Chop Suey wrote:
Larry inspired greatness with:

The LEDs being produced today for illumination do not require any resistor.
They are connected directly to the battery or other power source.


They may have a resistor packaged with it, but a LED is a *DIODE*, and
it requires a resistor.


For the sake of my education let me ask. Doesn't an LED have a
predictable voltage drop across the junction? I seem to recall 1.5
volts is that correct? If it is I only need as many LEDs in series as
will equal the voltage and they will need no external resistance. Is my
memory wrong?
--
Tom Horne

writes:

For the sake of my education let me ask. Doesn't an LED have a
predictable voltage drop across the junction? I seem to recall 1.5
volts is that correct? If it is I only need as many LEDs in series as
will equal the voltage and they will need no external resistance. Is my
memory wrong?

The voltage-current characteristic of an LED has a pretty sharp "knee"
characteristic of most diodes. Very little current flows as voltage
increases up to the turnon voltage, and then current rises very rapidly
with very little change in voltage. The actual voltage depends on the
type of LED (different colours use different chemistry and have
different voltage) and temperature.

LEDs are easy to drive from a constant-current power source. You set
the current you want, and the power supply adjusts its voltage to
whatever is needed to make the LED draw that amount of current. Put a
bunch in series, and all get the same current.

But most power sources are closer to constant-voltage instead of
constant-current. It's nearly impossible to set the voltage needed for
a particular current, since a small voltage change results in a large
current change, and the voltage needed depends on temperature (which
depends on current among other things!).

So you generally use a resistor in series with the LED, or string of
LEDs. The power supply voltage is equal to the sum of the voltages of
the LEDs in the string, plus a few extra volts. The resistor is sized
to give the desired LED current with the "extra" voltage applied across
it. This gives a system where small changes in LED turnon voltage
produce small changes in the voltage across the resistor, which produce
small changes in current through the LEDs. Without the resistor, you'd
get large and unwanted changes in current.

As others pointed out, it is possible to build a system using only a LED
and a battery *if* the battery's own internal resistance is enough to
stabilize the system.

Dave

On Thu, 15 Feb 2007 13:12:12 -0800, "Bob F"
wrote:


"Doug Miller" wrote in message
.net...
In article , "Bob F"
wrote:

If the voltage is insufficient, no power supply, no matter what current
it
can deliver, will light the bulb.

Oh, really? Let's see the explanation of that.


There are hundreds of amps available from a car battery. Ever notice you
don't get electrocuted when you touch both terminals? It's the same thing.

Bob


But you can do welding with one (of course that's not using your body
as part of the circuit).
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

On Fri, 16 Feb 2007 02:30:11 +0000 (UTC),
(Larry) wrote:

In article ,
Chop Suey none wrote:
Larry inspired greatness with:

The LEDs being produced today for illumination do not require any resistor.
They are connected directly to the battery or other power source.


They may have a resistor packaged with it, but a LED is a *DIODE*, and
it requires a resistor.


Well, yes that is true. However they do not require an external
resistor in order to make a complete lighting circuit. Whatever resistance
is needed in built into the unit itself.

I considered that. "Built-in" is not equivalent to "nonexistent".
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

On Fri, 16 Feb 2007 04:53:13 GMT, Thomas Horne
wrote:

Chop Suey wrote:
Larry inspired greatness with:

The LEDs being produced today for illumination do not require any resistor.
They are connected directly to the battery or other power source.


They may have a resistor packaged with it, but a LED is a *DIODE*, and
it requires a resistor.


For the sake of my education let me ask. Doesn't an LED have a
predictable voltage drop across the junction? I seem to recall 1.5
volts is that correct?

It varies depending on the LED. In an electronics catalog I got last
year, I see forward voltages ranging from 1.5V to 4.8V (that high one
was blue). I'd expect it to be constant for a particular LED.

If it is I only need as many LEDs in series as
will equal the voltage and they will need no external resistance. Is my
memory wrong?

Maybe if that voltage is EXACTLY right. Theoretically, you might see
this a few times during the life of the universe. In reality, don't
expect it.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

On Thu, 15 Feb 2007 20:02:12 +0000 (UTC), (Dave
Martindale) wrote:

Mark Lloyd writes:

I hooked them up in parallel and they seem to work just fine. Incredibly
bright too.

LEDs in parallel with each other? It's unlikely that all would light,
since the threshold voltages would be slightly different, and the one
that's lowest would prevent the others from lighting.

If they're all from the same batch, they're probably pretty well matched
in forward voltage, and it only takes a little bit of series resistance
within each LED to approximately balance the current between LEDs.
This isn't a *good* way of connecting multiple LEDs, but it's not
automatically doomed to failure.

Modern LEDs can appear very bright. I noticed that with the holiday
lights I had this year (yes, I know that's "last year", but it is
still less than 2 months ago). Those LEDs look brighter than the
miniature incandescent's.

The little 1 W and 3 W LEDs appearing in flashlights are now brighter
than anything I ever saw from any incandescent bulb in the same size of
flashlight (2 AA battery).


Yes. I have one of those 3W LED flashlights. It's brighter and whiter
than an incandescent flashlight.

Dave
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

On Thu, 15 Feb 2007 13:18:23 -0800, "Bob F"
wrote:


"Mark Lloyd" wrote in message

LEDs in parallel with each other? It's unlikely that all would light,
since the threshold voltages would be slightly different, and the one
that's lowest would prevent the others from lighting.

As someone said earlier, your LEDs probably are actually LED modules,
and come with built-in resistors. You have a separate resistor in
series with each LED.

He said 3V LEDs in one post - that implies the internal resister.


Not necessarily. As I said in another message, some LEDs have forward
voltages that high. That's the LEDs themselves.

Bob

--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

On Thu, 15 Feb 2007 19:07:45 GMT, (Doug Miller)
wrote:

In article , Mark Lloyd wrote:
On Thu, 15 Feb 2007 02:52:43 GMT, (Doug Miller)
wrote:

In article , Mark Lloyd
wrote:
On Thu, 15 Feb 2007 01:35:28 GMT, (Doug Miller)
wrote:

In article , Mark Lloyd
wrote:
On Wed, 14 Feb 2007 18:29:08 GMT, (Doug Miller)
wrote:

In article , "HK"
wrote:
Thanks everyone. Good information here.

This is a Grade 6 project and I was a little distressed to find that even
the basic concepts of electricity haven't been taught yet they are
supposed
to build a basic circuit and working model.
For instance, my daughter assumed that to power two 6v lights, she needed a
12v power supply. Yikes.

So what's the problem? Connect them in series.

Or in parallel if the power supply is big enough. C cells may not be.

If you connect two 6V lamps in parallel to a 12V supply, you're quite likely
to see the two lamps doing an excellent imitation of fuses -- especially "if
the power supply is big enough".

I have done that before. They usually do not burn out immediately, but
glow brighter (and whiter) for awhile.

Of course that is not what I was talking about before. Maybe you
missed that what I said was an ALTERNATIVE to using 12V.

?

I was responding to your suggestion, visible above, that "if the power supply
was big enough" he could connect two 6V lamps in parallel to a 12V supply.


That just might have worked IF I had forgotten what I said, and you
had edited my quote to say that. "12V" does not appear in that quote
at all.

Apparently you *did* forget what you said, or at least you forgot what you
were responding to -- which was the girl's belief that, to light two 6v
lamps, she should connect them to a 12v supply.

*I* said that would work fine if they were connected in series -- which is
true, as each lamp would see 6v.

*You* said "or connect them in parallel if the power supply is big enough".

That won't work for very long.

OK, I left something out (saying to use 6V, or clarifying what "big
enough" means). That's no excuse to stick in something that doesn't
belong.

I didn't stick *anything* in there, and you know it.

I do. That's something that would have helped make it look like I was
saying to use 12V.

I put in the word "OR" at the beginning of the line in question. Did
you forget what "or" means? This was something that would work
INSTEAD of "series and 12V". Why assume I meant something other than
what I said?
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

In article , Mark Lloyd wrote:
On Thu, 15 Feb 2007 19:07:45 GMT, (Doug Miller)
wrote:

In article , Mark Lloyd
wrote:
On Thu, 15 Feb 2007 02:52:43 GMT, (Doug Miller)
wrote:

In article , Mark Lloyd
wrote:
On Thu, 15 Feb 2007 01:35:28 GMT, (Doug Miller)
wrote:

In article , Mark Lloyd
wrote:
On Wed, 14 Feb 2007 18:29:08 GMT, (Doug Miller)
wrote:

In article , "HK"

wrote:
Thanks everyone. Good information here.

This is a Grade 6 project and I was a little distressed to find that
even
the basic concepts of electricity haven't been taught yet they are
supposed
to build a basic circuit and working model.
For instance, my daughter assumed that to power two 6v lights, she
needed a
12v power supply. Yikes.

So what's the problem? Connect them in series.

Or in parallel if the power supply is big enough. C cells may not be.

If you connect two 6V lamps in parallel to a 12V supply, you're quite
likely
to see the two lamps doing an excellent imitation of fuses -- especially
"if
the power supply is big enough".

I have done that before. They usually do not burn out immediately, but
glow brighter (and whiter) for awhile.

Of course that is not what I was talking about before. Maybe you
missed that what I said was an ALTERNATIVE to using 12V.

?

I was responding to your suggestion, visible above, that "if the power
supply
was big enough" he could connect two 6V lamps in parallel to a 12V supply.


That just might have worked IF I had forgotten what I said, and you
had edited my quote to say that. "12V" does not appear in that quote
at all.

Apparently you *did* forget what you said, or at least you forgot what you
were responding to -- which was the girl's belief that, to light two 6v
lamps, she should connect them to a 12v supply.

*I* said that would work fine if they were connected in series -- which is
true, as each lamp would see 6v.

*You* said "or connect them in parallel if the power supply is big enough".

That won't work for very long.

OK, I left something out (saying to use 6V, or clarifying what "big
enough" means). That's no excuse to stick in something that doesn't
belong.

I didn't stick *anything* in there, and you know it.

I do. That's something that would have helped make it look like I was
saying to use 12V.

You *were* saying to use 12V. Look at the sentences. OP said daughter thought
you needed to use a 12V supply for two 6V lamps. I said not a problem if you
connect them in series. You said "or in parallel."

And that's not correct. Do that, and you'll burn them out.

The discussion to that point had concerned itself only with 12V supplies;
thus, in the absence of any qualifying clauses specifying a different voltage,
subsequent statements must be taken as referring to the 12V supply under
discussion. If you *meant* "or in parallel, using a 6V supply" then you should
have *said* "... using a 6V supply."

I put in the word "OR" at the beginning of the line in question. Did
you forget what "or" means?

I know perfectly well what "or" means -- it means you were suggesting an
alternative. The flow of context makes it very plain that the meaning of your
suggested alternative was to connect them in parallel to a 12V supply, as
opposed to my suggestion to connect them in series.

This was something that would work
INSTEAD of "series and 12V". Why assume I meant something other than
what I said?

I *am* assuming you meant *exactly* what you said. The problem is that you
apparently didn't say what you meant -- if you *meant* 6V, then you needed to
*say* 6V.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

On Fri, 16 Feb 2007 12:58:43 -0600, Mark Lloyd
wrote:

On Fri, 16 Feb 2007 02:30:11 +0000 (UTC),
(Larry) wrote:

In article ,
Chop Suey none wrote:
Larry inspired greatness with:

The LEDs being produced today for illumination do not require any resistor.
They are connected directly to the battery or other power source.


They may have a resistor packaged with it, but a LED is a *DIODE*, and
it requires a resistor.


Well, yes that is true. However they do not require an external
resistor in order to make a complete lighting circuit. Whatever resistance
is needed in built into the unit itself.

I considered that. "Built-in" is not equivalent to "nonexistent".


If you're going to be pedantic about it, "a resistor" isn't
equivilent to "resistance", or to "limited current", either.

"Mark Lloyd" wrote in message
...
On Thu, 15 Feb 2007 13:12:12 -0800, "Bob F"
wrote:


"Doug Miller" wrote in message
.net...
In article , "Bob F"
wrote:

If the voltage is insufficient, no power supply, no matter what
current
it
can deliver, will light the bulb.

Oh, really? Let's see the explanation of that.


There are hundreds of amps available from a car battery. Ever notice you
don't get electrocuted when you touch both terminals? It's the same
thing.

Bob


But you can do welding with one (of course that's not using your body
as part of the circuit).

Actually, when I did welding with them, it took 3 car batteries
to do the job well.

Bob

"Mark Lloyd" wrote in message
...
On Thu, 15 Feb 2007 13:18:23 -0800, "Bob F"
wrote:


"Mark Lloyd" wrote in message

LEDs in parallel with each other? It's unlikely that all would light,
since the threshold voltages would be slightly different, and the one
that's lowest would prevent the others from lighting.

As someone said earlier, your LEDs probably are actually LED modules,
and come with built-in resistors. You have a separate resistor in
series with each LED.

He said 3V LEDs in one post - that implies the internal resister.


Not necessarily. As I said in another message, some LEDs have forward
voltages that high. That's the LEDs themselves.

Considering the circumstances, I doubt that that is the case here.

Bob

On Fri, 16 Feb 2007 13:54:18 -0800, "Bob F"
wrote:


"Mark Lloyd" wrote in message
.. .
On Thu, 15 Feb 2007 13:18:23 -0800, "Bob F"
wrote:


"Mark Lloyd" wrote in message

LEDs in parallel with each other? It's unlikely that all would light,
since the threshold voltages would be slightly different, and the one
that's lowest would prevent the others from lighting.

As someone said earlier, your LEDs probably are actually LED modules,
and come with built-in resistors. You have a separate resistor in
series with each LED.

He said 3V LEDs in one post - that implies the internal resister.


Not necessarily. As I said in another message, some LEDs have forward
voltages that high. That's the LEDs themselves.

Considering the circumstances, I doubt that that is the case here.

Bob


What I said definitely IS the case. What I said is just to disagree
that "3V" IIMPLIES "internal resistor". Nothing else.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

"Mark Lloyd" wrote in message
...
On Fri, 16 Feb 2007 13:54:18 -0800, "Bob F"
wrote:


"Mark Lloyd" wrote in message
.. .
On Thu, 15 Feb 2007 13:18:23 -0800, "Bob F"
wrote:


"Mark Lloyd" wrote in message

LEDs in parallel with each other? It's unlikely that all would
light,
since the threshold voltages would be slightly different, and the
one
that's lowest would prevent the others from lighting.

As someone said earlier, your LEDs probably are actually LED
modules,
and come with built-in resistors. You have a separate resistor in
series with each LED.

He said 3V LEDs in one post - that implies the internal resister.


Not necessarily. As I said in another message, some LEDs have forward
voltages that high. That's the LEDs themselves.

Considering the circumstances, I doubt that that is the case here.

Bob


What I said definitely IS the case. What I said is just to disagree
that "3V" IIMPLIES "internal resistor". Nothing else.

These are "3V LEDs" provided for a school project. These would
seem very likely to be LEDs designed to operate easily off of
3 volts, since LEDs are rarely described as "X volt" in any other
circumstance. Certainly not at the "consumer" level.

On Feb 15, 12:42 pm, Mark Lloyd wrote:

OK, I left something out (saying to use 6V, or clarifying what "big
enough" means). That's no excuse to stick in something that doesn't
belong.

Mark,

If I may interject here ... The discussion was about connecting 6V
lamps to a 12V supply. As you said, you didn't mention anything about
changing the supply to 6V. It's perfectly reasonable for Doug to
assume you were still talking about a 12V supply. End of story.

Respectful regards,

Mark

On 17 Feb 2007 06:44:49 -0800, "Redbelly"
wrote:

On Feb 15, 12:42 pm, Mark Lloyd wrote:

OK, I left something out (saying to use 6V, or clarifying what "big
enough" means). That's no excuse to stick in something that doesn't
belong.

Mark,

If I may interject here ... The discussion was about connecting 6V
lamps to a 12V supply. As you said, you didn't mention anything about
changing the supply to 6V.

The discussion was about 6V lamps, "12V" was a later addition, which I
posted an ALTERNATIVE to. Sorry for not making things clearer.

I didn't say 6V because it was ALREADY 6V, as appropriate for a 6V
lamp (perhaps I underestimated people's lack of knowledge). As I did
mention, it would have been better to have included that information
anyway. I apologize for the mistake I did make, just not for the one I
didn't.

It's perfectly reasonable for Doug to
assume you were still talking about a 12V supply. End of story.

Respectful regards,

Mark
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

In article , Mark Lloyd wrote:
On 17 Feb 2007 06:44:49 -0800, "Redbelly"
wrote:

On Feb 15, 12:42 pm, Mark Lloyd wrote:

OK, I left something out (saying to use 6V, or clarifying what "big
enough" means). That's no excuse to stick in something that doesn't
belong.

Mark,

If I may interject here ... The discussion was about connecting 6V
lamps to a 12V supply. As you said, you didn't mention anything about
changing the supply to 6V.

The discussion was about 6V lamps, "12V" was a later addition,

No, it was not -- the 12V is in the ORIGINAL POST.

which I
posted an ALTERNATIVE to. Sorry for not making things clearer.

I didn't say 6V because it was ALREADY 6V,

Wrong again. The ORIGINAL POST was complaining that the subject had been
explained so poorly in school that the OP's daughter thought she needed a 12V
supply to power two 6V lamps.

I said that was fine, if they were wired in series.

You said "or in parallel, if the power supply is big enough."

as appropriate for a 6V
lamp (perhaps I underestimated people's lack of knowledge).

The only lack of knowledge apparent here is yours. It's not quite clear
whether that lack is in the difference between series and parallel wiring, or
in reading comprehension.

As I did
mention, it would have been better to have included that information
anyway. I apologize for the mistake I did make, just not for the one I
didn't.

You still haven't figured out which mistake you really did make, and I'm still
waiting for your apology for accusing me of having put things into your post.

It's perfectly reasonable for Doug to
assume you were still talking about a 12V supply. End of story.

Respectful regards,

Mark

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.