all due respect this thread is going nowhere fast. You have

received enough information to realize that your logic is severely
flawed and that you need to do some research.
Now I'm suspecting troll activities.

Tom

 

[John Fields]

On Sun, 30 Jun 2013 19:43:17 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 29/06/2013 10:43 PM, John Fields wrote:
How would you rate its fusing current?

By just applying 3V over the LED, it would draw whatever current needed?

---
Good question, but no.
LEDs aren't anything like ohmic loads and they don't work like that.

Just for grins, let's assume you're not trolling and let's start over,
OK?

Remembering that an LED is a diode, go here:

http://en.wikipedia.org/wiki/Fileiode-IV-Curve.svg

Notice, in the upper right hand quadrant of the plane, that as the
forward voltage across the diode increases, a point will be reached
where a very small increase in voltage across the LED will result in a
very large increase in current through the LED.

Notice also the corollary: - and this is the crux of the argument -
that for a given current through the LED there will be a peculiar
voltage dropped across the LED.

OK so far?



--
JF

 

[Mr. Man-wai Chang]

OK, let's end this thread! I will use current-limiting resistor.

--
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[John Fields]

On Tue, 02 Jul 2013 01:00:48 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

OK, let's end this thread! I will use current-limiting resistor.

---
Oh, now that things are getting real and you're losing face, you want
to bail in order to cut your losses?

Figures...

Do you even know how to use a current-limiting resistor?

--
JF

 

[Mr. Man-wai Chang]

On 2/07/2013 4:35 AM, John Fields wrote:

Oh, now that things are getting real and you're losing face, you want
to bail in order to cut your losses?

"Face" was never my objective. I just want to use fewer parts.

Figures...
Do you even know how to use a current-limiting resistor?

V=IR?

--
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/ v \ Simplicity is Beauty! May the Force and farces be with you!
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[John Fields]

On Tue, 02 Jul 2013 21:54:24 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 2/07/2013 4:35 AM, John Fields wrote:
Oh, now that things are getting real and you're losing face, you want
to bail in order to cut your losses?

"Face" was never my objective. I just want to use fewer parts.

Figures...
Do you even know how to use a current-limiting resistor?

V=IR?

---
Well, that's a start, but what's missing is the trick about how to
determine the value of the resistance required to drop the voltage
from the supply to the value expected across the LED with rated
current through it.

The circuit looks like this: (View using a fixed-pitch font)

Vled
Iled-> /
..Vs>----[Rs]---+
.. |A
.. [LED]
.. |
..GND-----------+

where Vs is the supply voltage,
Iled is the LED forward current,
Vled is the LED forward voltage,

and the trick is... <DRUMROLL>:

Vs - Vled
Rs = -----------
Iled

That works because the current in a series circuit is everywhere the
same and if the LED drops a certain voltage with a peculiar current
through it, the current limiting resistor must drop the difference
between the supply voltage and the LED's voltage, across itself, with
that same current through itself.

For example, if we have a 12V supply and a white LED which drops 3.5V
when the current through it is 20mA, we can say:


Vs - Vled 12V - 3.5V
Rs = ----------- = ------------ = 425 ohms
Iled 0.02A

OK?

--
JF

 

[Mr. Man-wai Chang]

On 3/07/2013 5:01 AM, John Fields wrote:

That works because the current in a series circuit is everywhere the
same and if the LED drops a certain voltage with a peculiar current
through it, the current limiting resistor must drop the difference
between the supply voltage and the LED's voltage, across itself, with
that same current through itself.

Could I ignore the voltage drop at the LED and just use a resistor to
limit the current in the circuit to 20mA? That is,12V/20mA in your example.

The LED might be at its maximum brightness though....

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
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[John Fields]

On Wed, 03 Jul 2013 20:21:16 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 3/07/2013 5:01 AM, John Fields wrote:
That works because the current in a series circuit is everywhere the
same and if the LED drops a certain voltage with a peculiar current
through it, the current limiting resistor must drop the difference
between the supply voltage and the LED's voltage, across itself, with
that same current through itself.

Could I ignore the voltage drop at the LED and just use a resistor to
limit the current in the circuit to 20mA? That is,12V/20mA in your example.

---
Not if you wanted 20mA through the LED.
---

The LED might be at its maximum brightness though....

---
No.

If you wanted to ignore the LED's Vf, then that's the same as shorting
out the LED and you'd need a 600 ohm resistor to limit the current
through the resistor to 20mA if powered by a 12V source.

As soon as you removed the short though, the drop across the LED would
take away from the drop across the resistor, causing less current
through the circuit than in the shorted case, making the LED less
bright.

--
JF

 

[Jasen Betts]

On 2013-07-03, Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:

On 3/07/2013 5:01 AM, John Fields wrote:
That works because the current in a series circuit is everywhere the
same and if the LED drops a certain voltage with a peculiar current
through it, the current limiting resistor must drop the difference
between the supply voltage and the LED's voltage, across itself, with
that same current through itself.

Could I ignore the voltage drop at the LED and just use a resistor to
limit the current in the circuit to 20mA? That is,12V/20mA in your example.

You can do that, you won't get full current, and thefore won't get full
brightness. many times full brighness isn't needed or even desirable,
and by running the LED at reduced power you increase its lifetime.

The LED might be at its maximum brightness though....

huh? It definately won't.

--
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[Mr. Man-wai Chang]

On 4/07/2013 4:37 PM, Jasen Betts wrote:

On 2013-07-03, Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:
The LED might be at its maximum brightness though....

huh? It definately won't.

Dammit.. I left out "not" in that sentence... T.T

--
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/ v \ Simplicity is Beauty! May the Force and farces be with you!
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[Mr. Man-wai Chang]

On 4/07/2013 12:57 AM, John Fields wrote:

On Wed, 03 Jul 2013 20:21:16 +0800, "Mr. Man-wai Chang"
toylet.toylet@gmail.com> wrote:

The LED might be at its maximum brightness though....
As soon as you removed the short though, the drop across the LED would
take away from the drop across the resistor, causing less current
through the circuit than in the shorted case, making the LED less
bright.

Dammit.. I left out "not" in that sentence... T.T

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
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[Mr. Man-wai Chang]

On 2/07/2013 4:35 AM, John Fields wrote:

Oh, now that things are getting real and you're losing face, you want
to bail in order to cut your losses?
Figures...

Have you ever studied seriously about LED and high current from the
perspective of material science?

Could current-limiting resistor for LED be nothing but a superstition
among electronic engineers? Objective is to sell more resistors?

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
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