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

In article <H8vCj.21745$0w.9359@newssvr27.news.prodigy.net>,
"Anon" <nospam@vool.com> wrote:

> 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

Anon wrote:
> 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?

No. The purpose of the resistor is to set the current. You
need to find a higher voltage supply, or divide the LEDs
into two groups and run those groups in parallel, each with
its own current setting resistor.

> Does the order of the voltage drops matter to the current flow?
(snip)
No.

--
Regards,

John Popelish

"John Popelish" <jpopelish@rica.net> wrote in message
news:PLSdnUXxNdCaEUfanZ2dnUVZ_tbinZ2d@comcast.com...
> Anon wrote:
>> 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?
>
> No. The purpose of the resistor is to set the current. You need to find
> a higher voltage supply, or divide the LEDs into two groups and run those
> groups in parallel, each with its own current setting resistor.
>
>> Does the order of the voltage drops matter to the current flow?
> (snip)
> No.
>
> --
> Regards,
>
> John Popelish

John,

Thank you for your advice. Power supply is an AC to DC power brick
outputting 12V 1.5A. Plan B is to remove the 3.2V LED to free enough
voltage for the 1.9V LED and using the approporiate resistor value. Plan C,
if I find the red light too dim will be to split the string into two and
adding more reds.

Scott in Dunedin

"Ecnerwal" <LawrenceSMITH@SOuthernVERmont.NyET> wrote in message
news:LawrenceSMITH-9DE0B5.09480214032008@news.verizon.net...
> In article <H8vCj.21745$0w.9359@newssvr27.news.prodigy.net>,
> "Anon" <nospam@vool.com> wrote:
>
>> 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

In article <4HxCj.545$p24.253@nlpi061.nbdc.sbc.com>,
"Anon" <nospam@vool.com> wrote:

> 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

"Ecnerwal" <LawrenceSMITH@SOuthernVERmont.NyET> wrote in message
news:LawrenceSMITH-336F8E.16020714032008@news.verizon.net...
> In article <4HxCj.545$p24.253@nlpi061.nbdc.sbc.com>,
> "Anon" <nospam@vool.com> wrote:
>
>> 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

In article <tPBCj.16508$xq2.12636@newssvr21.news.prodigy.net>,
"Anon" <nospam@vool.com> wrote:

> "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.

Check the math on that before you get too worried. It's not much. 1/3 to
1/4 watt per string, total.

> 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.

Measure the voltage across the LEDs you want to run at 20mA when they
are running at 20mA. Calculate a resistor that would draw 10mA at that
voltage, and put that resistor in parallel with those LEDs. Put the
whole business in series with the resistor and LED running 30mA.

[v+]--[R1]--[LED30]---+---[R2]-------+
| |
+-[LED20]------+--[v-]

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

On Mar 15, 10:08=A0am, "Anon" <nos...@vool.com> wrote:
> "Ecnerwal" <LawrenceSM...@SOuthernVERmont.NyET> wrote in message
>
> news:LawrenceSMITH-336F8E.16020714032008@news.verizon.net...
>
> > In article <4HxCj.545$p24....@nlpi061.nbdc.sbc.com>,
> > "Anon" <nos...@vool.com> wrote:
>
> >> 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. =A0My gut says no, since two resist=
ors
> >> 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 tryin=
g
> to avoid too many resistors and the subsequent power waste/heat through
> multiple resistors.
>
If you don't want to waste power don't uses resistors at all. Just
wire multiple (same type) LEDS is series to give a voltage drop of
>12V e.g. if you use 1.7 V leds, then 12/1.7 is 7.05 so use 7 LEDS in
series and no resistor is needed.

Cheers

"PLOSSL" <Moreflaps@gmail.com> wrote in message
news:7ccf13b8-57e3-45a2-83e4-If you don't > want to waste power don't uses
resistors at all. Just
> wire multiple (same type) LEDS is series to give a voltage drop of
> >12V e.g. if you use 1.7 V leds, then 12/1.7 is 7.05 so use 7 LEDS in
> series and no resistor is needed.
>
> Cheers


A temperature increase of x in an LED causes a current increase of exp(x) if
the voltage across it is fixed. So, on a hot day, your current string may
start putting out enough heat to heat itself faster than it can cool off.
This will eventually cause one of the LEDs to fail.

However, if you put a resistance in series with it, any increases of current
are balanced by a decrease in voltage across the LEDs(since the increased
current causes the resistor to have more voltage across it). This will
decrease the current, and so provide negative feedback, preventing runaway
current increases.

A small resistance will be fine. For a 12V, string the LEDs in series, and
try out various resistances until you find one that works, starting with 1k
or so, and moving downwards in resistance until you get the maximum number
of LEDs along with the minimum resistance (down to about 47 ohms or so)

Regards,
Bob Monsen

On Mar 14, 12:26 pm, "Anon" <nos...@vool.com> wrote:
> "John Popelish" <jpopel...@rica.net> wrote in message
>
> news:PLSdnUXxNdCaEUfanZ2dnUVZ_tbinZ2d@comcast.com...
>
>
>
> > Anon wrote:
> >> 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?
>
> > No. The purpose of the resistor is to set the current. You need to find
> > a higher voltage supply, or divide the LEDs into two groups and run those
> > groups in parallel, each with its own current setting resistor.
>
> >> Does the order of the voltage drops matter to the current flow?
> > (snip)
> > No.
>
> > --
> > Regards,
>
> > John Popelish
>
> John,
>
> Thank you for your advice. Power supply is an AC to DC power brick
> outputting 12V 1.5A. Plan B is to remove the 3.2V LED to free enough
> voltage for the 1.9V LED and using the approporiate resistor value. Plan C,
> if I find the red light too dim will be to split the string into two and
> adding more reds.
>
> Scott in Dunedin

When you say the supply is 12V, do you mean 12.0 V? You've specified
diode voltages to the nearest 0.1V, but what really is the supply
voltage to this accuracy? Did you measure it, or are you just going
by what is written on the supply?

For that matter, did you measure the diode voltages, or are you just
going by what is written on the diode packages?

Best to measure all voltages, and use those numbers for calculating
your resistor. A "12V" supply rated for 1.5A, when drawing only 20
mA, can very well have a significantly higher voltage.

Regards,

Mark

On Mar 16, 5:42=A0pm, "Bob Monsen" <rcmon...@gmail.com> wrote:
> "PLOSSL" <Morefl...@gmail.com> wrote in message
>
> news:7ccf13b8-57e3-45a2-83e4-If you don't > want to waste power don't uses=

> resistors at all. Just
>
> > wire multiple (same type) LEDS is series to give a voltage drop of
> > >12V e.g. if you use 1.7 V leds, then 12/1.7 is 7.05 so use 7 LEDS in
> > series and no resistor is needed.
>
> > Cheers
>
> A temperature increase of x in an LED causes a current increase of exp(x) =
if
> the voltage across it is fixed. So, on a hot day, your current string may
> start putting out enough heat to heat itself faster than it can cool off.
> This will eventually cause one of the LEDs to fail.
>
> However, if you put a resistance in series with it, any increases of curre=
nt
> are balanced by a decrease in voltage across the LEDs(since the increased
> current causes the resistor to have more voltage across it). This will
> decrease the current, and so provide negative feedback, preventing runaway=

> current increases.
>
> A small resistance will be fine. For a 12V, string the LEDs in series, and=

> try out various resistances until you find one that works, starting with 1=
k
> or so, and moving downwards in resistance until you get the maximum number=

> of LEDs along with the minimum resistance (down to about 47 ohms or so)
>

I take your point on termal runaway but if you keep within the current
spec of the LED (measure it) they should not run away. If you put
enough in series that you don't need to drop 12V-> 1.7 across 1
resistor you'll be much more efficient. So as a better solution how
about ust add up the nominal LED voltages to be less than the PS
voltage and select a small resisior to control current. e.g. if they
are 1.7 V use 6 LEDS in serries to give 10.4V drop and then drop the
remaining 1.6 V at 30 mA across a resistor?

Cheers