Resistor value for LED

T

Tom

Guest
I have a led that needs 2.3 to 2.6 V max, I want to run it off a 5V
supply at close to its maximum voltage.

The exact led is here:

http://tinyurl.com/cmleb

Could someone tell me what value resistor I need to use?

Thanks
Tom
 
In article <42712E86.9A60113F@address.invalid>,
Tom <no@address.invalid> wrote:

I have a led that needs 2.3 to 2.6 V max, I want to run it off a 5V
supply at close to its maximum voltage.
Click to expand...
What the data sheet tells us is that the LED's absolute forward
current level is 50 mA continuous, or 100 mA if fed a low-duty-cycle
high-speed pulse train. More current than that will damage it.

Running an LED near to its absolute-maximum rating is, I believe,
likely to shorten its lifetime significantly. I usually prefer to
keep them at half-maximum or a bit less. This LED's brightness (which
is considerable) is rated by the manufacturer at a forward current of
20 mA, so that's what I'd suggest using.

At that current level, the LED is going to have a forward voltage of
2.3 to 2.6 volts. Let's use 2.5 volts as a working figure (it won't
matter very much if the particular LED you use is a bit higher or
lower).

So... a 5-volt supply, with 2.5 volts appearing across the diode.
This means that we must choose a resistor which will have (5.0 - 2.5)
= 2.5 volts at a forward current level of 20 mA.

You can solve for this by using Ohm's Law, which states that E=I*R, or
(by solving for R) R=E/I.

R = 2.5 / .02 (if we do it in volts and amps) or
R = 2500 / 20 (if we do it millivolts and milliamps)

In either case, R works out to be 125 ohms.

Let's figure out the power dissipation. P = E*I. We know that E
(across the resistor) is 2.5, and I is .02, so P = .05 watts of heat
dissipated in the resistor. It's generally a good idea to "de-rate"
resistors by 2:1 (that is, don't ask them to dissipate more than half
of their rated capability) to ensure long life. In this particular
case that's no problem, since the smallest standard leaded resistors
are 1/8 watt.

So - choose a resistor value in the neighborhood of 125 ohms (whatever
you have that's closest, plus or minus 10-20%), in whatever size is
convenient (1/8 watt or larger), and you should be fine.


--
Dave Platt <dplatt@radagast.org> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
 
tlbs wrote:
Notice in the data sheet, that the forward current (If) listed for
those maximum voltages is 20 mA. You need to drop half of your voltage
across the resistor (~2.5 V), so use Ohms law.

R = V / I
R = 2.5 / 0.02
R = 125 Ohm

The resistor power is calculated thus:
P = V^2 / R
P = 2.5^2 / 125
P = 0.05 W

so a 1/8 W resistor or larger will do just fine.

Since the LED will handle up to 50 mA continuously (although I would
not recommend operating it like that) the resistor value can go as low
as 50 Ohms. The LED will PROBABLY light up dimly with as little as 10
mA, so the resistor value can go as high as 250 Ohms.
Click to expand...

Thanks for the excellent replies!
 
"Tom" <no@address.invalid> wrote in message
news:42712E86.9A60113F@address.invalid...
I have a led that needs 2.3 to 2.6 V max, I want to run it off a 5V
supply at close to its maximum voltage.

The exact led is here:

http://tinyurl.com/cmleb

Could someone tell me what value resistor I need to use?
Click to expand...
Looks like 68 ohms would be a good choice. Use at least a quarter watt
resistor.

Thanks
Tom
Click to expand...
 
I'm astonished, nay FLABBERGASTED, that none of the regulars on this
newsgroup told this fellow to use a microcontroller to do the job.


{;-)


Jim



Could someone tell me what value resistor I need to use?

Thanks
Tom
Click to expand...
 
"mc" <mc_no_spam@uga.edu> wrote in message
news:42714f60$1@mustang.speedfactory.net...
"Tom" <no@address.invalid> wrote in message
news:42712E86.9A60113F@address.invalid...
I have a led that needs 2.3 to 2.6 V max, I want to run it off a 5V
supply at close to its maximum voltage.

The exact led is here:

http://tinyurl.com/cmleb

Could someone tell me what value resistor I need to use?

Typical operation is with a current of 20 mA. I am surprised that
they say
2.3 volts, because the normal forward voltage drop of a red LED is 1.8
volts, and that's what I'll assume.
Click to expand...
The older GaAs LEDs were lower, maybe below 2VDC. But I took a bunch of
indicator grade red LEDs, maybe just a few mCd, and measured them at
20mA. They were all 2VDC or even more. Last night I took a single high
brightness red LED and put 25mA thru it, and it dropped 2.3VDC. All the
newer GaInAs whatever LEDs seem to be well over 2V drop.

I put this high brightness red LED into a '50s vintage 'jewel'
indicator. These have a red glass multifaceted 'jewel' in a threaded
brass base a half inch across, which was mounted on the equipment panel
with a #47 lamp behind it. The red LED looks really good behind the
jewel. Maybe a bit too bright at 20mA, tho. I looked into it and had
spots in my vision for a minute afterwards. :p

You need a voltage drop of 3.2 volts (to take 5.0 down to 1.8 volts)
at 20
mA = 0.020 A. Ohm's Law:

R = E / I = 3.2 / 0.020 = 160 ohms.

They don't make 160-ohm resistors, so use either 150 or 220.
Click to expand...
Again, you've missed the boat. The 160, 1.6k, etc., 5% resistor values
are no harder to get than 1k or 2.2k. Standard resistor values can be
found here. _Read_and_remember_them_.
http://www.rfcafe.com/references/electrical/resistor_values.htm

Now... Will an ordinary 1/8-watt resistor handle the current? Let's
see.

P = E I = 3.2 * 0.020 = 0.06 watt

Yes.
Click to expand...
 
"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on this
newsgroup told this fellow to use a microcontroller to do the job.


{;-)
Click to expand...
Make fun if you like, but it's way more power efficient than a resistor,
even if it's not cheaper.....yet. ;-) AFAIK, even an exceptional SMPS
couldn't do it any more efficiently. IME a nano-watt PIC running at
32kHz should be ~99% efficient driving the LED with at least 10mA
average current.

How difficult would it be to beat that using another method?
 
On Sun, 01 May 2005 17:12:17 GMT, the renowned "Anthony Fremont"
<spam@anywhere.com> wrote:

"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on this
newsgroup told this fellow to use a microcontroller to do the job.


{;-)

Make fun if you like, but it's way more power efficient than a resistor,
even if it's not cheaper.....yet. ;-) AFAIK, even an exceptional SMPS
couldn't do it any more efficiently. IME a nano-watt PIC running at
32kHz should be ~99% efficient driving the LED with at least 10mA
average current.
Click to expand...
By your definition a resistor is 100% efficient, then?



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:hu3a71lfrv4tdm5928cahbdks0nlnck9gq@4ax.com...
On Sun, 01 May 2005 17:12:17 GMT, the renowned "Anthony Fremont"
spam@anywhere.com> wrote:


"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on
this
newsgroup told this fellow to use a microcontroller to do the job.


{;-)

Make fun if you like, but it's way more power efficient than a
resistor,
even if it's not cheaper.....yet. ;-) AFAIK, even an exceptional
SMPS
couldn't do it any more efficiently. IME a nano-watt PIC running at
32kHz should be ~99% efficient driving the LED with at least 10mA
average current.

By your definition a resistor is 100% efficient, then?
Click to expand...
I don't know what you mean. I was thinking the dropping resistor
approach was much less than 50% efficient since the resistor ends up
dissipating more power than does the LED. Is that not correct?

I was thinking that by PWM'ng the LED and using a bit of inductance to
limit the current surges thru the i/o pin, would be much more efficient.
A nano-watt 16f88 claims to dissipate a max of 40uA at 5V running at
32kHz. That's only 200uW of power used by the PIC. The LED dissipates
many times (at least 100?) as much power. What am I doing wrong?
 
On Sun, 01 May 2005 17:27:19 GMT, the renowned "Anthony Fremont"
<spam@anywhere.com> wrote:

"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:hu3a71lfrv4tdm5928cahbdks0nlnck9gq@4ax.com...
On Sun, 01 May 2005 17:12:17 GMT, the renowned "Anthony Fremont"
spam@anywhere.com> wrote:


"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on
this
newsgroup told this fellow to use a microcontroller to do the job.


{;-)

Make fun if you like, but it's way more power efficient than a
resistor,
even if it's not cheaper.....yet. ;-) AFAIK, even an exceptional
SMPS
couldn't do it any more efficiently. IME a nano-watt PIC running at
32kHz should be ~99% efficient driving the LED with at least 10mA
average current.

By your definition a resistor is 100% efficient, then?

I don't know what you mean. I was thinking the dropping resistor
approach was much less than 50% efficient since the resistor ends up
dissipating more power than does the LED. Is that not correct?
Click to expand...
Yes.

I was thinking that by PWM'ng the LED and using a bit of inductance to
limit the current surges thru the i/o pin, would be much more efficient.
A nano-watt 16f88 claims to dissipate a max of 40uA at 5V running at
32kHz. That's only 200uW of power used by the PIC. The LED dissipates
many times (at least 100?) as much power. What am I doing wrong?
Click to expand...
The increased efficiency would be because of the inductor and
switch(s) or switch + diode, not the control circuitry. I didn't see
either taken into account in your efficiency calculation. They'll not
likely be very close to being ideal.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:R58de.35651$h6.10828@tornado.texas.rr.com...
"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on this
newsgroup told this fellow to use a microcontroller to do the job.


{;-)

Make fun if you like, but it's way more power efficient than a resistor,
even if it's not cheaper.....yet. ;-) AFAIK, even an exceptional SMPS
couldn't do it any more efficiently. IME a nano-watt PIC running at
32kHz should be ~99% efficient driving the LED with at least 10mA
average current.

How difficult would it be to beat that using another method?
Click to expand...

The learning curve. When someone asks how to turn on a LED.
There was a space constraint to the project box as I recall.
Most of everyone thinks of a DC source,1 resistor and the LED.
Simple enough. Someone posts the actual calculations, E=IR and
W=EI . Without knowing about E=IR or W=EI, you expect to jump
into Microcontrollers? Writing uC programs ?
How long will it be before you research about PICs, build or buy a pic
programmer, learn to program a PIC. There's a big jump between
a transistor battery and a PIC project, don't you think?
To run, you 1st got to learn to crawl & stand up.
 
"EN" <res808c4@earthlink.net> wrote in message
news:sgade.2636$7F4.2403@newsread2.news.atl.earthlink.net...
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:R58de.35651$h6.10828@tornado.texas.rr.com...

"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on
this
newsgroup told this fellow to use a microcontroller to do the job.

{;-)

Make fun if you like, but it's way more power efficient than a
resistor,
even if it's not cheaper.....yet. ;-) AFAIK, even an exceptional
SMPS
couldn't do it any more efficiently. IME a nano-watt PIC running at
32kHz should be ~99% efficient driving the LED with at least 10mA
average current.

How difficult would it be to beat that using another method?


The learning curve. When someone asks how to turn on a LED.
There was a space constraint to the project box as I recall.
Most of everyone thinks of a DC source,1 resistor and the LED.
Simple enough. Someone posts the actual calculations, E=IR and
W=EI . Without knowing about E=IR or W=EI, you expect to jump
into Microcontrollers? Writing uC programs ?
How long will it be before you research about PICs, build or buy a pic
programmer, learn to program a PIC. There's a big jump between
a transistor battery and a PIC project, don't you think?
To run, you 1st got to learn to crawl & stand up.
Click to expand...
My exact sentiments. However there are a lotta pic programmers who
don't have a clue about analog electronics. They could connect point A
to point B and make a digital circuit (and with the limited number of
pins on a pic, it's really easy), but wouldn't have a clue as to how to
calculate the resistor for the LED.

I'm not suggesting that they _get_ a clue. They do just fine without
one.
 
"EN" <res808c4@earthlink.net> wrote in message
news:sgade.2636$7F4.2403@newsread2.news.atl.earthlink.net...
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:R58de.35651$h6.10828@tornado.texas.rr.com...

"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on
this
newsgroup told this fellow to use a microcontroller to do the job.


{;-)

Make fun if you like, but it's way more power efficient than a
resistor,
even if it's not cheaper.....yet. ;-) AFAIK, even an exceptional
SMPS
couldn't do it any more efficiently. IME a nano-watt PIC running at
32kHz should be ~99% efficient driving the LED with at least 10mA
average current.

How difficult would it be to beat that using another method?


The learning curve. When someone asks how to turn on a LED.
There was a space constraint to the project box as I recall.
Most of everyone thinks of a DC source,1 resistor and the LED.
Simple enough. Someone posts the actual calculations, E=IR and
W=EI . Without knowing about E=IR or W=EI, you expect to jump
into Microcontrollers? Writing uC programs ?
Click to expand...
Whoa nellie, I didn't suggest the OP do any such thing. ;-) I was only
following up on Jim's comment about PICs.

How long will it be before you research about PICs, build or buy a pic
programmer, learn to program a PIC. There's a big jump between
a transistor battery and a PIC project, don't you think?
To run, you 1st got to learn to crawl & stand up.
Click to expand...
I really wish that every mention of a PIC didn't have to result in this
statement. It's sorta becoming the Godwin's law of electronics. It's
almost like saying, "everything about electronics is worth learning, no
matter how difficult or expensive, as long as it doesn't involve micros
because they are too hard and too expensive". At any rate, I didn't
suggest using one for the OP's problem, I only mentioned that it could
be quite efficient, as a response to Jims comment.

I wasn't looking to beat that dead horse again, I was only hoping that
someone could/would (if possible) show me something more efficient using
"conventional" components.
 
In article <1177ni69odjseba@corp.supernews.com>, Watson A.Name - \"Watt
Sun, the Dark Remover\" wrote:
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:d07771dgjdgkjekceojour6c58dc3l1b9m@4ax.com...
On Thu, 28 Apr 2005 16:25:05 -0400, the renowned "mc"
mc_no_spam@uga.edu> wrote:


"Tom" <no@address.invalid> wrote in message
news:42712E86.9A60113F@address.invalid...
I have a led that needs 2.3 to 2.6 V max, I want to run it off a 5V
supply at close to its maximum voltage.

The exact led is here:

http://tinyurl.com/cmleb

Could someone tell me what value resistor I need to use?

Typical operation is with a current of 20 mA. I am surprised that
they say
2.3 volts, because the normal forward voltage drop of a red LED is
1.8
volts, and that's what I'll assume.

Um, you're assuming the datasheet is *wrong*?

I just measured a 635nm red (that's less orange) of similar
brightness, and it came to 2.1V @ 20mA.

Generally, forward voltages seem to be creeping up (for the same color
of emitted light) as efficiencies increase.
Click to expand...
Also consider that voltage varies surprisingly little with
wavelength/color within a given chip chemistry. For example,
less-yellowish green LEDs and red ones of the same chemistry (InGaAlP,
or GaP) have surprisingly similar voltage drop.
One thing that does usually happen is that efficiency falls bigtime if
the wavelength gets shorter to an extent that has the photon energy in
electron volts exceeds the voltage drop in volts.

Yeah, and as the number of elements increase: gallium, indium, antimony,
aluminum, arsenic, rat poison...

And that doesn't even get into the various phosphors they use to get
white LEDs.
Click to expand...
Phosphors don't affect the voltage drop...

But most white LEDs have blue LED chips and phosphor. So white LEDs
will have the same voltage drop as related blue ones.

- Don Klipstein (don@misty.com)
 
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:ixgde.35869$h6.34224@tornado.texas.rr.com...
"EN" <res808c4@earthlink.net> wrote in message
news:sgade.2636$7F4.2403@newsread2.news.atl.earthlink.net...

"Anthony Fremont" <spam@anywhere.com> wrote in message
news:R58de.35651$h6.10828@tornado.texas.rr.com...

"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on
this
newsgroup told this fellow to use a microcontroller to do the
job.


{;-)

Make fun if you like, but it's way more power efficient than a
resistor,
even if it's not cheaper.....yet. ;-) AFAIK, even an exceptional
SMPS
couldn't do it any more efficiently. IME a nano-watt PIC running
at
32kHz should be ~99% efficient driving the LED with at least 10mA
average current.

How difficult would it be to beat that using another method?


The learning curve. When someone asks how to turn on a LED.
There was a space constraint to the project box as I recall.
Most of everyone thinks of a DC source,1 resistor and the LED.
Simple enough. Someone posts the actual calculations, E=IR and
W=EI . Without knowing about E=IR or W=EI, you expect to jump
into Microcontrollers? Writing uC programs ?

Whoa nellie, I didn't suggest the OP do any such thing. ;-) I was
only
following up on Jim's comment about PICs.

How long will it be before you research about PICs, build or buy a
pic
programmer, learn to program a PIC. There's a big jump between
a transistor battery and a PIC project, don't you think?
To run, you 1st got to learn to crawl & stand up.

I really wish that every mention of a PIC didn't have to result in
this
statement. It's sorta becoming the Godwin's law of electronics. It's
almost like saying, "everything about electronics is worth learning,
no
matter how difficult or expensive, as long as it doesn't involve
micros
because they are too hard and too expensive". At any rate, I didn't
suggest using one for the OP's problem, I only mentioned that it could
be quite efficient, as a response to Jims comment.

I wasn't looking to beat that dead horse again, I was only hoping that
someone could/would (if possible) show me something more efficient
using
"conventional" components.
Click to expand...
It has _nothing_ to do with "..micros because they are too hard and too
expensive". What it has to do with is deception.

Deception. If there were money being exchanged, it could be called
something illegal, like fraud. But there's no money being exchanged.
The deceiver is advocating the use of something because it's his
fervent, almost religious, belief that it's _the_ solution to all of the
OP's problems. Which may be true.

But the deceptive part is that the deceiver has not told the OP (who is
usually a neophyte and probably knows little about electronics OR pics)
that there is a price he will have to pay in up front costs for the
hardware and software to program the pic. And there is a learning curve
that the OP must go thru to learn how to program and debug the software.
It is simply NOT Plug and Play!

I have nothing against using the right tool for the job. But it's
deceptive and unfair to tell someone to use something without telling
them what they will have to pay and what they will be going thru to
complete their project.

It just so happened that microcontrollers were the subject in past
discussions. Four decades ago it could have been using a laser as a
pointer for presentations. We all know nowadays that the little
hand-held ones do an excellent job. Back then, 40 years ago, you would
have had to blow your own glass, fill it with helium and neon, and use a
high voltage power supply to excite it. If a presenter could do all
that, he could then pull out his gatling gun sized pointer and look like
the termninator as he pointed to his presentation. But it would be
deceptive to not tell him that he would have to jump thru all those
hoops to get this cool looking presentation tool.

And there is *nothing* Godwinesque about it. Deception is deception.
 
"Watson A.Name - "Watt Sun, the Dark Remover"" <NOSPAM@dslextreme.com>
wrote in message news:117bk88qlcead6f@corp.supernews.com...
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:ixgde.35869$h6.34224@tornado.texas.rr.com...

I really wish that every mention of a PIC didn't have to result in
this
statement. It's sorta becoming the Godwin's law of electronics.
It's
almost like saying, "everything about electronics is worth learning,
no
matter how difficult or expensive, as long as it doesn't involve
micros
because they are too hard and too expensive". At any rate, I didn't
suggest using one for the OP's problem, I only mentioned that it
could
be quite efficient, as a response to Jims comment.

I wasn't looking to beat that dead horse again, I was only hoping
that
someone could/would (if possible) show me something more efficient
using
"conventional" components.

It has _nothing_ to do with "..micros because they are too hard and
too
expensive". What it has to do with is deception.
Click to expand...
Here we go again. :-(

Deception. If there were money being exchanged, it could be called
something illegal, like fraud. But there's no money being exchanged.
The deceiver is advocating the use of something because it's his
fervent, almost religious, belief that it's _the_ solution to all of
the
OP's problems. Which may be true.
Click to expand...
So, suggesting the use of a micro is now tantamount to fraud? The
zealot thing is cute too. Not very realistic, but cute.

But the deceptive part is that the deceiver has not told the OP (who
is
usually a neophyte and probably knows little about electronics OR
pics)
that there is a price he will have to pay in up front costs for the
hardware and software to program the pic. And there is a learning
curve
that the OP must go thru to learn how to program and debug the
software.
It is simply NOT Plug and Play!
Click to expand...
I don't recall anyone (and especially not me) saying that it was PnP.
Of course there is a learning curve, and I believe it is simply another
step in someone's education in electronics. As I've said before, if I
was recommending one to a newbie, I'd likely be giving him the code and
offering to flash the chip for them.

To be clear, I never suggested the OP use a micro. I just wanted to
respond to Jim's comment and hopefully spawn some conversation about
more efficient ways to drive the LED. It looks like that was simply a
pipe dream on my part.

I have nothing against using the right tool for the job. But it's
deceptive and unfair to tell someone to use something without telling
them what they will have to pay and what they will be going thru to
complete their project.
Click to expand...
One might also consider it deceptive and unfair for someone to suggest
that micros are as difficult and unreachable as you describe.

It just so happened that microcontrollers were the subject in past
discussions. Four decades ago it could have been using a laser as a
pointer for presentations. We all know nowadays that the little
hand-held ones do an excellent job. Back then, 40 years ago, you
would
have had to blow your own glass, fill it with helium and neon, and use
a
high voltage power supply to excite it. If a presenter could do all
that, he could then pull out his gatling gun sized pointer and look
like
the termninator as he pointed to his presentation. But it would be
deceptive to not tell him that he would have to jump thru all those
hoops to get this cool looking presentation tool.
Click to expand...
The primary difference being that your anology is absurd. Learning
micros or purchasing the needed equipment is no where near the
difficulty you describe. Ten or twenty hours of self study and $75 for
a programmer is not quite in the same league as ramping up to make your
own LASER tubes.

And there is *nothing* Godwinesque about it. Deception is deception.
Click to expand...
If there is nothing Godwinesque about it, then why is the same old tired
argument used to shout down the micro crowd every time?

One final time, I wasn't wishing to debate this topic (as it's like
talking religion or politics to some people), I was looking for some
clever analog or digital based current source that would be more
efficient. Any ideas? ;-)
 
"Watson A.Name - "Watt Sun, the Dark Remover"" wrote

"Anthony Fremont" <spam@anywhere.com> wrote
Click to expand...
<Snipped pointless rhetoric about using microcontrollers>

As I have now said multiple times, I do not wish to have this
"religious" debate.

Very well, I take it that you wish not to contribute anything useful to
the subthread about driving an LED with extreme power efficiency.
 
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:d07771dgjdgkjekceojour6c58dc3l1b9m@4ax.com...

Typical operation is with a current of 20 mA. I am surprised that they
say
2.3 volts, because the normal forward voltage drop of a red LED is 1.8
volts, and that's what I'll assume.

Um, you're assuming the datasheet is *wrong*?

I just measured a 635nm red (that's less orange) of similar
brightness, and it came to 2.1V @ 20mA.

Generally, forward voltages seem to be creeping up (for the same color
of emitted light) as efficiencies increase.
Click to expand...
I wonder why. I thought the voltage was related to the wavelength by a
basic law of physics. Is the internal resistance also increasing, I wonder?
 
"RST Engineering (jw)" <jim@rstengineering.com> wrote in message
news:1177ap112ioto34@corp.supernews.com...
I'm astonished, nay FLABBERGASTED, that none of the regulars on this
newsgroup told this fellow to use a microcontroller to do the job.
Click to expand...
AVR or PIC ? :)
 
R = E / I = 3.2 / 0.020 = 160 ohms.

They don't make 160-ohm resistors, so use either 150 or 220.

Again, you've missed the boat. The 160, 1.6k, etc., 5% resistor values
are no harder to get than 1k or 2.2k. Standard resistor values can be
found here. _Read_and_remember_them_.
http://www.rfcafe.com/references/electrical/resistor_values.htm
Click to expand...
I know what the standard values. Some people have to shop at Radio Shack,
which has 220 but not 160 or 150 available singly. Radio Shack has
assortments that have 150 but not 160. Admittedly, "they don't make" was an
erroneous oversimplification.

I have actually not seen 160 (or 1.6, 16, 1600, etc.) very often at all.
Not all of the 5% standard series is equally widely available.
Long-established good practice, even with 5% resistors, is to prefer the
values in the 20% series first, then the 10% series, then the 5% series.

The 20% series goes: 1.0, 1.5, 2.2, 3.3, 4.7, 6.8, 10.

I spent a long time writing construction project articles for magazines and
developed a fairly strong aversion to hard-to-get parts. That, and I
started designing things back when resistors were 10% carbon composition!
 
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