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

[Mr. Man-wai Chang]

I supposed it should work fine...

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
/( _ )\ (Fedora 18 i686) Linux 3.9.6-200.fc18.i686
^ ^ 21:27:01 up 1:53 0 users load average: 0.36 0.12 0.07
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

 

[Rheilly Phoull]

On 24/06/13 21:32, Mr. Man-wai Chang wrote:

I supposed it should work fine...

Hmmm, some are more typical than others. If you had Blue in mind it

might be tricky using conventional methods.

 

[Mr. Man-wai Chang]

On 24/06/2013 9:41 PM, Rheilly Phoull wrote:

On 24/06/13 21:32, Mr. Man-wai Chang wrote:
I supposed it should work fine...

Hmmm, some are more typical than others. If you had Blue in mind it
might be tricky using conventional methods.

Was considering to connect two 3mm LEDs to the 5V of a molex connector
inside a PC chassis. Nothing fancy!

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
/( _ )\ (Fedora 18 i686) Linux 3.9.6-200.fc18.i686
^ ^ 22:00:01 up 2:26 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

 

[John Fields]

On Mon, 24 Jun 2013 21:32:09 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

I supposed it should work fine...

---
Depends on the LED chemistry:

http://www.oksolar.com/led/led_color_chart.htm

--
JF

 

[Peter Bennett]

On Mon, 24 Jun 2013 22:08:16 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

On 24/06/2013 9:41 PM, Rheilly Phoull wrote:
On 24/06/13 21:32, Mr. Man-wai Chang wrote:
I supposed it should work fine...

Hmmm, some are more typical than others. If you had Blue in mind it
might be tricky using conventional methods.

Was considering to connect two 3mm LEDs to the 5V of a molex connector
inside a PC chassis. Nothing fancy!

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.

Blue and while LEDs have a voltage drop of 3 volts or more, so you
couldn't run two of them in series from 5 volts, but you could run two
sets of LED and current limiting resistor in series, in parallel from
5 volts.


--
Peter Bennett, VE7CEI Vancouver BC
peterbb (at) telus.net
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Daniel Pitts]

On 6/24/13 6:32 AM, Mr. Man-wai Chang wrote:

I supposed it should work fine...

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.

Diodes work differently than other types of loads. They always drop a
pretty consistent voltage, regardless of current. This means that if
nothing else resists the current, then it basically looks like a short.

That's a simplified model of diodes, but for the most part a very useful
one. As others have mentioned, the voltage that LEDs drop depends on the
chemistry. As does the color.

Usually, when you buy LEDs, the spec sheet tells you the important
characteristics:

1. max forward current, If(max).
2. Typical forward current, If(typ)
3. Typical forward voltage drop. Vf

To figure out how much resistance to use, you want to make sure that the
total current is less than the max forward current, and preferably
closer to the typical forward current.

Let's use a concrete example. From the first google result for "Blue LED
spec sheet" <http://www.lc-led.com/products/500tsb4d.html>

Peak current is necessary to consider if you have capacitor in the
circuit, or some other kind of timing dependent circuitry. Let's ignore
that for now.

Max continuous forward current is 30ma. So we want to keep it below
that. Let's aim for 20ma.

Typical forward voltage drop is 3.6 (already 2.5v is too low).

Take your total supply voltage (5v in your case), and subtract the rated
typical forward voltage drop. 5v - 3.6v = 1.4v

Now we know that the LED drops 3.6v, and the "rest" of the series
circuit needs to drop 1.4v. Using ohm's law V=I*R, we know that we want
R such that 1.4v=20ma*R. 1.4v/20ma = 70ohms. That is the minimum
resistor you need. The next standard size up from 70Ί is 75Ί. You could
also go up to 100Ί and end up with 14ma through the LED, which may be
bright enough.

In professional applications, you'll also need to consider the power
dissipation, but that's another few pages of discussion ;-)

Hopefully this helps,
Daniel.

P.S. I'm not an expert in the field, so I may have forgotten something
or gotten something wrong. Anyone else should feel free to correct me.

 

[Jim Thompson]

On Mon, 24 Jun 2013 10:32:29 -0700, Daniel Pitts
<newsgroup.nospam@virtualinfinity.net> wrote:

On 6/24/13 6:32 AM, Mr. Man-wai Chang wrote:

I supposed it should work fine...

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.

Diodes work differently than other types of loads. They always drop a
pretty consistent voltage, regardless of current. This means that if
nothing else resists the current, then it basically looks like a short.

That's a simplified model of diodes, but for the most part a very useful
one. As others have mentioned, the voltage that LEDs drop depends on the
chemistry. As does the color.

Usually, when you buy LEDs, the spec sheet tells you the important
characteristics:

1. max forward current, If(max).
2. Typical forward current, If(typ)
3. Typical forward voltage drop. Vf

To figure out how much resistance to use, you want to make sure that the
total current is less than the max forward current, and preferably
closer to the typical forward current.

Let's use a concrete example. From the first google result for "Blue LED
spec sheet" <http://www.lc-led.com/products/500tsb4d.html

Peak current is necessary to consider if you have capacitor in the
circuit, or some other kind of timing dependent circuitry. Let's ignore
that for now.

Max continuous forward current is 30ma. So we want to keep it below
that. Let's aim for 20ma.

Typical forward voltage drop is 3.6 (already 2.5v is too low).

Take your total supply voltage (5v in your case), and subtract the rated
typical forward voltage drop. 5v - 3.6v = 1.4v

Now we know that the LED drops 3.6v, and the "rest" of the series
circuit needs to drop 1.4v. Using ohm's law V=I*R, we know that we want
R such that 1.4v=20ma*R. 1.4v/20ma = 70ohms. That is the minimum
resistor you need. The next standard size up from 70? is 75?. You could
also go up to 100? and end up with 14ma through the LED, which may be
bright enough.

In professional applications, you'll also need to consider the power
dissipation, but that's another few pages of discussion ;-)

Hopefully this helps,
Daniel.

P.S. I'm not an expert in the field, so I may have forgotten something
or gotten something wrong. Anyone else should feel free to correct me.

There once was a chip, LM3909... from an old post...

The LM3909, originally called "The Miser", was designed by fellow MIT
class of 1962 grad Robert A. Hirschfeld back in the '60's.

Then he went on to become a lawyer, espoused father's rights in child
custody cases, and was ultimately disbarred here in Arizona a few
years ago ;-)

One of my own lawyers is married to Hirschfeld's daughter.

John F. Banzhaf III was also an EE graduate of the class of 1962.
He's now the lawyer behind all the suits against junk food. Gives you
an idea what a weird graduating class that was ;-)

Back to the LM3909...

The data sheet schematic is a reasonably good representation of the
actual circuit... seems you young bucks might copy it as a discrete
circuit and embellish it for blue LEDs.

(It only has five transistors.)

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Jim Thompson]

On Mon, 24 Jun 2013 11:33:17 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Monday, June 24, 2013 1:32:29 PM UTC-4, Daniel Pitts wrote:
On 6/24/13 6:32 AM, Mr. Man-wai Chang wrote:



I supposed it should work fine...



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.

Diodes work differently than other types of loads. They always drop a
pretty consistent voltage, regardless of current. This means that if
nothing else resists the current, then it basically looks like a short.

Hey, that raises an interesting question. I always figured LED's are like other diodes (I-V curve is an exponential.)
So I've got an amber (590nm) LED... here's the voltage drop vs bias current.

Current Forward Voltage
10nA 1.273V
100nA 1.387V
1uA 1.498V
10uA 1.605V
100uA 1.706V
1mA 1.806V
10mA 1.923V

Looks pretty exponential to me until it gets a noticeable resistive
term... mid-range looks to be _around_ 100mV/decade.

[snip]

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Tim Wescott]

On Mon, 24 Jun 2013 22:08:16 +0800, Mr. Man-wai Chang wrote:

On 24/06/2013 9:41 PM, Rheilly Phoull wrote:
On 24/06/13 21:32, Mr. Man-wai Chang wrote:
I supposed it should work fine...

Hmmm, some are more typical than others. If you had Blue in mind it
might be tricky using conventional methods.

Was considering to connect two 3mm LEDs to the 5V of a molex connector
inside a PC chassis. Nothing fancy!

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.

If you have the data sheets then check the current and voltage ratings of
the things and size your resistors accordingly. If not, try about 220
ohms.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

 

[George Herold]

On Monday, June 24, 2013 1:32:29 PM UTC-4, Daniel Pitts wrote:

On 6/24/13 6:32 AM, Mr. Man-wai Chang wrote:



I supposed it should work fine...



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.

Diodes work differently than other types of loads. They always drop a
pretty consistent voltage, regardless of current. This means that if
nothing else resists the current, then it basically looks like a short.

Hey, that raises an interesting question. I always figured LED's are like other diodes (I-V curve is an exponential.)
So I've got an amber (590nm) LED... here's the voltage drop vs bias current..

Current Forward Voltage
10nA 1.273V
100nA 1.387V
1uA 1.498V
10uA 1.605V
100uA 1.706V
1mA 1.806V
10mA 1.923V

With a dark room I can just see the light at 1uA. 10uA is no problem.

I've also heard tell you can get a feel for Plancks constant by plotting
forward voltage vs led color (for a bunch of different leds).
e*V = h*freq = h*c/wavlength
But this is not right.... first off, for the above 590 nm LED I get 2.09V
And secondly what current do I pick?

Hey it's gotta be that the LED is just using those electrons at the high energy end of the Boltzmann distribution... Can you see any LED cooling of the junction? At 1mA and ~0.2 V (2.0 - 1.8) that's ~0.2mW

George H.

That's a simplified model of diodes, but for the most part a very useful
one. As others have mentioned, the voltage that LEDs drop depends on the
chemistry. As does the color.



Usually, when you buy LEDs, the spec sheet tells you the important

characteristics:



1. max forward current, If(max).

2. Typical forward current, If(typ)

3. Typical forward voltage drop. Vf



To figure out how much resistance to use, you want to make sure that the

total current is less than the max forward current, and preferably

closer to the typical forward current.



Let's use a concrete example. From the first google result for "Blue LED

spec sheet" <http://www.lc-led.com/products/500tsb4d.html



Peak current is necessary to consider if you have capacitor in the

circuit, or some other kind of timing dependent circuitry. Let's ignore

that for now.



Max continuous forward current is 30ma. So we want to keep it below

that. Let's aim for 20ma.



Typical forward voltage drop is 3.6 (already 2.5v is too low).



Take your total supply voltage (5v in your case), and subtract the rated

typical forward voltage drop. 5v - 3.6v = 1.4v



Now we know that the LED drops 3.6v, and the "rest" of the series

circuit needs to drop 1.4v. Using ohm's law V=I*R, we know that we want

R such that 1.4v=20ma*R. 1.4v/20ma = 70ohms. That is the minimum

resistor you need. The next standard size up from 70Ί is 75Ί. You could

also go up to 100Ί and end up with 14ma through the LED, which may be

bright enough.



In professional applications, you'll also need to consider the power

dissipation, but that's another few pages of discussion ;-)



Hopefully this helps,

Daniel.



P.S. I'm not an expert in the field, so I may have forgotten something

or gotten something wrong. Anyone else should feel free to correct me.

 

[Jamie]

Mr. Man-wai Chang wrote:

I supposed it should work fine...

depends on make, model and color!

Jamie

 

[John Larkin]

On Mon, 24 Jun 2013 21:32:09 +0800, "Mr. Man-wai Chang"
<toylet.toylet@gmail.com> wrote:

I supposed it should work fine...

Here are some points:

https://dl.dropboxusercontent.com/u/53724080/Optos/OsramCurrents.JPG

The greens are visible at 1 uA in office light, barely perceptable to
the close-up, dark-adapted eye at about 1 nA.



--

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

 

[Mr. Man-wai Chang]

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.

Blue and while LEDs have a voltage drop of 3 volts or more, so you
couldn't run two of them in series from 5 volts, but you could run two
sets of LED and current limiting resistor in series, in parallel from
5 volts.

Thanks!

--
@~@ 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):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa

 

[Mr. Man-wai Chang]

On 25/06/2013 8:24 AM, John Larkin wrote:

https://dl.dropboxusercontent.com/u/53724080/Optos/OsramCurrents.JPG
The greens are visible at 1 uA in office light, barely perceptable to
the close-up, dark-adapted eye at about 1 nA.

Thanks

--
@~@ 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):
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa

 

[Mr. Man-wai Chang]

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?

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
/( _ )\ (Fedora 18 i686) Linux 3.9.6-200.fc18.i686
^ ^ 14:33:01 up 2 min 0 users load average: 0.20 0.19 0.08
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]

On 24/06/2013 11:55 PM, John Fields wrote:

http://www.oksolar.com/led/led_color_chart.htm

Thanks

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
/( _ )\ (Fedora 18 i686) Linux 3.9.6-200.fc18.i686
^ ^ 14:45:01 up 14 min 0 users load average: 0.01 0.07 0.09
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]

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?

--
@~@ Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty! May the Force and farces be with you!
/( _ )\ (Fedora 18 i686) Linux 3.9.6-200.fc18.i686
^ ^ 14:45:01 up 14 min 0 users load average: 0.01 0.07 0.09
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

 

[Daniel Pitts]

On 6/24/13 11:50 PM, Mr. Man-wai Chang 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?

No, that's not how parallel circuitry works. In a parallel circuit, the

voltage is equal across the branches. In your case, that's the 5v will
always be across each of the LED+resistor circuits. If you had equal
current going through both branches, and then one of the branches
"died", resulting in an open circuit, your total current would just be
cut in half.

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. In a parallel circuit, the current wouldn't likely go
through the other LED.

 

[Peter Bennett]

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.



--
Peter Bennett, VE7CEI Vancouver BC
peterbb (at) telus.net
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Daniel Pitts]

On 6/24/13 11:39 PM, Mr. Man-wai Chang 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?

Yes. Unless the voltage drop equals exactly 5v, *and* the voltage source

will never go above 5v, you will have some extra voltage left over. If
you have no resistance (or the very low resistance of the wire), you
will have a run-away current.

It may even appear to work for a while, but you could be damaging your
power-supply.