White/Blue LED Driver

[Tom @ HollyLodge]

I am intending to build a set of garden (path) lights from White/Blue LEDs
[not bulbs, due to power/life reasons].
What is the best method of driving these?
I was thinking along the lines of 555 Monostable driving 555 Astable to
produce narrow pulses, at a high frequency, so its not seen that they are
flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue LEDs so
that not too much power is lost as heat.
LEDs will probably be in parallel, with one common cathode resistor going
to -ve/gnd

Thanks In Advance
Tom

 

[happyhobit]

Hi Tom,

Why do you want to drive them?

Jay

 

[R.Lewis]

I am intending to build a set of garden (path) lights from White/Blue LEDs
[not bulbs, due to power/life reasons].
What is the best method of driving these?
I was thinking along the lines of 555 Monostable driving 555 Astable to
produce narrow pulses, at a high frequency, so its not seen that they are
flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue LEDs so
that not too much power is lost as heat.
LEDs will probably be in parallel, with one common cathode resistor going
to -ve/gnd

Thanks In Advance
Tom

There is a frequently re-occurring myth in this NG that by waggling around
with the supply to an led some magic is performed and the led thereby
becomes more efficacious.

It doesn't.

Use a switcher to reduce I*R losses (do your sums to see if this is so in
your case) and forget the magic.

 

[John Fields]

I am intending to build a set of garden (path) lights from White/Blue LEDs
[not bulbs, due to power/life reasons].
What is the best method of driving these?
I was thinking along the lines of 555 Monostable driving 555 Astable to
produce narrow pulses, at a high frequency, so its not seen that they are
flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue LEDs so
that not too much power is lost as heat.
LEDs will probably be in parallel, with one common cathode resistor going
to -ve/gnd

---
Very bad idea to run them in parallel. One will have a lower forward
voltage than the rest and will hog the current until it dies. If it
dies open, the one with the next lowest Vf will hog the current until it
dies and on and on... If any of them die shorted the rest of them will
go out and you may exceed the power dissipation rating of the resistor,
blowing it up as well. The proper way to do what you want to do is to
connect several of the LEDs in series with a current limiting resistor
in series with the string, then to connect the strings in parallel.

The 555 is also a bad idea because you don't need it. if you reduce the
duty cycle you'll have to increase the current during the ON portion of
the pulse to maintain the same apparent average brightness, so what's
the point?

I think the best way to drive them would be from the full-wave rectified
secondary of a 24 volt control transformer, with appropriate current
limiting resistors in each string and transient protection across the
transformer's secondary.

How many LEDs are you planning to drive?

 

[Ian Stirling]

Sci.med.vision,sci.optics added to newsgroups.

The 555 is also a bad idea because you don't need it. if you reduce the
duty cycle you'll have to increase the current during the ON portion of
the pulse to maintain the same apparent average brightness, so what's
the point?

If you'r willing to live with flickering, what's the best way for a point
source to flicker to attract attention, with a given number of photons.

I'd guess you don't care about how long the pulse is, as long as it's
short enough that it forms a point on the retina, rather than a streak.

(Does the brian use the direction of the streak on the retina during saccades
to locate point light sources?)

And it's got to be bright enough for that point to stand out over the
background during the integration time of the eye.

I think this means the optimum flicker frequency varies with both light
level and where in the optical field it is.

--
http://inquisitor.i.am/ | mailto:[email protected] | Ian Stirling.
---------------------------+-------------------------+--------------------------
To do is to be
To be is to do
Do be do be do do

 

[Jim Yanik]

I am intending to build a set of garden (path) lights from White/Blue
LEDs [not bulbs, due to power/life reasons].
What is the best method of driving these?
I was thinking along the lines of 555 Monostable driving 555 Astable
to produce narrow pulses, at a high frequency, so its not seen that
they are flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue
LEDs so that not too much power is lost as heat.
LEDs will probably be in parallel, with one common cathode resistor
going to -ve/gnd

Thanks In Advance
Tom



---
Outgoing mail is certified Virus Free.
AVG Free Edition
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.515 / Virus Database: 313 - Release Date: 01/09/2003

Check out Linear Technology's LT1932;*6 parts* to boost V and current
regulate several white/blue LEDs. A very simple circuit,and very small.

 

[Stepan Novotill]

On Thu, 18 Sep 2003 01:11:12 +0000 (UTC), Ian Stirling

Random flicker is most visible in my opinion. Pulse widths less than
10ms or so, do not achieve full brightness on the retina. I strive for
a bright 15ms pulse if I want it to be seen. This achieves full visual
perception at minimal power consumption.
....Stepan

 

[Trevor Wilson]

I am intending to build a set of garden (path) lights from White/Blue LEDs
[not bulbs, due to power/life reasons].
What is the best method of driving these?
I was thinking along the lines of 555 Monostable driving 555 Astable to
produce narrow pulses, at a high frequency, so its not seen that they are
flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue LEDs so
that not too much power is lost as heat.
LEDs will probably be in parallel, with one common cathode resistor going
to -ve/gnd

**Just use DC. In any event, be aware that white LEDs have a very definite
life span, due to their construction. Blue LEDs do not. I've used some white
LEDs, in my doorbell @ 20mA for 3 years. They're down to around 20% of their
original brightness. My preference, in white LEDs is with the Luxeon
devices. Superb spread of light and usefully bright. Very expensive,
however.

 

[Louis Boyd]

What sort of "attention" are you trying to attract? A warning to avoid?
A beacon to come and investigate? A "low oil" warning alert? A target
to be aimed at precisely? Strobes are very noticble relative to their
average power consumption but you'd have a difficult time aiming a rifle
at one. A phosporescent wall plate doesn't grab your attention but it
allows you to walk right to it in a dark room with little energy used.
You might get a better answer if you stated what you're trying to
achieve.

 

[Fred Bloggs]

I am intending to build a set of garden (path) lights from White/Blue
LEDs [not bulbs, due to power/life reasons]. What is the best method
of driving these? I was thinking along the lines of 555 Monostable
driving 555 Astable to produce narrow pulses, at a high frequency, so
its not seen that they are flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue
LEDs so that not too much power is lost as heat. LEDs will probably
be in parallel, with one common cathode resistor going to -ve/gnd

The method of pulsing current into the LEDs makes sense for the
red/green types because the luminous efficiency, ratio of light output
to power consumed, is best at high currents. This is not the case for
blue/white variety where the efficiency peaks at low currents like 5mA
and tends to roll off at higher currents, making a pulsed scheme
pointless unless multiplexing or flashing serves some other purpose. For
a straight illumination application, the most practical way would be to
produce a large enough DC voltage that can drive all the LEDs within a
physical cluster as a series connection with a separate current
regulation circuit for each cluster. The relatively recent miniaturized
switching regulators are excellent for this application and are easily
adapted to this service by using a small current sensing resistor in
series with the LED series cluster to develop the feedback FB voltage
for the switcher, which then adjusts the voltage across the cluster so
as to maintain the current at a constant level. See
http://www.national.com and the Web Bench simulator for some excellent
and low cost candidates. By using switchers with frequency at 200KHz and
above, you end up with very small inductors, filter caps, and overall
circuit volume. Your "bulb" then consists of an input rectifier, this
circuit, and the LED cluster which you drive with that standard beefy
low voltage lighting transformer in parallel with all the other "bulbs."

 

[Helmut Wabnig]

Sci.med.vision,sci.optics added to newsgroups.


If you'r willing to live with flickering, what's the best way for a point
source to flicker to attract attention, with a given number of photons.

I'd guess you don't care about how long the pulse is, as long as it's
short enough that it forms a point on the retina, rather than a streak.

(Does the brian use the direction of the streak on the retina during saccades
to locate point light sources?)

And it's got to be bright enough for that point to stand out over the
background during the integration time of the eye.

I think this means the optimum flicker frequency varies with both light
level and where in the optical field it is.

Once I read a treatise about that, somewhere.
They suggested 1/e for reasons derived from
information theory.
This is roughly 1 / 3 .

If I only could remember, ON or OFF?

w.

 

[Ian Stirling]

I am intending to build a set of garden (path) lights from White/Blue LEDs
[not bulbs, due to power/life reasons].
What is the best method of driving these?

**Just use DC. In any event, be aware that white LEDs have a very definite
life span, due to their construction. Blue LEDs do not. I've used some white
LEDs, in my doorbell @ 20mA for 3 years. They're down to around 20% of their
original brightness. My preference, in white LEDs is with the Luxeon
devices. Superb spread of light and usefully bright. Very expensive,
however.

It depends on the sort of LED, some have better lifes than others.

--
http://inquisitor.i.am/ | mailto:[email protected] | Ian Stirling.
---------------------------+-------------------------+--------------------------
<Squawk> Pieces of eight!
<Squawk> Pieces of eight!
<Squawk> Pieces of eight!
<Squawk> Pieces of eight!
<Squawk> Pieces of eight!
<Squawk> Pieces of nine!
<SYSTEM HALTED: parroty error!>

 

[Trevor Wilson]

I am intending to build a set of garden (path) lights from White/Blue LEDs
[not bulbs, due to power/life reasons].
What is the best method of driving these?

**Just use DC. In any event, be aware that white LEDs have a very definite
life span, due to their construction. Blue LEDs do not. I've used some white
LEDs, in my doorbell @ 20mA for 3 years. They're down to around 20% of their
original brightness. My preference, in white LEDs is with the Luxeon
devices. Superb spread of light and usefully bright. Very expensive,
however.

It depends on the sort of LED, some have better lifes than others.

**Tell me more. AFAIK, they all use the same (at least similar) technology.
It is, of course, the fluorescent coating which fails, over time.

 

[N. Thornton]

Tom @ HollyLodge wrote:

I am intending to build a set of garden (path) lights from White/Blue
LEDs [not bulbs, due to power/life reasons]. What is the best method
of driving these? I was thinking along the lines of 555 Monostable
driving 555 Astable to produce narrow pulses, at a high frequency, so
its not seen that they are flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue
LEDs so that not too much power is lost as heat. LEDs will probably
be in parallel, with one common cathode resistor going to -ve/gnd

The method of pulsing current into the LEDs makes sense for the
red/green types because the luminous efficiency, ratio of light output
to power consumed, is best at high currents. This is not the case for
blue/white variety where the efficiency peaks at low currents like 5mA
and tends to roll off at higher currents, making a pulsed scheme
pointless unless multiplexing or flashing serves some other purpose. For
a straight illumination application, the most practical way would be to
produce a large enough DC voltage that can drive all the LEDs within a
physical cluster as a series connection with a separate current
regulation circuit for each cluster. The relatively recent miniaturized
switching regulators are excellent for this application and are easily
adapted to this service by using a small current sensing resistor in
series with the LED series cluster to develop the feedback FB voltage
for the switcher, which then adjusts the voltage across the cluster so
as to maintain the current at a constant level. See
http://www.national.com and the Web Bench simulator for some excellent
and low cost candidates. By using switchers with frequency at 200KHz and
above, you end up with very small inductors, filter caps, and overall
circuit volume. Your "bulb" then consists of an input rectifier, this
circuit, and the LED cluster which you drive with that standard beefy
low voltage lighting transformer in parallel with all the other "bulbs."

Hi

If youre running them off the mains, much of this is unnecesary. From
a small transformer one only needs rectifier, reservoir cap and
resistor. Switched mode stuff is really only worthwhile if youre
running on batteries.

And if you want white, you can mix R G and B to get higher output at
lower cost and higher energy efficiency.


Regards, NT

 

[Fred Bloggs]

Tom @ HollyLodge wrote:

I am intending to build a set of garden (path) lights from White/Blue
LEDs [not bulbs, due to power/life reasons]. What is the best method
of driving these? I was thinking along the lines of 555 Monostable
driving 555 Astable to produce narrow pulses, at a high frequency, so
its not seen that they are flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue
LEDs so that not too much power is lost as heat. LEDs will probably
be in parallel, with one common cathode resistor going to -ve/gnd

The method of pulsing current into the LEDs makes sense for the
red/green types because the luminous efficiency, ratio of light output
to power consumed, is best at high currents. This is not the case for
blue/white variety where the efficiency peaks at low currents like 5mA
and tends to roll off at higher currents, making a pulsed scheme
pointless unless multiplexing or flashing serves some other purpose. For
a straight illumination application, the most practical way would be to
produce a large enough DC voltage that can drive all the LEDs within a
physical cluster as a series connection with a separate current
regulation circuit for each cluster. The relatively recent miniaturized
switching regulators are excellent for this application and are easily
adapted to this service by using a small current sensing resistor in
series with the LED series cluster to develop the feedback FB voltage
for the switcher, which then adjusts the voltage across the cluster so
as to maintain the current at a constant level. See
http://www.national.com and the Web Bench simulator for some excellent
and low cost candidates. By using switchers with frequency at 200KHz and
above, you end up with very small inductors, filter caps, and overall
circuit volume. Your "bulb" then consists of an input rectifier, this
circuit, and the LED cluster which you drive with that standard beefy
low voltage lighting transformer in parallel with all the other "bulbs."

Hi

If youre running them off the mains, much of this is unnecesary. From
a small transformer one only needs rectifier, reservoir cap and
resistor. Switched mode stuff is really only worthwhile if youre
running on batteries.

Switchmode allows a better circuit that regulates against line and load
fluctuations using a standard low voltage lighting transformer- no heat
sink should be required and the number of bulbs can vary over a greater
range. But if you just want a basic "thing" that illuminates- why bother
asking on SED- go substitute resistors to suit.

 

[N. Thornton]

Switchmode allows a better circuit that regulates against line and load
fluctuations using a standard low voltage lighting transformer- no heat
sink should be required and the number of bulbs can vary over a greater
range.

It does, but I think this is overkill for a garden path. Household
light bulbs dont usually have regulation.

Regards, NT

 

[Fred Bloggs]

It does, but I think this is overkill for a garden path. Household
light bulbs dont usually have regulation.

Regards, NT

This is not a household bulb, it is a LED- notice the technology is
different.

 

[Ian Stirling]

I am intending to build a set of garden (path) lights from White/Blue LEDs
[not bulbs, due to power/life reasons].
What is the best method of driving these?

**Just use DC. In any event, be aware that white LEDs have a very definite
life span, due to their construction. Blue LEDs do not. I've used some white
LEDs, in my doorbell @ 20mA for 3 years. They're down to around 20% of their
original brightness. My preference, in white LEDs is with the Luxeon
devices. Superb spread of light and usefully bright. Very expensive,
however.

It depends on the sort of LED, some have better lifes than others.

**Tell me more. AFAIK, they all use the same (at least similar) technology.
It is, of course, the fluorescent coating which fails, over time.

The technology is indeed similar.
It's the little wrinkles.
The older ones tended to have poorer lifes, as competition wasn't so
hard, and you had to live with what you could get, plus not as much
was known about why they'd dim over time.
Now they are paying attention to ways to get the longest life.

It'd be very nice if they'd get LEDs to the efficiancy of fluorescant
lights.
The best of them is around 50lm/W, and compares to the low-pressure sodium
yellow light (coincidentally at a similar colour) at 180lm/W, or around
a quarter as good.
For white light, it's not even that good.
Common-or-garden fluorescant tubes get around 100 lumens a watt, and the
best around 130.
Compared to a luxeon white device at around 20, for a fifth to a sixth.
This is around the same efficiancy as a halogen light.

As a comparison, the highest efficiency that I know of is for some IR
laser diodes, at around 55%.
If this could be done for green, you'd get around 300lm/W, or about six
times better than the best LED in terms of brightness.

 

[Zak]

As a comparison, the highest efficiency that I know of is for some IR
laser diodes, at around 55%.
If this could be done for green, you'd get around 300lm/W, or about six
times better than the best LED in terms of brightness.

Green fluorescents also have enormous efficiency. I remember 2 or even 4
times as good as white. They were used in copiers.

But I can't find the info on Google...


Thomas

 

[R.Lewis]

Tom @ HollyLodge wrote:

I am intending to build a set of garden (path) lights from White/Blue
LEDs [not bulbs, due to power/life reasons]. What is the best method
of driving these? I was thinking along the lines of 555 Monostable
driving 555 Astable to produce narrow pulses, at a high frequency, so
its not seen that they are flashing, but it reduces the duty cycle.
Once the 555 Astable is built, how do i best drive the white / blue
LEDs so that not too much power is lost as heat. LEDs will probably
be in parallel, with one common cathode resistor going to -ve/gnd

The method of pulsing current into the LEDs makes sense for the
red/green types because the luminous efficiency, ratio of light output
to power consumed, is best at high currents. This is not the case for
blue/white variety where the efficiency peaks at low currents like 5mA
and tends to roll off at higher currents, making a pulsed scheme
pointless unless multiplexing or flashing serves some other purpose. For
a straight illumination application, the most practical way would be to
produce a large enough DC voltage that can drive all the LEDs within a
physical cluster as a series connection with a separate current
regulation circuit for each cluster. The relatively recent miniaturized
switching regulators are excellent for this application and are easily
adapted to this service by using a small current sensing resistor in
series with the LED series cluster to develop the feedback FB voltage
for the switcher, which then adjusts the voltage across the cluster so
as to maintain the current at a constant level. See
http://www.national.com and the Web Bench simulator for some excellent
and low cost candidates. By using switchers with frequency at 200KHz and
above, you end up with very small inductors, filter caps, and overall
circuit volume. Your "bulb" then consists of an input rectifier, this
circuit, and the LED cluster which you drive with that standard beefy
low voltage lighting transformer in parallel with all the other "bulbs."

Hi

If youre running them off the mains, much of this is unnecesary. From
a small transformer one only needs rectifier, reservoir cap and
resistor. Switched mode stuff is really only worthwhile if youre
running on batteries.

And if you want white, you can mix R G and B to get higher output at
lower cost and higher energy efficiency.

I think you may need to check that last statement.