Again! Current-limiting resistor for LEDs

Ian Field · Jul 25, 2013

"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:ksrbs5$gh3$1@dont-email.me...

On 24/07/2013 10:43 PM, John Fields wrote:
The reason for that is because the VI curve for an LED is steep and if
you apply 3.5V across an LED which drops 3V with 20mA through it you
could easily overcurrent the LED.

So if the input voltage is guaranteed to be lower than the forward voltage
drop of a LED, no current-limiting resistor would be needed?

You could say that - but the LED won't do much.

Mr. Man-wai Chang · Jul 25, 2013

On 25/07/2013 10:39 PM, Ian Field wrote:

So if the input voltage is guaranteed to be lower than the forward
voltage drop of a LED, no current-limiting resistor would be needed?

You could say that - but the LED won't do much.

How do you prevent a LED from shorting itself?

Is the value of input voltage related?

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty!
/( _ )\ May the Force and farces be with you!
^ ^ (x86_64 Ubuntu 9.10) Linux 2.6.39.3
ä¸ĺč˛¸! ä¸čŠé¨! ä¸ć´äş¤! ä¸ćäş¤! ä¸ćĺŤ! ä¸čŞćŽş! čŤčćŽçść´ (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa

Ian Field · Jul 25, 2013

"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:ksrd3n$nd2$1@dont-email.me...

On 25/07/2013 10:39 PM, Ian Field wrote:
So if the input voltage is guaranteed to be lower than the forward
voltage drop of a LED, no current-limiting resistor would be needed?

You could say that - but the LED won't do much.

How do you prevent a LED from shorting itself?

Is the value of input voltage related?

LEDs are current operated - a fixed voltage is the least suitable way to
drive them.

You have been told this by several different people, if you still don't
understand - take up woodwork!

Mr. Man-wai Chang · Jul 25, 2013

On 25/07/2013 11:03 PM, Ian Field wrote:

LEDs are current operated - a fixed voltage is the least suitable way to
drive them.

What's wrong about that?

All those motherboards out there uses fixed voltage to drive indicator
LEDs, I supposed.

You have been told this by several different people, if you still don't
understand - take up woodwork!

I think you are asking for too much from simple LED applications.

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
/( _ )\ (Fedora 19 i686) Linux 3.9.9-302.fc19.i686
^ ^ 23:15:02 up 5 min 1 user load average: 0.70 0.55 0.30
ä¸ĺč˛¸! ä¸čŠé¨! ä¸ć´äş¤! ä¸ćäş¤! ä¸ćĺŤ! ä¸čŞćŽş! čŤčćŽçść´ (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa

Mr. Man-wai Chang · Jul 25, 2013

On 25/07/2013 11:23 PM, Mr. Man-wai Chang wrote:

You have been told this by several different people, if you still don't
understand - take up woodwork!

I think you are asking for too much from simple LED applications.

I am talking about those simple 3mm/5mm LEDs here, not those hi-power
LED lamps....

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
/( _ )\ (Fedora 19 i686) Linux 3.9.9-302.fc19.i686
^ ^ 23:15:02 up 5 min 1 user load average: 0.70 0.55 0.30
ä¸ĺč˛¸! ä¸čŠé¨! ä¸ć´äş¤! ä¸ćäş¤! ä¸ćĺŤ! ä¸čŞćŽş! čŤčćŽçść´ (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa

Ian Field · Jul 25, 2013

"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:ksrflh$5fg$2@dont-email.me...

On 25/07/2013 11:23 PM, Mr. Man-wai Chang wrote:
You have been told this by several different people, if you still don't
understand - take up woodwork!

I think you are asking for too much from simple LED applications.

I am talking about those simple 3mm/5mm LEDs here, not those hi-power LED
lamps....

Those are even more so likely to be fed via a current limiting resistor.

The fixed voltage comes in where you have predictable volt drop - and
current through the series resistor.

P E Schoen · Jul 26, 2013

"Ian Field" wrote in message news:TMaIt.53908$lg3.21222@fx32.am4...

"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:ksrbs5$gh3$1@dont-email.me...

So if the input voltage is guaranteed to be lower than the forward
voltage drop of a LED, no current-limiting resistor would be needed?

You could say that - but the LED won't do much.

Maybe you could add negative resistance?

When you apply less than the forward voltage drop, the device becomes a
Darkness Emitting Diode, or DED, and it really sucks (light).

Paul

John Fields · Jul 26, 2013

On Thu, 25 Jul 2013 22:20:47 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 24/07/2013 10:43 PM, John Fields wrote:
The reason for that is because the VI curve for an LED is steep and if
you apply 3.5V across an LED which drops 3V with 20mA through it you
could easily overcurrent the LED.

So if the input voltage is guaranteed to be lower than the forward
voltage drop of a LED, no current-limiting resistor would be needed?

---
Indeed, and neither would be the LED.

--
JF

John Fields · Jul 27, 2013

On Thu, 25 Jul 2013 23:23:45 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 25/07/2013 11:03 PM, Ian Field wrote:
LEDs are current operated - a fixed voltage is the least suitable way to
drive them.

What's wrong about that?

All those motherboards out there uses fixed voltage to drive indicator
LEDs, I supposed.

You have been told this by several different people, if you still don't
understand - take up woodwork!

I think you are asking for too much from simple LED applications.

---
It appears we're _all_ asking too much from Simple Simon.

--
JF

John Larkin · Jul 27, 2013

On Fri, 26 Jul 2013 17:13:42 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

"Ian Field" wrote in message news:TMaIt.53908$lg3.21222@fx32.am4...

"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:ksrbs5$gh3$1@dont-email.me...

So if the input voltage is guaranteed to be lower than the forward
voltage drop of a LED, no current-limiting resistor would be needed?

You could say that - but the LED won't do much.

Maybe you could add negative resistance?

When you apply less than the forward voltage drop, the device becomes a
Darkness Emitting Diode, or DED, and it really sucks (light).

Paul

It becomes a solar cell! Just not a very good one.

--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

John Fields · Jul 27, 2013

On Sat, 27 Jul 2013 13:52:52 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

On Fri, 26 Jul 2013 17:13:42 -0400, "P E Schoen" <paul@peschoen.com
wrote:

"Ian Field" wrote in message news:TMaIt.53908$lg3.21222@fx32.am4...

"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:ksrbs5$gh3$1@dont-email.me...

So if the input voltage is guaranteed to be lower than the forward
voltage drop of a LED, no current-limiting resistor would be needed?

You could say that - but the LED won't do much.

Maybe you could add negative resistance?

When you apply less than the forward voltage drop, the device becomes a
Darkness Emitting Diode, or DED, and it really sucks (light).

Paul

It becomes a solar cell! Just not a very good one.

---
Isn't it inherently PV with no bias?

--
JF

Shaun · Aug 11, 2013

"Mr. Man-wai Chang" wrote in message news:ksrd3n$nd2$1@dont-email.me...

On 25/07/2013 10:39 PM, Ian Field wrote:

So if the input voltage is guaranteed to be lower than the forward
voltage drop of a LED, no current-limiting resistor would be needed?

You could say that - but the LED won't do much.

How do you prevent a LED from shorting itself?

Is the value of input voltage related?

Why don't you look up basic electronics on the internet (a course) or go to
a bookstore a buy a book on basic electronics instead of asking all these
dumb questions.

Shaun

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty!
/( _ )\ May the Force and farces be with you!
^ ^ (x86_64 Ubuntu 9.10) Linux 2.6.39.3
ä¸ĺč˛¸! ä¸čŠé¨! ä¸ć´äş¤! ä¸ćäş¤! ä¸ćĺŤ! ä¸čŞćŽş! čŤčćŽçść´ (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa

Mr. Man-wai Chang · Aug 11, 2013

On 11/08/2013 1:41 PM, Shaun wrote:

Is the value of input voltage related?
Why don't you look up basic electronics on the internet (a course) or go
to a bookstore a buy a book on basic electronics instead of asking all
these dumb questions.

I expected short answers here. Don't worry, I always google if I know
the right set of keywords.

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
/( _ )\ (Fedora 19 i686) Linux 3.10.5-201.fc19.i686
^ ^ 21:30:02 up 1 day 2:51 0 users load average: 0.00 0.03 0.05
ä¸ĺč˛¸! ä¸čŠé¨! ä¸ć´äş¤! ä¸ćäş¤! ä¸ćĺŤ! ä¸čŞćŽş! čŤčćŽçść´ (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa

Again! Current-limiting resistor for LEDs

Ian Field

Guest

Mr. Man-wai Chang

Guest

Ian Field

Guest

Mr. Man-wai Chang

Guest

Mr. Man-wai Chang

Guest

Ian Field

Guest

P E Schoen

Guest

John Fields

Guest

John Fields

Guest

John Larkin

Guest

John Fields

Guest

Shaun

Guest

Mr. Man-wai Chang

Guest

Welcome to EDABoard.com

Sponsor

Online statistics

Forum statistics

Again! Current-limiting resistor for LEDs

Ian Field

Guest

Mr. Man-wai Chang

Guest

Ian Field

Guest

Mr. Man-wai Chang

Guest

Mr. Man-wai Chang

Guest

Ian Field

Guest

P E Schoen

Guest

John Fields

Guest

John Fields

Guest

John Larkin

Guest

John Fields

Guest

Shaun

Guest

Mr. Man-wai Chang

Guest

Log in

Welcome to EDABoard.com

Sponsor