Can 2.5V to light up a typical 3mm LED?

On Tue, 25 Jun 2013 14:50:19 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 25/06/2013 3:37 AM, Tim Wescott wrote:
Connect a resistor in series with each one, then connect the two resistor-
LED pairs in parallel. That's the conventional, safe, and easy way to do
it.

What if one LED in the parallel circuit died? Would the remaining LED
receiving more than its share of current, even with a current-limiting
resistor?
Click to expand...
When you connect two or more LED/resistor combinations in parallel,
the current in each parallel branch is independent - adding or
removing branches will not affect the currents in the remaining
branches.

--
Peter Bennett, VE7CEI Vancouver BC
peterbb (at) telus.net
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
On Tue, 25 Jun 2013 14:39:08 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 25/06/2013 1:32 AM, Daniel Pitts wrote:
Always use a current limiting resistor. If you don't, and you're lucky,
you'll just burn out your LED. If you're unlucky, it'll burn out your
power supply and/or other circuitry in the path.

Do I need that resister if I am to connect *TWO* LEDs in series over the 5V?
Click to expand...
Yes.

--
Peter Bennett, VE7CEI Vancouver BC
peterbb (at) telus.net
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
"Peter Bennett" <peterbb@somewhere.invalid> wrote in message
news:0pijs81o2kcc00pvddv7o75rck2v1uib4o@news.supernews.com...
On Tue, 25 Jun 2013 14:32:09 +0800, "Mr. Man-wai Chang"
toylet.toylet@gmail.com> wrote:

On 25/06/2013 12:21 AM, Peter Bennett wrote:
Red, yellow and green LEDs have voltage drops around 2 volts, so two
of them in series, with a current limiting resistor, would work on 5
volts.

It is working! A 3mm green LED in series with a 5mm red LED connected to
5V.


It is not safe to put two LEDs in series across a fixed supply without
a current limiting resistor.

Without a current-limiting resistor, any variation in supply voltage
or LED voltage drops (and those voltage drops do change with
temperature) can cause the current to vary from too little to light
the LEDs to more than they can handle.
Click to expand...
Thermal runaway is the real issue - if the LEDs are passing enough current
to heat up, the Vf will lower causing them to conduct yet more current.
 
On 26/06/2013 12:59 AM, Peter Bennett wrote:
When you connect two or more LED/resistor combinations in parallel,
the current in each parallel branch is independent - adding or
removing branches will not affect the currents in the remaining
branches.
Click to expand...
I remember now: 1/R(parallel) = 1/R1 + 1/R2

--
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On 26/06/2013 12:55 AM, Daniel Pitts wrote:
Now, if you had a series circuit, and one LED died, then both LEDs would
appear off, since no current could flow at all.
OTOH, in a series circuit, if either LED failed by shorting, you'd end
up with too much current through the other remaining LED, and likely fry
them both.
Click to expand...
Would a LED suddenly short itself? Not very likely, I supposed...

--
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/ v \ Simplicity is Beauty! May the Force and farces be with you!
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"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:kqdqqj$hjc$3@dont-email.me...
On 26/06/2013 12:55 AM, Daniel Pitts wrote:
Now, if you had a series circuit, and one LED died, then both LEDs would
appear off, since no current could flow at all.
OTOH, in a series circuit, if either LED failed by shorting, you'd end
up with too much current through the other remaining LED, and likely fry
them both.

Would a LED suddenly short itself? Not very likely, I supposed...
Click to expand...
Without current limiting a failed short-circuit is not only likely but
probable.

Having said that - I recently aquired a pc case that some muppet had wired
the power LED to a spare drive Molex - it took some dismantling to discover
there was no current limiting resistor. Due to thermal shock, the LED die
bond wire had fractured - as the die thermally expanded and contracted, the
fractured bond wire made or break so the LED flashed (for a while).
 
On 26/06/2013 11:24 PM, Ian Field wrote:
Having said that - I recently aquired a pc case that some muppet had
wired the power LED to a spare drive Molex - it took some dismantling to
discover there was no current limiting resistor. Due to thermal shock,
the LED die bond wire had fractured - as the die thermally expanded and
contracted, the fractured bond wire made or break so the LED flashed
(for a while).
Click to expand...
Really? It didn't short itself, did it?

Another question:

LED is a diode.

Can large current make a diode to behave like conductor and hence short
the +5V and GND?

--
@~@ 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" <toylet.toylet@gmail.com> wrote in message
news:kqf9ui$2os$1@dont-email.me...
On 26/06/2013 11:24 PM, Ian Field wrote:
Having said that - I recently aquired a pc case that some muppet had
wired the power LED to a spare drive Molex - it took some dismantling to
discover there was no current limiting resistor. Due to thermal shock,
the LED die bond wire had fractured - as the die thermally expanded and
contracted, the fractured bond wire made or break so the LED flashed
(for a while).

Really? It didn't short itself, did it?

Another question:

LED is a diode.

Can large current make a diode to behave like conductor and hence short
the +5V and GND?
Click to expand...
How long is a piece of string!

How big is the diode - how much current can the PSU supply?
 
On 27/06/2013 2:38 AM, Ian Field wrote:
LED is a diode.

Can large current make a diode to behave like conductor and hence
short the +5V and GND?

How long is a piece of string!

How big is the diode - how much current can the PSU supply?
Click to expand...
But would the short piece of string melt like a FUSE **BEFORE** the
large current could get through?

If it did melt *FAST* enough, then there was nothing to worry about... . :)

--
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/ v \ Simplicity is Beauty! May the Force and farces be with you!
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On 27/06/2013 2:45 AM, Mr. Man-wai Chang wrote:
On 27/06/2013 2:38 AM, Ian Field wrote:
LED is a diode.

Can large current make a diode to behave like conductor and hence
short the +5V and GND?

How long is a piece of string!

How big is the diode - how much current can the PSU supply?

But would the short piece of string melt like a FUSE **BEFORE** the
large current could get through?

If it did melt *FAST* enough, then there was nothing to worry about... . :)
Click to expand...
BTW, are we talking about circuitry or material science? ;)

--
@~@ 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" <toylet.toylet@gmail.com> wrote in message
news:kqfcmb$j8f$2@dont-email.me...
On 27/06/2013 2:45 AM, Mr. Man-wai Chang wrote:
On 27/06/2013 2:38 AM, Ian Field wrote:
LED is a diode.

Can large current make a diode to behave like conductor and hence
short the +5V and GND?

How long is a piece of string!

How big is the diode - how much current can the PSU supply?

But would the short piece of string melt like a FUSE **BEFORE** the
large current could get through?

If it did melt *FAST* enough, then there was nothing to worry about... .
:)


BTW, are we talking about circuitry or material science? ;)
Click to expand...
Troll detection - and you have been.
 
On 27/06/2013 4:45 AM, Ian Field wrote:
But would the short piece of string melt like a FUSE **BEFORE** the
large current could get through?
If it did melt *FAST* enough, then there was nothing to worry
about... . :)
BTW, are we talking about circuitry or material science? ;)

Troll detection - and you have been.
Click to expand...
Of course not!!!!

**IF** an LED could become a conductor by slight fluctuation in the +5V
supply, a limiting resistor would not help, would it? It would still
short itself even whether you connected it serial or in parallel, with
or without a resistor!

So the question would demand an answer relating 3 objects:

1. diode/LED (properties)
2. conductor
3. fuse

How would you explain the relationship between these words?

--
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"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:kqhaei$op3$1@dont-email.me...
On 27/06/2013 4:45 AM, Ian Field wrote:
But would the short piece of string melt like a FUSE **BEFORE** the
large current could get through?
If it did melt *FAST* enough, then there was nothing to worry
about... . :)
BTW, are we talking about circuitry or material science? ;)

Troll detection - and you have been.

Of course not!!!!

**IF** an LED could become a conductor by slight fluctuation in the +5V
supply, a limiting resistor would not help, would it? It would still short
itself even whether you connected it serial or in parallel, with or
without a resistor!

So the question would demand an answer relating 3 objects:

1. diode/LED (properties)
2. conductor
3. fuse

How would you explain the relationship between these words?
Click to expand...
Obfuscation.
 
On 27/06/2013 9:44 PM, Ian Field wrote:
How would you explain the relationship between these words?

Obfuscation.
Click to expand...
Anyway, I agree that a limiting resistor is a safer bet.

That still does NOT explain whether the LED would short itself.... :)

--
@~@ 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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On Wed, 26 Jun 2013 12:36:33 +0800, Mr. Man-wai Chang wrote:

On 26/06/2013 12:55 AM, Daniel Pitts wrote:
Now, if you had a series circuit, and one LED died, then both LEDs
would appear off, since no current could flow at all.
OTOH, in a series circuit, if either LED failed by shorting, you'd end
up with too much current through the other remaining LED, and likely
fry them both.

Would a LED suddenly short itself? Not very likely, I supposed...
Click to expand...
Even if it did (I've never seen one fail that way) the current limiting
resistor would still limit current in that leg. You'd have to have both
the LED and the resistor in one leg short out -- and resistors just don't
fail short.

You have more to worry about from meteors hitting your noggin.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
 
On 27/06/2013 11:33 PM, Tim Wescott wrote:
Would a LED suddenly short itself? Not very likely, I supposed...

Even if it did (I've never seen one fail that way) the current limiting
resistor would still limit current in that leg. You'd have to have both
the LED and the resistor in one leg short out -- and resistors just don't
fail short.
You have more to worry about from meteors hitting your noggin.
Click to expand...
I acknowledge the safety measure, but is the threat real?

--
@~@ 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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On 2013-06-27, Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:
On 27/06/2013 11:33 PM, Tim Wescott wrote:
Would a LED suddenly short itself? Not very likely, I supposed...

Even if it did (I've never seen one fail that way) the current limiting
resistor would still limit current in that leg. You'd have to have both
the LED and the resistor in one leg short out -- and resistors just don't
fail short.
You have more to worry about from meteors hitting your noggin.


I acknowledge the safety measure, but is the threat real?
Click to expand...
Not with ordinary resin encapsulated thru-hole LEDs. the die bonding
tends to fail before thermal runaway can transform the die into a
bulk resistor

rectifier diodes have stronger bonding and do often fail shorted.

--
⚂⚃ 100% natural

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On 29/06/2013 7:04 AM, Jasen Betts wrote:
On 2013-06-27, Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:
I acknowledge the safety measure, but is the threat real?

Not with ordinary resin encapsulated thru-hole LEDs. the die bonding
tends to fail before thermal runaway can transform the die into a
bulk resistor
Click to expand...
So I should not expect a diode to behave like a fuse or a resistor? OK....

rectifier diodes have stronger bonding and do often fail shorted.
Click to expand...
Let's just talk about average LED. I don't need rectifier for the
meantime...

--
@~@ 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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On 25/06/2013 1:32 AM, Daniel Pitts wrote:
In professional applications, you'll also need to consider the power
dissipation, but that's another few pages of discussion ;-)
Click to expand...
Talking about the wattage over the current-limiting resistor?

http://www.elmassian.com/trains/dcc-battery-rc-electronics/misc-electronics/led-lighting-basics

--
@~@ 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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