# Ignorant LED questions (low power LED usage)

I

#### Ignoramus21592

Jan 1, 1970
0
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Are there any LED driver kinds of chips that allow LEDs to be used in
a low power usage kind of situation (as opposed to running from a
truck alternator). That replace those resistors with something smarter.

The reason for my question is that I have a mailbox that used to be
often hit by vehicles. I have changed it to a higher visibility,
swingaway mailbox:

http://igor.chudov.com/projects/Mailbox/

but I would like to also add solar powered LEDs to it based on a WHY
NOT principle. (why not do it)

For this thing to work properly, it has to use as little power as
possible, so, I would like to know how LEDs are used in low power
situations.

i

T

#### Tim Wescott

Jan 1, 1970
0
Ignoramus21592 said:
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Are there any LED driver kinds of chips that allow LEDs to be used in
a low power usage kind of situation (as opposed to running from a
truck alternator). That replace those resistors with something smarter.

The reason for my question is that I have a mailbox that used to be
often hit by vehicles. I have changed it to a higher visibility,
swingaway mailbox:

http://igor.chudov.com/projects/Mailbox/

but I would like to also add solar powered LEDs to it based on a WHY
NOT principle. (why not do it)

For this thing to work properly, it has to use as little power as
possible, so, I would like to know how LEDs are used in low power
situations.

i

Igor, juvenile hoodlums will thank you. Leaning out of a car window
clutching a baseball bat while going fast in the dark makes it very hard
to see mailboxes. Having one lit up by LEDs would be a blessing.

A switching power supply would be the way to go. I doubt that any are
made specifically for LED supplies, and finding one that'll be efficient
at the low power levels you're looking at will be a challenge. So you
have your work cut out for you -- but you're known to be energetic.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

J

#### Jan Panteltje

Jan 1, 1970
0
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Well, you could have put some LEDs in series on that 12V, and create a
constant current source with some FET or transistor.
This is the classic low component count LED constant current source:

k a k a k a k a
------ LED1--LED2--LED3..... LEDn--------- PLUS 12 V
|
|---
---->| JFET
| |---
| | select
| [ ] R for 25 mA
| |
---------------------------------------------------- MINUS

Are there any LED driver kinds of chips that allow LEDs to be used in
a low power usage kind of situation (as opposed to running from a
truck alternator). That replace those resistors with something smarter.

See above.
More efficient even is a switchmode current regulator ....

The reason for my question is that I have a mailbox that used to be
often hit by vehicles. I have changed it to a higher visibility,
swingaway mailbox:

http://igor.chudov.com/projects/Mailbox/
but I would like to also add solar powered LEDs to it based on a WHY
NOT principle. (why not do it)

Well light at night, sun during the day, you need at least a battery of sorts.

For this thing to work properly, it has to use as little power as
possible, so, I would like to know how LEDs are used in low power
situations.

Long story, many solutions, late at night, whatdoyouknow.
Over to somebody else.

D

#### default

Jan 1, 1970
0
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Are there any LED driver kinds of chips that allow LEDs to be used in
a low power usage kind of situation (as opposed to running from a
truck alternator). That replace those resistors with something smarter.

The reason for my question is that I have a mailbox that used to be
often hit by vehicles. I have changed it to a higher visibility,
swingaway mailbox:

http://igor.chudov.com/projects/Mailbox/

but I would like to also add solar powered LEDs to it based on a WHY
NOT principle. (why not do it)

For this thing to work properly, it has to use as little power as
possible, so, I would like to know how LEDs are used in low power
situations.

i
There are some chips made for the purpose but they tend to be hard to
find, costly, and are primarily for high power leds.

Easiest way to save some power is to just pulse the LED with a much
higher current for a much shorter duty cycle - or use one of the
flashing LEDs or get a hold of a LM3904 and use that to flash your
leds.

Solar powered - you'd want it to shut off while light outside.

D

#### D from BC

Jan 1, 1970
0
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Are there any LED driver kinds of chips that allow LEDs to be used in
a low power usage kind of situation (as opposed to running from a
truck alternator). That replace those resistors with something smarter.

The reason for my question is that I have a mailbox that used to be
often hit by vehicles. I have changed it to a higher visibility,
swingaway mailbox:

http://igor.chudov.com/projects/Mailbox/

but I would like to also add solar powered LEDs to it based on a WHY
NOT principle. (why not do it)

For this thing to work properly, it has to use as little power as
possible, so, I would like to know how LEDs are used in low power
situations.

i

Could one say that impedance matching is required?

Match load and supply or use something acting like an imaginary DC to
DC transformer (ex. smps).
D from BC

D

#### Don Lancaster

Jan 1, 1970
0
Ignoramus21592 said:
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Are there any LED driver kinds of chips that allow LEDs to be used in
a low power usage kind of situation (as opposed to running from a
truck alternator). That replace those resistors with something smarter.

The reason for my question is that I have a mailbox that used to be
often hit by vehicles. I have changed it to a higher visibility,
swingaway mailbox:

http://igor.chudov.com/projects/Mailbox/

but I would like to also add solar powered LEDs to it based on a WHY
NOT principle. (why not do it)

For this thing to work properly, it has to use as little power as
possible, so, I would like to know how LEDs are used in low power
situations.

i

LED's are efficiently driven by switchmode current supplies.
There are now dozens of different chips available from just about every
major player.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552

Please visit my GURU's LAIR web site at http://www.tinaja.com

D

Jan 1, 1970
0
I

#### Ignoramus21592

Jan 1, 1970
0
Igor, juvenile hoodlums will thank you. Leaning out of a car window
clutching a baseball bat while going fast in the dark makes it very hard
to see mailboxes. Having one lit up by LEDs would be a blessing.

I know what you mean, but so far, all my mailbox accidents were NOT
due to juvenile hoodlums. They were due to people not knowing how to
drive. Once it was a snow plow, and another time it was some odiot who
left his or her car mirror lying next to the mailbox.

A switching power supply would be the way to go. I doubt that any are
made specifically for LED supplies, and finding one that'll be efficient
at the low power levels you're looking at will be a challenge. So you
have your work cut out for you -- but you're known to be energetic.

Thanks Tim... I am still hoping that this is a standard problem...

i

I

#### Ignoramus21592

Jan 1, 1970
0
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Well, you could have put some LEDs in series on that 12V, and create a
constant current source with some FET or transistor.
This is the classic low component count LED constant current source:

k a k a k a k a
|
|---
| |---
| | select
| [ ] R for 25 mA
| |

That's a great idea, very simple and elegant. Thanks.
Well light at night, sun during the day, you need at least a battery of sorts.

Definitely, yes. Battery plus solar charger.

i

D

#### Don Lancaster

Jan 1, 1970
0
Ignoramus21592 said:
Well, you could have put some LEDs in series on that 12V, and create a
constant current source with some FET or transistor.
This is the classic low component count LED constant current source:

k a k a k a k a
|
|---
| |---
| | select
| [ ] R for 25 mA
| |

That's a great idea, very simple and elegant. Thanks.

Also very wrong.
The efficiency would be negligible.

The trick is to provide JUST ENOUGH VOLTAGE to allow a desired current
to flow through both the LEDs and their (hopefully very small) current
sensing resistor.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552

Please visit my GURU's LAIR web site at http://www.tinaja.com

J

#### Jan Panteltje

Jan 1, 1970
0
Ignoramus21592 said:
Well, you could have put some LEDs in series on that 12V, and create a
constant current source with some FET or transistor.
This is the classic low component count LED constant current source:

k a k a k a k a
|
|---
| |---
| | select
| [ ] R for 25 mA
| |

That's a great idea, very simple and elegant. Thanks.

Also very wrong.
The efficiency would be negligible.

Not really, a few volts drop with 12V and a few more when battery goes way up
say to 14 V or more, will protect the LEDs, keep teh current constant very well.
It is more constant and more efficient then each LED a resistor.
It is not as efficient as _most_ switchmodes.
And it is not wrong.
The only thing that is wrong is that your prediction I would have LED lights
in my house by now never happened.

And will not happen in the coming years.
LOL

J

#### James Arthur

Jan 1, 1970
0
Ignoramus21592 said:
Well, you could have put some LEDs in series on that 12V, and create a
constant current source with some FET or transistor.
This is the classic low component count LED constant current source:
k a k a k a k a
|
|---
| |---
| | select
| [ ] R for 25 mA
| |
That's a great idea, very simple and elegant. Thanks.

Also very wrong.
The efficiency would be negligible.

Greetings Don!
Perhaps you missed that there were many LEDs in series? Although
not as good as a switcher, it would be many-fold better than a single
LED resistored to 12v.

Cheers,
James Arthur

R

#### Rich Grise

Jan 1, 1970
0
Ignoramus21592 wrote: ....

Igor, juvenile hoodlums will thank you. Leaning out of a car window
clutching a baseball bat while going fast in the dark makes it very hard
to see mailboxes. Having one lit up by LEDs would be a blessing.

A switching power supply would be the way to go. I doubt that any are
made specifically for LED supplies, and finding one that'll be efficient
at the low power levels you're looking at will be a challenge. So you
have your work cut out for you -- but you're known to be energetic.

I've seen ordinary bicycle retroreflectors on mailboxes. They take zero
power.

However, if the vandal is driving with no lights, I guess it wouldn't
help much. ;-)

(and if you put a solar panel on top of your mailbox, it would probably
get stolen. =:-O )

Good Luck!
Rich

R

#### Rich Grise

Jan 1, 1970
0
I know what you mean, but so far, all my mailbox accidents were NOT due to
juvenile hoodlums. They were due to people not knowing how to drive. Once
it was a snow plow, and another time it was some odiot who left his or her
car mirror lying next to the mailbox.

Thanks Tim... I am still hoping that this is a standard problem...

I don't know where you live, but in snow country I've seen mailbox posts
set back 4-6' from the shoulder, with the box on a cantilevered arm, like
a fence gate. It stays in place by ordinary friction, or maybe there's a
piece of breakaway wire. This does two things - the snowplows don't plow
the post down, and if the box gets hit, it just swings out of the way.

Hmm - this suggests an additional scenario: the box bounces off the front
of their car, swings around, and breaks their back window. ;-)

Cheers!
Rich

I

#### Ignoramus26157

Jan 1, 1970
0
Ignoramus21592 said:
Well, you could have put some LEDs in series on that 12V, and create a
constant current source with some FET or transistor.
This is the classic low component count LED constant current source:
k a k a k a k a
|
|---
| |---
| | select
| [ ] R for 25 mA
| |
That's a great idea, very simple and elegant. Thanks.

Also very wrong.
The efficiency would be negligible.

Greetings Don!
Perhaps you missed that there were many LEDs in series? Although
not as good as a switcher, it would be many-fold better than a single
LED resistored to 12v.

it could be, in fact, much closer to 100% than to 0%. Depending on
voltage drop per diode.

i

I

#### Ignoramus26157

Jan 1, 1970
0
I don't know where you live, but in snow country I've seen mailbox posts
set back 4-6' from the shoulder, with the box on a cantilevered arm, like
a fence gate. It stays in place by ordinary friction, or maybe there's a
piece of breakaway wire. This does two things - the snowplows don't plow
the post down, and if the box gets hit, it just swings out of the way.

That's exactly what I did.

http://igor.chudov.com/projects/Mailbox/

i

E

#### ehsjr

Jan 1, 1970
0
Ignoramus21592 said:
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Are there any LED driver kinds of chips that allow LEDs to be used in
a low power usage kind of situation (as opposed to running from a
truck alternator). That replace those resistors with something smarter.

The reason for my question is that I have a mailbox that used to be
often hit by vehicles. I have changed it to a higher visibility,
swingaway mailbox:

http://igor.chudov.com/projects/Mailbox/

but I would like to also add solar powered LEDs to it based on a WHY
NOT principle. (why not do it)

For this thing to work properly, it has to use as little power as
possible, so, I would like to know how LEDs are used in low power
situations.

i

Cat# SPL05 is a solar powered, 3 LED lite -
charges in daylight, turns on automatically
at night. $4.50 http://www.allelectronics.com/ I don't think you can beat that. Even if it is less power efficient than some pulser or switcher design, it's a cheap "done deal" that seems to fit your needs. Besides it provides the solar cell you'll need if you build one. Ed I #### Ignoramus21592 Jan 1, 1970 0 Cat# SPL05 is a solar powered, 3 LED lite - charges in daylight, turns on automatically at night.$4.50
http://www.allelectronics.com/

I don't think you can beat that. Even if
it is less power efficient than some pulser
or switcher design, it's a cheap "done deal"
that seems to fit your needs. Besides it
provides the solar cell you'll need if you
build one.

Ed

Ed, looks very interesting (the number is actually SPL-05). Thanks.

i

J

#### jasen

Jan 1, 1970
0
So, I completed putting LEDs on my trailer. (LEDs are used to
indicate power and braking status) They are current limited by simple
500 ohm resistors, to approximately 25 mA. No big deal.

That made me think: most of the power to LEDs is wasted worthlessly in
those resistors. Only a small fraction of it is used to make
light. (my guess is about 85% of power is wasted)

Are there any LED driver kinds of chips that allow LEDs to be used in
a low power usage kind of situation (as opposed to running from a
truck alternator). That replace those resistors with something smarter.

some sort of "switcher" eg "joule thief" (google)

those cheap solar led lights often use a similar approach too.

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