Simple LED questions...

[Noozer]

I'm looking at building a couple circuits and have some questions regarding
LEDs...

#1:
I've been given the following formula to figure out what size current
limiting resistor to use when working with LEDs:

R = ( Vs - Vl ) / I

So, if I want to power a 3v, 20mA LED from a 12v source (such as an
automobile) I would simply need to use a 470K (closest value to 450)
resistor? Do I need to worry that my source is 12v and the LED is only 3v?

450 = ( 12 - 3 ) / .02


#2
How does one determine what wattage resistor to use as a current limiting
resistor?

Using the above example, would it be 12v * 20mA = .24 watt?


#3
I know that putting LEDs in parallel is bad. If I put LEDs in parallel, and
ensure that each LED has it's own current limiting resistor it should be
fine, right? Same for strings of LEDs?

In other words, if I want to light 25 LEDs, can I string five LEDs together
in series with a single current limiting resistor, and repeat that four more
times in parallel?

 

[John Popelish]

I'm looking at building a couple circuits and have some questions regarding
LEDs...

#1:
I've been given the following formula to figure out what size current
limiting resistor to use when working with LEDs:

R = ( Vs - Vl ) / I

So, if I want to power a 3v, 20mA LED from a 12v source (such as an
automobile) I would simply need to use a 470K (closest value to 450)
resistor? Do I need to worry that my source is 12v and the LED is only 3v?

450 = ( 12 - 3 ) / .02

Right, except that there is no k after that 450. So 470
ohms would work. (yellow, violet, brown)

But generally one does not operate LEDs all the way to their
maximum current specification, and an automotive electrical
system, runs closer to 14 volts than 12, when the engine is
running. So I generally assume 14 volts and drive the LEDs
at only half rated current, for long life.

1100 ohms = (14 - 3) / 0.01

1k would be close enough.

#2
How does one determine what wattage resistor to use as a current limiting
resistor?

Using the above example, would it be 12v * 20mA = .24 watt?

That would be the total power used by both the LED and
resistor. Substitute the voltage drop across the resistor
in that formula, and you have the resistor power. about
9 * 0.02 = 0.18 watt. A quarter watt or half watt resistor
would do fine.

#3
I know that putting LEDs in parallel is bad. If I put LEDs in parallel, and
ensure that each LED has it's own current limiting resistor it should be
fine, right? Same for strings of LEDs?

Yes.

Strings of LEDs are forced to share exactly the same current
and use the extra voltage more efficiently. But you should
reserve at least a quarter of the voltage for the resistor,
so that it still controls the current, as the LEDs warm up
and change voltage drop. For instance in an automotive
application (14 volts, approximately, you might put 3 each 3
volt LEDs in series that will use up about 9 volts, and pick
a resistor that limits the current to the desired amount
while it burns up the remaining 5 volts or so. For instance
if you pick and operating current of 0.01 amp, that would
require a resistor of about 500 ohms. Either a 470 or 510
ohm would work.

In other words, if I want to light 25 LEDs, can I string five LEDs together
in series with a single current limiting resistor, and repeat that four more
times in parallel?

If you have enough extra voltage in each string to allow
enough resistor drop so that the resistor dominates the
current control situation. 5 times 3 volt LEDs would drop
about 15 volts. That string would work well with a 24 volt
supply, but not a 12-14 volt supply.

 

[Noozer]

<snip>

Exactly the information I was looking for. It's GREATLY appreciated. Makes
PERFECT sense to me.

Thank you!

 

[Peter Bennett]

I'm looking at building a couple circuits and have some questions regarding
LEDs...

#1:
I've been given the following formula to figure out what size current
limiting resistor to use when working with LEDs:

R = ( Vs - Vl ) / I

So, if I want to power a 3v, 20mA LED from a 12v source (such as an
automobile) I would simply need to use a 470K (closest value to 450)
resistor? Do I need to worry that my source is 12v and the LED is only 3v?

450 = ( 12 - 3 ) / .02

That's correct - and the resistor drops the excess voltage.

By the way - the LED current rating is probably a maximum, and it will
probably make sufficient light, and last longer (and the battery will
last longer) with a lower current - I usually aim for 10 mA (it makes
the math easier, too )

#2
How does one determine what wattage resistor to use as a current limiting
resistor?

Using the above example, would it be 12v * 20mA = .24 watt?

There will be 9 volts across the resistor so power is 9 * .02 = .18
watt, so a 1/4 watt would be fine.

#3
I know that putting LEDs in parallel is bad. If I put LEDs in parallel, and
ensure that each LED has it's own current limiting resistor it should be
fine, right? Same for strings of LEDs?

If you want to put LEDs in parallel, each LED needs its own current
limiting resistor.

If you put LEDs in series, you only need one resistor for the lot.

In other words, if I want to light 25 LEDs, can I string five LEDs together
in series with a single current limiting resistor, and repeat that four more
times in parallel?

Yes - but when putting LEDs in series like that, their voltages add,
and you have to leave some voltage for the series resistor. For your
3 volt LEDs, and a 12 volt supply, you could only put three LEDs in
series (for 9 volts), and drop the remaining 3 volts in the resistor.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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[HapticZ]

http://www.gizmology.net/index.htm

many other very helpful links there

 

[Anthony Fremont]

Noozer wrote:
But generally one does not operate LEDs all the way to their
maximum current specification, and an automotive electrical
system, runs closer to 14 volts than 12, when the engine is
running. So I generally assume 14 volts and drive the LEDs
at only half rated current, for long life.

1100 ohms = (14 - 3) / 0.01

1k would be close enough.

Thanks for pointing that out John. It's kind of a pet peeve of mine that
there seems to be some unwritten law that 20mA is the "correct" current for
any and all LED driving situations. ISTM that most modern LEDs will
illuminate brightly with just a few mA.

 

[HapticZ]

http://www.gizmology.net/LEDs.htm

reposted, sorry!