| Author |
Message |
Chris W
Guest
|
 Posted:
Wed Mar 16, 2005 12:19 pm Post subject:
Re: The Results of everyones help. |
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Robert Monsen wrote:
| Quote: | Chris W wrote:
John Fields wrote:
LEDs don't want voltage, they want a certain amount of current, and
when that much current is flowing through them the voltage across them
will be the specified Vf, or forward voltage.
I know that Vf is different for different LEDs, but doesn't E = I*R
apply to LED's.
R is simply the ratio of voltage to current. For a resistor, R is
designed to be nearly constant. However, this is not true for almost
anything else you can put current through.
The current of an LED is given by the Shockley diode equation:
I = Is * (exp(-Vf/(N*Vt)) - 1)
|
Thanks, that makes it very clear. It's not that I can't supply the
correct voltage to get the correct current, just that the value is very
critical and probably varies from batch to batch, with temperature and
other factors beyond the control of my voltage supply. In this case I
am looking to get as much light out of the LED's as I can, but if I
weren't, couldn't I just supply a voltage that was enough lower than the
Vf to guarantee If was below 20ma and still get plenty of light for a
indicator or just testing on a prototype board? That way I wouldn't
have to have a resistor for every LED.
I'm glad I am getting this information before I get a regulator to
regulate the battery power going to my the LED's and circuit to control
the LED's in my RC plane.
--
Chris W
Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com
|
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Robert Monsen
Guest
|
| Posted:
Wed Mar 16, 2005 12:55 pm Post subject:
Re: The Results of everyones help. |
|
|
Chris W wrote:
| Quote: | Robert Monsen wrote:
Chris W wrote:
John Fields wrote:
LEDs don't want voltage, they want a certain amount of current, and
when that much current is flowing through them the voltage across them
will be the specified Vf, or forward voltage.
I know that Vf is different for different LEDs, but doesn't E = I*R
apply to LED's.
R is simply the ratio of voltage to current. For a resistor, R is
designed to be nearly constant. However, this is not true for almost
anything else you can put current through.
The current of an LED is given by the Shockley diode equation:
I = Is * (exp(-Vf/(N*Vt)) - 1)
Thanks, that makes it very clear.
|
You forgot the smiley...
| Quote: | It's not that I can't supply the
correct voltage to get the correct current, just that the value is very
critical and probably varies from batch to batch, with temperature and
other factors beyond the control of my voltage supply. In this case I
am looking to get as much light out of the LED's as I can, but if I
weren't, couldn't I just supply a voltage that was enough lower than the
Vf to guarantee If was below 20ma and still get plenty of light for a
indicator or just testing on a prototype board? That way I wouldn't
have to have a resistor for every LED.
I'm glad I am getting this information before I get a regulator to
regulate the battery power going to my the LED's and circuit to control
the LED's in my RC plane.
|
One problem with putting an LED on a voltage source is that the forward
voltage of the LED decreases as it heats up. Thus, if it starts to heat
up, it'll pass *more* current, causing it to heat up even more. This can
lead to failure.
Also, if you put a bunch of LEDs in parallel, one of them may pass
slightly more current, and will heat up, possibly leading to the runaway
scenario given above. Some will be dim. You just don't know. Also,
passing more current through an LED leads to premature dimming.
However, they are your LEDs. It may work if the voltage is adjusted just
right. I'll bet that the LEDs won't be consistently bright, but give it
a try, and let us know what happens.
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God. |
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John Fields
Guest
|
| Posted:
Wed Mar 16, 2005 4:50 pm Post subject:
Re: The Results of everyones help. |
|
|
On Wed, 16 Mar 2005 00:19:24 -0600, Chris W <1qazse4@cox.net> wrote:
| Quote: | Robert Monsen wrote:
Chris W wrote:
John Fields wrote:
LEDs don't want voltage, they want a certain amount of current, and
when that much current is flowing through them the voltage across them
will be the specified Vf, or forward voltage.
I know that Vf is different for different LEDs, but doesn't E = I*R
apply to LED's.
R is simply the ratio of voltage to current. For a resistor, R is
designed to be nearly constant. However, this is not true for almost
anything else you can put current through.
The current of an LED is given by the Shockley diode equation:
I = Is * (exp(-Vf/(N*Vt)) - 1)
Thanks, that makes it very clear. It's not that I can't supply the
correct voltage to get the correct current, just that the value is very
critical and probably varies from batch to batch, with temperature and
other factors beyond the control of my voltage supply. In this case I
am looking to get as much light out of the LED's as I can, but if I
weren't, couldn't I just supply a voltage that was enough lower than the
Vf to guarantee If was below 20ma and still get plenty of light for a
indicator or just testing on a prototype board? That way I wouldn't
have to have a resistor for every LED.
|
---
You could, but unless you use a couple of different supplies you could
run into problems driving the logic with the reduced voltage you'd be
using to run the LEDs. If you decide to use the Allegro parts, though,
you'll sidestep the problem since you'll only need one resistor to set
the current into all 16 LEDs each chip drives.
---
| Quote: | I'm glad I am getting this information before I get a regulator to
regulate the battery power going to my the LED's and circuit to control
the LED's in my RC plane.
|
---
Since your battery voltage will be pretty stable, and predictable, you
may not need a regulator. What did you have in mind?
--
John Fields
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Roger Johansson
Guest
|
| Posted:
Wed Mar 16, 2005 6:32 pm Post subject:
Re: The Results of everyones help. |
|
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Chris W <1qazse4@cox.net> wrote:
| Quote: | Maybe this explanation is simple enough to understand for beginners:
A little condescending don't you think?
|
No. I don't have that kind of personality. I never feel offended by too
simple explanations. I like such explanations which let me work as
little as possible to understand something. And if I already understand
that issue I just jump to the next chapter.
If you are not a beginner, why do you read an article which starts with
the line above? And if you do, why complain when you find that it is
exactly that, an explanation for beginners?
| Quote: | Just because I don't have any
real education or experience in electronics, doesn't mean I'm an idiot.
|
I have no idea who you are or what education you have. I don't know if
you are the person who started this thread or somebody else. I reply to
questions in a newsgroup called sci.electronics.basics, intended for
beginners and hobbyists, and try to develop ways to explain things for
people with very little knowledge.
If you don't like explanations for beginners you can use a newsgroup
where you are expected to know the basics already, like
sci.electronics.design.
The most common question in sci.electronics.basics is "how do I use
an LED?" We see it seven times a week. So we try to find out how to reply
to that question in the most enlightening way possible, to people who
don't know anything about electronics, or about Google.
And especially to all the people with little knowledge who try to feed a
voltage instead of a current to a LED.
I don't talk to just one person when I write, there are other readers,
and people who search the usenet archives in the future, and I talk to
all of them. This is not email or a chatroom, it is usenet and I write
articles for all the readers, in the present and in the future.
I assume that other writers in the field of electronics read my
articles as well as I read their articles, and we learn new teacher
tricks and explanations from each other all the time. It is like an
experimental workshop in technical writing, and the person who started a
thread, usually referred to as "the other person" or the OP, because we
are too lazy to remember the names of all people who pass by, is maybe
less important than he thinks.
--
Roger J. |
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Chris W
Guest
|
| Posted:
Wed Mar 16, 2005 11:07 pm Post subject:
Re: The Results of everyones help. |
|
|
John Fields wrote:
| Quote: | On Wed, 16 Mar 2005 00:19:24 -0600, Chris W <1qazse4@cox.net> wrote:
Robert Monsen wrote:
Chris W wrote:
John Fields wrote:
LEDs don't want voltage, they want a certain amount of current, and
when that much current is flowing through them the voltage across them
will be the specified Vf, or forward voltage.
I know that Vf is different for different LEDs, but doesn't E = I*R
apply to LED's.
R is simply the ratio of voltage to current. For a resistor, R is
designed to be nearly constant. However, this is not true for almost
anything else you can put current through.
The current of an LED is given by the Shockley diode equation:
I = Is * (exp(-Vf/(N*Vt)) - 1)
Thanks, that makes it very clear. It's not that I can't supply the
correct voltage to get the correct current, just that the value is very
critical and probably varies from batch to batch, with temperature and
other factors beyond the control of my voltage supply. In this case I
am looking to get as much light out of the LED's as I can, but if I
weren't, couldn't I just supply a voltage that was enough lower than the
Vf to guarantee If was below 20ma and still get plenty of light for a
indicator or just testing on a prototype board? That way I wouldn't
have to have a resistor for every LED.
---
You could, but unless you use a couple of different supplies you could
run into problems driving the logic with the reduced voltage you'd be
using to run the LEDs. If you decide to use the Allegro parts, though,
you'll sidestep the problem since you'll only need one resistor to set
the current into all 16 LEDs each chip drives.
for this circuit it isn't an issue but I was thinking of future projects. |
| Quote: | ---
I'm glad I am getting this information before I get a regulator to
regulate the battery power going to my the LED's and circuit to control
the LED's in my RC plane.
---
Since your battery voltage will be pretty stable, and predictable, you
may not need a regulator. What did you have in mind?
|
Lets say I use a 4 cell NiCd pack. Fully charged the voltage can be as
high 5.8. By the time it reaches 4.4 under load, it is almost dead.
From what I was reading about that LED driver chip, it sounds like it
uses what would be considered a liner regulator. It says that if Vf on
the LED's is much higher than it is needed the chip will be dissipating
a lot of power. To avoid wasting battery power heating up that chip, I
was thinking of having using 2 switching regulators, one set at 4V for
the white LEDs and one at around 2.5 for the other colors. Also the
RCM3100 that I am using requires 3.15 to 3.45V. The 6276 chip lists the
typical 4.5 to 5.5 supply voltage limits, since the battery will very
quickly drop from 5.8 to less than 5.5 once a load is applied, I guess
battery voltage will run that chip fine. Unless you took the battery
off the charger less than an hour before hooking it up to the circuit,
it will probably be below 5.5v before you put a load on it anyway. I
guess that means 3 regulators. Maybe I will drive the lower voltage
LEDs with the same 3.3V I supply to the RCM3100, that will keep it to
just 2 regulators.
--
Chris W
Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com |
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Roger Johansson
Guest
|
| Posted:
Thu Mar 17, 2005 6:10 am Post subject:
Re: The Results of everyones help. |
|
|
Rich Grise <richgrise@example.net> wrote:
| Quote: | "the other person" or the OP,
Ahem. "Original Poster".
|
This is the first time I have heard that interpretation of OP, and I
have heard the expression "the other person" at least a few thousand
times here in usenet. But I have nothing against that interpretation, it
sounds plausible too.
"other person"
765,000
"original poster"
1,330,000
That's funny, a search on groups google seems to support your
interpretation stronger than mine. :-)
Maybe this is a suitable occasion to quote Isaac Asimov:
"The most exciting phrase to hear in science, the one that heralds new
discoveries, is not 'Eureka!' (I've found it!), but 'That's funny..."
--
Roger J. |
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Rich Grise
Guest
|
| Posted:
Thu Mar 17, 2005 6:10 am Post subject:
Re: The Results of everyones help. |
|
|
On Wed, 16 Mar 2005 13:32:07 +0100, Roger Johansson wrote:
| Quote: | I assume that other writers in the field of electronics read my
articles as well as I read their articles, and we learn new teacher
tricks and explanations from each other all the time. It is like an
experimental workshop in technical writing, and the person who started a
thread, usually referred to as "the other person" or the OP, because we
are too lazy to remember the names of all people who pass by, is maybe
less important than he thinks.
|
Ahem. "Original Poster".
;-)
Cheers!
Rich |
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Chris W
Guest
|
| Posted:
Thu Mar 17, 2005 6:10 am Post subject:
Re: The Results of everyones help. |
|
|
Roger Johansson wrote:
| Quote: | If you are not a beginner, why do you read an article which starts with
the line above? And if you do, why complain when you find that it is
exactly that, an explanation for beginners?
It wasn't the term beginner that made it seem condescending to me, it |
was the assumption that because I was a beginner in electronics, that I
was incapable of comprehending a "real" explanation. The mathematical
relationship that another poster gave was a much more useful and
succinct explanation and gave as good of in site into why you need to
regulate the current as I think you could have. After all isn't the
reason we use news groups because we want a quick answer and don't want
to take the time to "read the book"?
| Quote: | I have no idea who you are or what education you have. I don't know if
you are the person who started this thread or somebody else.
A quick look at the from header in the message would tell you that I am |
the one who started this thread.
| Quote: | I reply to
questions in a newsgroup called sci.electronics.basics, intended for
beginners and hobbyists, and try to develop ways to explain things for
people with very little knowledge.
If you don't like explanations for beginners you can use a newsgroup
where you are expected to know the basics already, like
sci.electronics.design.
somewhere in between would probably be a better fit for me. |
| Quote: | I don't talk to just one person when I write, there are other readers,
and people who search the usenet archives in the future, and I talk to
all of them. This is not email or a chatroom, it is usenet and I write
articles for all the readers, in the present and in the future.
When I respond to NG posts I generally try to get a feel for what the |
poster already knows, and gear my reply to that.
| Quote: | I assume that other writers in the field of electronics read my
articles as well as I read their articles, and we learn new teacher
tricks and explanations from each other all the time. It is like an
experimental workshop in technical writing, and the person who started a
thread, usually referred to as "the other person" or the OP, because we
are too lazy to remember the names of all people who pass by, is maybe
less important than he thinks.
|
Well anyway I think you should use a pressure relief valve instead of a
damn for your analogy. Having some experience in hydraulics, that
sounds like a better fit. Of course, like any analogy, if you try to
take it too far, it will fall apart as well. One of the reasons I like
the pressure relief valve is, just like an LED, if you pass to much
current through it, it will get hot fast. Then you could also relate a
current regulator to a pressure compensated flow control valve, but that
is starting to get too deep into hydraulics, unless of course you are
already familiar with hydraulics.
--
Chris W
Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com |
|
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|
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Roger Johansson
Guest
|
| Posted:
Thu Mar 17, 2005 6:10 am Post subject:
Re: The Results of everyones help. |
|
|
Roger Johansson <no-email@no.invalid> wrote:
| Quote: | "other person"
765,000
"original poster"
1,330,000
That's funny, a search on groups google seems to support your
interpretation stronger than mine. :-)
|
Restricting the search to this newsgroup gives the numbers 48 and 50.
Still to your advantage but at least a lot closer to fifty/fifty.
--
Roger J. |
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|
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Roger Johansson
Guest
|
| Posted:
Thu Mar 17, 2005 6:10 am Post subject:
Re: The Results of everyones help. |
|
|
Chris W <1qazse4@cox.net> wrote:
| Quote: | The mathematical
relationship that another poster gave was a much more useful and
succinct explanation
|
Very few people who ask questions here are able to read and understand such a
mathematical expression. If I recall correctly you even got a comment as if
your simple acceptance was sarcasm. That was another person who did not
expect a mathematical understanding at that level from you. So I wasn't
alone in misjudging you.
looking up and quoting:
..........
| Quote: | I = Is * (exp(-Vf/(N*Vt)) - 1)
Thanks, that makes it very clear.
|
You forgot the smiley...
...........
| Quote: | Well anyway I think you should use a pressure relief valve instead of a
damn for your analogy. Having some experience in hydraulics, that
sounds like a better fit. Of course, like any analogy, if you try to
take it too far, it will fall apart as well. One of the reasons I like
the pressure relief valve is, just like an LED, if you pass to much
current through it, it will get hot fast. Then you could also relate a
current regulator to a pressure compensated flow control valve, but
that is starting to get too deep into hydraulics, unless of course you
are already familiar with hydraulics.
|
An analogy for a diode in general is a wall in a tube with a door which
opens only one way. That model allows for many properties of a real
diode, like backward leakage, backward breakdown, exponential forward
conduction (if the door is kept shut by a spring), etc..
I think such analogies are valuable for beginners, others say they are
more confusing than enlightening, and they think the math expressions are
better for beginners.
--
Roger J. |
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John Fields
Guest
|
| Posted:
Thu Mar 17, 2005 10:41 pm Post subject:
Re: The Results of everyones help. |
|
|
On 17 Mar 2005 05:12:44 +0100, Roger Johansson <no-email@no.invalid>
wrote:
| Quote: | Roger Johansson <no-email@no.invalid> wrote:
"other person"
765,000
"original poster"
1,330,000
That's funny, a search on groups google seems to support your
interpretation stronger than mine. :-)
Restricting the search to this newsgroup gives the numbers 48 and 50.
Still to your advantage but at least a lot closer to fifty/fifty.
|
---
If it were "Other Person" it would be too ambiguous to make any sense.
Which "Other Person" on the thread for instance? It could be anyone.
On the other hand, "Original Poster" is very specific and refers to
the person who started the thread, the original poster.
--
John Fields |
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