Hobby Electronics Basics LEDs different current/voltage

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Subject Author Date
LEDs different current/voltage Anon 03-14-08
Posted by Anon on March 14, 2008, 9:33 am
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Hello,

From what I've read in the many basic tutorials on LEDs, you can wire
different color LEDs in series on a single resistor, regardless of their
individual voltage, if the current requirements are the same.

I have a single 100 Ohm resistor in line with a 3.2V, a 3.4V and a 3.5V LED
in series on a 12V source that works great. These LEDs are all rated at
30mA, with an operating current of 20mA.

I also have a 1.9V red LED, rated at 50mA, with an operating current of
20mA. Adding this LED to the other three brings the voltage drop to 12V.
Can I simply omit the resistor or is a minimal one needed to limit the
current to 20mA? It has the same operating current as the others, but is
rated for a higher max, does this matter?

Does the order of the voltage drops matter to the current flow?

Thanks in advance for any advice before I fry a few components in
experimentation.

Scott in Dunedin






Posted by Ecnerwal on March 14, 2008, 9:48 am
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> I also have a 1.9V red LED, rated at 50mA, with an operating current of
> 20mA. Adding this LED to the other three brings the voltage drop to 12V.
> Can I simply omit the resistor or is a minimal one needed to limit the
> current to 20mA?

The main problem is that LED voltage at any particular currrent is not a
well-specified number, and also varies with temperature. Thus, if your
string is made up of only LEDs, the current may vary considerably. If
you're driving it off a car-type battery, which is effectively dead if
it's actually at 12V, the current may be more than you want, even if the
voltage drop for your particular string is actually 12V at 20 mA. Order
does not matter. Better LED driving schemes approximate, or are, current
sources. LED current is a fairly well-specified parameter, so
controlling that and letting the voltage do what it will is a good
paradigm.

> It has the same operating current as the others, but is
> rated for a higher max, does this matter?

Other than it not putting out as much of its potential output (ie, it's
running at 40% rated current while the others are running at 66%), no.

> Thanks in advance for any advice before I fry a few components in
> experimentation.

That's the spirit. They don't cost much.

--
Cats, coffee, chocolate...vices to live by

Posted by Anon on March 14, 2008, 12:26 pm
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>
>> I also have a 1.9V red LED, rated at 50mA, with an operating current of
>> 20mA. Adding this LED to the other three brings the voltage drop to 12V.
>> Can I simply omit the resistor or is a minimal one needed to limit the
>> current to 20mA?
>
> The main problem is that LED voltage at any particular currrent is not a
> well-specified number, and also varies with temperature. Thus, if your
> string is made up of only LEDs, the current may vary considerably. If
> you're driving it off a car-type battery, which is effectively dead if
> it's actually at 12V, the current may be more than you want, even if the
> voltage drop for your particular string is actually 12V at 20 mA. Order
> does not matter. Better LED driving schemes approximate, or are, current
> sources. LED current is a fairly well-specified parameter, so
> controlling that and letting the voltage do what it will is a good
> paradigm.

Source is a AC to DC power brick; output 12V 1.5A
I am also experimenting with 12 VAC transformer to drive two of these
strings on each cycle.

>
>> It has the same operating current as the others, but is
>> rated for a higher max, does this matter?
>
> Other than it not putting out as much of its potential output (ie, it's
> running at 40% rated current while the others are running at 66%), no.

Makes sense. Thanks for the advice, Lawrence. I ponder whether its
possible to give 30mA for the red and 20mA for two others, within the same
serial string, using two resistors. My gut says no, since two resistors in
series would be added together.
+ ---100R---30mA---160R---20mA---20mA--- -


Plan B is to remove the 3.2V LED to free up voltage for the 1.9V red LED,
but may have to go to (Plan C) two strings with multiple reds driven at the
higher current for showing the shorter wavelength light better.

>
>> Thanks in advance for any advice before I fry a few components in
>> experimentation.
>
> That's the spirit. They don't cost much.
> --
> Cats, coffee, chocolate...vices to live by

Thanks!
Scott



Posted by Ecnerwal on March 14, 2008, 4:02 pm
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> Source is a AC to DC power brick; output 12V 1.5A
...
> I ponder whether its
> possible to give 30mA for the red and 20mA for two others, within the same
> serial string, using two resistors. My gut says no, since two resistors in
> series would be added together.

You would put one in parallel with the LEDs that are to get lower
current. ie, you want 30 mA through the red one, and then 20 mA through
the others, with 10mA in parallel going through a resistor. However:

Other than elegance, you have no reason not to run as many parallel
strings as you like (well, up to 50-75 of them, at 30-20 mA per string),
given 1500 mA of available supply. Use one resistor and 1-3 LEDs per
string. If your end product requires some sort of balance, one LED per
string allows tuning the current of each (by choice of resistor)
independent of the others.

"Elegance" in this case being "minimal use of power for the same result"
- one LED per string means more power wasted as heat in resistors for
the same light out. But it's fairly minor if this is not a 24/7/365
product.

--
Cats, coffee, chocolate...vices to live by

Posted by Anon on March 14, 2008, 5:08 pm
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>
>> Source is a AC to DC power brick; output 12V 1.5A
> ...
>> I ponder whether its
>> possible to give 30mA for the red and 20mA for two others, within the
>> same
>> serial string, using two resistors. My gut says no, since two resistors
>> in
>> series would be added together.
>
> You would put one in parallel with the LEDs that are to get lower
> current. ie, you want 30 mA through the red one, and then 20 mA through
> the others, with 10mA in parallel going through a resistor.

<slaps forehead>
I'll have to refresh my memory on resistors in parallel.
Thank you so much for your advice, it has put me on the right track.

> However:
> Other than elegance, you have no reason not to run as many parallel
> strings as you like (well, up to 50-75 of them, at 30-20 mA per string),
> given 1500 mA of available supply. Use one resistor and 1-3 LEDs per
> string. If your end product requires some sort of balance, one LED per
> string allows tuning the current of each (by choice of resistor)
> independent of the others.

I intended on using 12 of these strings, so the 1500mA transformer is
probably overkill, but it does leave me open for further experimentation.

>
> "Elegance" in this case being "minimal use of power for the same result"
> - one LED per string means more power wasted as heat in resistors for
> the same light out. But it's fairly minor if this is not a 24/7/365
> product.
>
> --
> Cats, coffee, chocolate...vices to live by

"Minimal use of power for the same result" is what I am after; I was trying
to avoid too many resistors and the subsequent power waste/heat through
multiple resistors.

Again thank you very much for your assistance. I'm tempted to ask how to
calculate the parallel resistor value to obtain a 10mA reduction, but its
not something I want to waste your time on.

Much appreciated!
Scott in Dunedin



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