LEDs different current/voltage

[Anon]

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

 

[Ecnerwal]

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

 

[John Popelish]

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

 

[Anon]

"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

 

[Anon]

"John Popelish" <jpopelish@rica.net> wrote in message
newsLSdnUXxNdCaEUfanZ2dnUVZ_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]

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

 

[Anon]

"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

 

[Ecnerwal]

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

 

[PLOSSL]

On Mar 15, 10:08 am, "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.  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.

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

 

[Bob Monsen]

"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

 

[redbelly]

On Mar 14, 12:26 pm, "Anon" <nos...@vool.com> wrote:

"John Popelish" <jpopel...@rica.net> wrote in message

newsLSdnUXxNdCaEUfanZ2dnUVZ_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

 

[Varactor]

On Mar 16, 5:42 pm, "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 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)

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

 

[Anon]

"Ecnerwal" <LawrenceSMITH@SOuthernVERmont.NyET> wrote in message
news:LawrenceSMITH-DBC360.08041115032008@news.verizon.net...

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

Ecnerwal,

Thank you for your reply. I just wanted to let you know the preliminary
experiments worked out without any immediate failure, and I very happy with
the results so far. Although I do not have my notes handy, I did have to
tweak the math a little for the resistors immediately available to me.
Thinking of the current instead of voltage gave me a better view of what I
was working toward. I still feel they may be underpowered because I
determined the minimum voltage required for each LED and worked up from
there, maintaining the 30mA and 20mA currents.

Thank you for your assistance.
Scott in Dunedin

 

[Anon]

"Varactor" <Moreflaps@gmail.com> wrote in message
news:b862dbda-b266-441c-ba64->2e5b003a2cff@s37g2000prg.googlegroups.com...
On Mar 16, 5:42 pm, "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
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)


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

PLOSSL, Thank you for your reply. Indeed my original experiments used no
resistors and simply relied upon the even distribution of voltage. Your (or
Varactor's) suggestion of a small regulating resitor on a string of similar
spec LEDs was what I was attempting to minimize variations in voltage.
However, since I wanted to use a red LED within the short strings (3 or 4
LEDs), controlling the current instead of the voltage was the way to go,
since it is not a similar spec LED.

Bob Monsen, Resistors as a thermal regulator it turns out, is an added
benefit - excellent point. My earlier experiments did seem to run hot during
the day. And many of my LEDs have been fried from overvoltage from a
non-regulated power supply pushing just a little too much.

Thanks for your help!
Scott in Dunedin

 

[Anon]

"redbelly" <redbelly98@yahoo.com> wrote in message
news:7cd3c38d-4e89-4849-9a7b-adc7e0570c46@x30g2000hsd.googlegroups.com...

On Mar 14, 12:26 pm, "Anon" <nos...@vool.com> wrote:
"John Popelish" <jpopel...@rica.net> wrote in message

newsLSdnUXxNdCaEUfanZ2dnUVZ_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

Mark,

Thanks for your reply. My 12V 1.5A DC power supply outputs 11.8V to 12.2V
with a 20mA draw (during the times I've sampled it, though it's probably
even more variable). My 12V AC power supply (88Watt)outputs between 11.8V
and 12.4V depending on the load. I've been using 12.5V in my calculations
to be on the safe side.

The LED voltages were simply specs from the manufacturer, however
experimentation has revealed maximum voltages 0.5V higher than spec (at some
risk of early failure), and minimum voltages much lower (1.2V less than
spec's typical voltage drop) than I expected. I will keep your suggestion
in mind as it is more accurate to calculate with actual values than with
specs. Specs are convenient as a starting point though.

Thanks!
Scott in Dunedin