Again! Current-limiting resistor for LEDs

M

Mr. Man-wai Chang

Guest
The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)
 
On 7/22/13 4:18 AM, Mr. Man-wai Chang wrote:
The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)
Click to expand...
You'll likely end up with a fairly dim (or completely dark) LED, if the
voltage source is exactly the LED voltage drop. That's because you won't
have any current at all.

Try the experiments I suggested earlier. Use a variable voltage source
and a variable resistance, so that you can play around with both values.
 
"Daniel Pitts" <newsgroup.nospam@virtualinfinity.net> wrote in message
news:IceHt.57941$qA.44498@fx01.iad...
On 7/22/13 4:18 AM, Mr. Man-wai Chang wrote:
The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)


You'll likely end up with a fairly dim (or completely dark) LED, if the
voltage source is exactly the LED voltage drop. That's because you won't
have any current at all.

Try the experiments I suggested earlier. Use a variable voltage source and
a variable resistance, so that you can play around with both values.
Click to expand...
Having read a few of this guy's posts - I think he should take up woodwork.
 
On 7/22/2013 4:13 PM, Ian Field wrote:

Having read a few of this guy's posts - I think he should take up woodwork.
Click to expand...
Please Ian, he would need sharp tools!
 
On Mon, 22 Jul 2013 16:22:20 -0400, Tom Biasi <tombiasi@optonline.net>
wrote:

On 7/22/2013 4:13 PM, Ian Field wrote:

Having read a few of this guy's posts - I think he should take up woodwork.

Please Ian, he would need sharp tools!
Click to expand...
Ian should avoid sharp tools... like pencils... >:-}

...Jim Thompson
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| James E.Thompson | mens |
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| San Tan Valley, AZ 85142 Skype: Contacts Only | |
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I love to cook with wine. Sometimes I even put it in the food.
 
On 7/22/13 1:13 PM, Ian Field wrote:
"Daniel Pitts" <newsgroup.nospam@virtualinfinity.net> wrote in message
news:IceHt.57941$qA.44498@fx01.iad...
On 7/22/13 4:18 AM, Mr. Man-wai Chang wrote:
The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)


You'll likely end up with a fairly dim (or completely dark) LED, if
the voltage source is exactly the LED voltage drop. That's because you
won't have any current at all.

Try the experiments I suggested earlier. Use a variable voltage source
and a variable resistance, so that you can play around with both values.

Having read a few of this guy's posts - I think he should take up woodwork.
Perhaps, but I think his questions have helped me to devise explanations
Click to expand...
and experiments to validate the assertions we've been making.
 
On Mon, 22 Jul 2013 19:18:52 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)
Click to expand...
Zero ohms.

Sounds like a lot of cheap LED flashlights and key-ring lights:
Battery, switch, LED.

LED voltage-current curves aren't brick walls. The blue (and white)
ones can be quite ohmic.



--

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
 
On 23/07/2013 1:45 AM, Daniel Pitts wrote:
On 7/22/13 4:18 AM, Mr. Man-wai Chang wrote:
The supplied voltage is 3V.
The forward voltage drop over LED is 3V.
What is the resistance of current-limiting resistor? :)

You'll likely end up with a fairly dim (or completely dark) LED, if the
voltage source is exactly the LED voltage drop. That's because you won't
have any current at all.
Click to expand...
Then do I need a current-limiting resistor when connecting two 3V LEDs
in series to a 6V supply voltage? :)

Yes, sort of going back to my question I posted days ago! :)

Try the experiments I suggested earlier. Use a variable voltage source
and a variable resistance, so that you can play around with both values.
Click to expand...
Later... A LED may be dirt cheap, but blowing one up is still money. :)

--
@~@ 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 23/07/2013 9:10 AM, John Larkin wrote:
On Mon, 22 Jul 2013 19:18:52 +0800, "Mr. Man-wai Chang"
toylet.toylet@gmail.com> wrote:

The supplied voltage is 3V.
The forward voltage drop over LED is 3V.
What is the resistance of current-limiting resistor? :)

Zero ohms.
Click to expand...
Should I worry about the LED shorting itself in this scenario? :)

--
@~@ 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
^ ^ 22:36:02 up 1:17 0 users load average: 0.03 0.04 0.05
不借貸! 不詐騙! 不援交! 不打交! 不打劫! 不自殺! 請考慮綜援 (CSSA):
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On 23/07/2013 4:52 AM, Daniel Pitts wrote:
Perhaps, but I think his questions have helped me to devise explanations
and experiments to validate the assertions we've been making.
Click to expand...
It could be fun to ask EE students to write a short paper about this
simple LED thingy... :)

--
@~@ 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
^ ^ 22:36:02 up 1:17 0 users load average: 0.03 0.04 0.05
不借貸! 不詐騙! 不援交! 不打交! 不打劫! 不自殺! 請考慮綜援 (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa
 
"Mr. Man-wai Chang" <toylet.toylet@gmail.com> wrote in message
news:ksm57q$ajl$2@dont-email.me...
On 23/07/2013 9:10 AM, John Larkin wrote:
On Mon, 22 Jul 2013 19:18:52 +0800, "Mr. Man-wai Chang"
toylet.toylet@gmail.com> wrote:

The supplied voltage is 3V.
The forward voltage drop over LED is 3V.
What is the resistance of current-limiting resistor? :)

Zero ohms.

Should I worry about the LED shorting itself in this scenario? :)
Click to expand...
Not if the batteries have enough internal resistance to protect the LED.

Some makes of 3D LED flashlight have no limiting resistor, but it doesn't do
the LED any good and the light output falls off long before the batteries
are fully used up.

You can get 1 or 2 cell LED flashlights very cheaply, they have a flyback
boost converter which is better for the LED and keeps on going until the
batteries are finished.
 
On Tue, 23 Jul 2013 22:56:49 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 23/07/2013 9:10 AM, John Larkin wrote:
On Mon, 22 Jul 2013 19:18:52 +0800, "Mr. Man-wai Chang"
toylet.toylet@gmail.com> wrote:

The supplied voltage is 3V.
The forward voltage drop over LED is 3V.
What is the resistance of current-limiting resistor? :)

Zero ohms.

Should I worry about the LED shorting itself in this scenario? :)
Click to expand...
It's certainly trickier than using a current limiter of some sort. The loaded
battery voltage has to match up with the desired operating point on the LED
curve. Any voltage mismatch changes the current by an amount deltaV/Re, where Re
is the sum of the battery internal resistance and the slope resistance of the
LED curve.

I had a key-ring light that had two 3-volt Li batteries and one white LED. I
guess the "designer" figured that the battery internal resistance would limit
the current. It didn't. The LED started flashing at high brightness for a while,
then failed. I've seen this flash-fail thing happen elsewhere, too.





--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
 
On Tuesday, July 23, 2013 7:55:42 AM UTC-7, Mr. Man-wai Chang wrote:
On 23/07/2013 1:45 AM, Daniel Pitts wrote:

On 7/22/13 4:18 AM, Mr. Man-wai Chang wrote:

The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)

Then do I need a current-limiting resistor when connecting two 3V LEDs
in series to a 6V supply voltage? :)
Click to expand...
It's not enough information.

Your '6V' nominal supply has a tolerance, and so does the '3V' forward
voltage on the LED. If the LED were a perfect diode, it'd double its forward
current every 15 mV or so, and 6.15V would, while being
nominally still '6V', force about a thousand times the intended current
through the LED.

Your LED choice, and battery choice, includes internal resistances
and temperature coefficients, and measured-value tolerances. LED
direct to battery CAN work, but you have to examine all the vertices
(i.e. all the extreme values of temperature, voltage, etc.) and not
just one point which is the 'nominal' set of values.
 
On 2013-07-23, Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:
On 23/07/2013 1:45 AM, Daniel Pitts wrote:
On 7/22/13 4:18 AM, Mr. Man-wai Chang wrote:
The supplied voltage is 3V.
The forward voltage drop over LED is 3V.
What is the resistance of current-limiting resistor? :)

You'll likely end up with a fairly dim (or completely dark) LED, if the
voltage source is exactly the LED voltage drop. That's because you won't
have any current at all.

Then do I need a current-limiting resistor when connecting two 3V LEDs
in series to a 6V supply voltage? :)
Click to expand...
It depends on the 6V and on the LEDs.

Yes, sort of going back to my question I posted days ago! :)

Try the experiments I suggested earlier. Use a variable voltage source
and a variable resistance, so that you can play around with both values.

Later... A LED may be dirt cheap, but blowing one up is still money. :)
Click to expand...
it's a cheap education.

--
⚂⚃ 100% natural

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
 
On 23/07/2013 10:57 PM, Mr. Man-wai Chang wrote:
It could be fun to ask EE students to write a short paper about this
simple LED thingy... :)
Click to expand...
I remember: "essay", not "short paper"! ;)

--
@~@ 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
^ ^ 18:18:01 up 16:30 0 users load average: 0.00 0.01 0.05
不借貸! 不詐騙! 不援交! 不打交! 不打劫! 不自殺! 請考慮綜援 (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa
 
On Mon, 22 Jul 2013 19:18:52 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)
Click to expand...
---
You seem to have missed the point, which is that with a fixed current
through the LED the voltage drop across it will be in the range of the
voltages specified by the data sheet.

Take, for instance, a white LED with a nominal drop of 3.5V and a
range of 3 to 4V with 20 Ma through it.

That means that with 20mA through those LEDs most will drop 3.5V, but
the drop can vary from 3 to 4 volts for any of them.

With that in mind you should understand that what's important is to
force the desired current through the LED instead of applying a
certain voltage across it and expecting the current through it to be
what you want.

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.

--
JF
 
"Cydrome Leader" <presence@MUNGEpanix.com> wrote in message
news:kspgb1$8e$3@reader1.panix.com...
Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:
The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)

nice one.

what's the resistance of your 3 volt power source, and forward current of
the LED?
Click to expand...
There are plenty of cheap keychain lights in the discount stores; red LED +
CR2032, or blue/white LED + 2x CR2016 each with no limiting resistor.

The lithium coin cells are rated about 3.6V nominal.

If its 2x alkaline cells - they produce just over 1.5V per cell - 2 cells
will give a pale light from a blue LED.

My first LED flashlight was a 3D with no limiting resistor - with fresh
cells its blindingly bright, but from the point that the beam becomes weak,
the batteries will still serve some duty in a non LED application.

Many discount stores carry 1 or 2 cell flashlights that contain a small
boost converter - probably based on the PR4401 chip.



http://www.prema.com/pdf/pr4401.pdf
 
Mr. Man-wai Chang <toylet.toylet@gmail.com> wrote:
The supplied voltage is 3V.

The forward voltage drop over LED is 3V.

What is the resistance of current-limiting resistor? :)
Click to expand...
nice one.

what's the resistance of your 3 volt power source, and forward current of
the LED?
 
On 25/07/2013 5:39 AM, Ian Field wrote:
Many discount stores carry 1 or 2 cell flashlights that contain a small
boost converter - probably based on the PR4401 chip.
http://www.prema.com/pdf/pr4401.pdf
Click to expand...
Sounds interesting... :)

--
@~@ 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
^ ^ 13:18:01 up 1 day 11:30 0 users load average: 0.08 0.04 0.05
不借貸! 不詐騙! 不援交! 不打交! 不打劫! 不自殺! 請考慮綜援 (CSSA):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa
 
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.
Click to expand...
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?

--
@~@ 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
不借貸! 不詐騙! 不援交! 不打交! 不打劫! 不自殺! 請考慮綜援 (CSSA):
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