Pulsing LEDs to achieve maximum output?

[Myron Samila]

Ok,

I've read some discussion here about pulsing LEDs to achieve higher output without burning
the stupid things out. (Instead of CW).

I'm just working on a circuit for general purpose low voltage illumination, but want to
achieve maximum output.

I suppose a pulse of 100Hz would be decent? This would allow more voltage to be applied
to the LED without overheating it.

So I was thinking of using a 555timer triggering a transistor (of what type?) handling
approx 25 LEDs (no specific data on the LED yet, I haven't gotten that far ie: current,
voltage, etc..., I'm researching that now).

I have a basic schematic that shows a 555 triggering such a circuit, just what transistor
should work for me if I want to lets say switch a 6VDC/1A load?

Any info would be great, thanks!


BTW: I believe it was "Watson" that was telling us about his white LEDs that weren't
lasting very long, well, I've been seeing a lot more white LED applications such as
crossing lights, while the duty cycle is quite low, I suppose they expect these crossing
lights (you know the picture of the person walking!) to last quite a while.

From what I understand, the Osram LEDs are of excellent quality, and priced a wee bit
higher than the rest as well.

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19

 

[R.Lewis]

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
news:HuIfc.31271$vF3.1794669@news20.bellglobal.com...

Ok,

I've read some discussion here about pulsing LEDs to achieve higher output
without burning
the stupid things out. (Instead of CW).

I'm just working on a circuit for general purpose low voltage
illumination, but want to
achieve maximum output.

I suppose a pulse of 100Hz would be decent? This would allow more voltage
to be applied
to the LED without overheating it.

So I was thinking of using a 555timer triggering a transistor (of what
type?) handling
approx 25 LEDs (no specific data on the LED yet, I haven't gotten that far
ie: current,
voltage, etc..., I'm researching that now).

I have a basic schematic that shows a 555 triggering such a circuit, just
what transistor
should work for me if I want to lets say switch a 6VDC/1A load?

Any info would be great, thanks!


BTW: I believe it was "Watson" that was telling us about his white LEDs
that weren't
lasting very long, well, I've been seeing a lot more white LED
applications such as
crossing lights, while the duty cycle is quite low, I suppose they expect
these crossing
lights (you know the picture of the person walking!) to last quite a
while.

From what I understand, the Osram LEDs are of excellent quality, and
priced a wee bit
higher than the rest as well.

Read the last thread and you will see that pulsing leds REDUCES the output
compared to that obtained by the same average current at dc.

 

[Myron Samila]

"R.Lewis" <h.lewis-not this bit-@connect-2.co.uk> wrote in message
news:c5p5f6$t7n$1@thorium.cix.co.uk...

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
news:HuIfc.31271$vF3.1794669@news20.bellglobal.com...
Ok,

I've read some discussion here about pulsing LEDs to achieve higher output
without burning
the stupid things out. (Instead of CW).

I'm just working on a circuit for general purpose low voltage
illumination, but want to
achieve maximum output.

I suppose a pulse of 100Hz would be decent? This would allow more voltage
to be applied
to the LED without overheating it.

So I was thinking of using a 555timer triggering a transistor (of what
type?) handling
approx 25 LEDs (no specific data on the LED yet, I haven't gotten that far
ie: current,
voltage, etc..., I'm researching that now).

I have a basic schematic that shows a 555 triggering such a circuit, just
what transistor
should work for me if I want to lets say switch a 6VDC/1A load?

Any info would be great, thanks!


BTW: I believe it was "Watson" that was telling us about his white LEDs
that weren't
lasting very long, well, I've been seeing a lot more white LED
applications such as
crossing lights, while the duty cycle is quite low, I suppose they expect
these crossing
lights (you know the picture of the person walking!) to last quite a
while.

From what I understand, the Osram LEDs are of excellent quality, and
priced a wee bit
higher than the rest as well.

Read the last thread and you will see that pulsing leds REDUCES the output
compared to that obtained by the same average current at dc.


I guess the theory is you could supply more V at the junction, letting it cool in the off

period, and get higher output, but you're also switching it on and off (quite possibly
reducing output than if CW)

The idea is to get the LEDs to last long, ideally, my simple illumination project would
consist of nothing more than a simple bank of LEDs, and a voltage regulator! (skipping
any of the pulsing)

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19

 

[Gordon Youd]

Why not use a few more LEDs then run at a lower setting.

More light less loss!!

Regards, Gordon.

-----------------------------------
"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
newsaWfc.28968$2Z6.1176839@news20.bellglobal.com...

"R.Lewis" <h.lewis-not this bit-@connect-2.co.uk> wrote in message
news:c5p5f6$t7n$1@thorium.cix.co.uk...

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
news:HuIfc.31271$vF3.1794669@news20.bellglobal.com...
Ok,

I've read some discussion here about pulsing LEDs to achieve higher
output
without burning
the stupid things out. (Instead of CW).

I'm just working on a circuit for general purpose low voltage
illumination, but want to
achieve maximum output.

I suppose a pulse of 100Hz would be decent? This would allow more
voltage
to be applied
to the LED without overheating it.

So I was thinking of using a 555timer triggering a transistor (of what
type?) handling
approx 25 LEDs (no specific data on the LED yet, I haven't gotten that
far
ie: current,
voltage, etc..., I'm researching that now).

I have a basic schematic that shows a 555 triggering such a circuit,
just
what transistor
should work for me if I want to lets say switch a 6VDC/1A load?

Any info would be great, thanks!


BTW: I believe it was "Watson" that was telling us about his white
LEDs
that weren't
lasting very long, well, I've been seeing a lot more white LED
applications such as
crossing lights, while the duty cycle is quite low, I suppose they
expect
these crossing
lights (you know the picture of the person walking!) to last quite a
while.

From what I understand, the Osram LEDs are of excellent quality, and
priced a wee bit
higher than the rest as well.

Read the last thread and you will see that pulsing leds REDUCES the
output
compared to that obtained by the same average current at dc.


I guess the theory is you could supply more V at the junction, letting it
cool in the off
period, and get higher output, but you're also switching it on and off
(quite possibly
reducing output than if CW)

The idea is to get the LEDs to last long, ideally, my simple illumination
project would
consist of nothing more than a simple bank of LEDs, and a voltage
regulator! (skipping
any of the pulsing)

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19

 

[CWatters]

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
newsaWfc.28968$2Z6.1176839@news20.bellglobal.com...

I guess the theory is you could supply more V at the junction, letting it
cool in the off
period, and get higher output, but you're also switching it on and off
(quite possibly
reducing output than if CW)

The problem is the eye sees the average brightness. Put simply if you double
the power into the LED but for only half the time you get no change. (Even
that assumes that the LED power efficiency is constant for all power
inputs.)

This trick does work under certain conditions but typically the frequency
and mark space ratio has to be low enough for the flashing to be seen. For
example 0.25 second ON, 2 seconds OFF.

The idea is to get the LEDs to last long, ideally, my simple illumination
project would
consist of nothing more than a simple bank of LEDs, and a voltage
regulator!

Actually you need a current regulator not a voltage regulator but that's
being picky.

Colin

 

[Myron Samila]

"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:wSWfc.73754$FD1.4804650@phobos.telenet-ops.be...

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
newsaWfc.28968$2Z6.1176839@news20.bellglobal.com...

I guess the theory is you could supply more V at the junction, letting it
cool in the off
period, and get higher output, but you're also switching it on and off
(quite possibly
reducing output than if CW)

The problem is the eye sees the average brightness. Put simply if you double
the power into the LED but for only half the time you get no change. (Even
that assumes that the LED power efficiency is constant for all power
inputs.)

This trick does work under certain conditions but typically the frequency
and mark space ratio has to be low enough for the flashing to be seen. For
example 0.25 second ON, 2 seconds OFF.

The idea is to get the LEDs to last long, ideally, my simple illumination
project would
consist of nothing more than a simple bank of LEDs, and a voltage
regulator!

Actually you need a current regulator not a voltage regulator but that's
being picky.

Colin

Hi Colin,

Curious, lets say I have 10 LEDs that draw 20mA each, so 200mA total at lets say the
breakover voltage is 2.0V, wouldn't a 7802 (if they even have regulators that small, 7805
common enough) be able to regulate it?

With the "7802" regulator, you'd be able to handle 50 LEDs (1A) I suppose, if my
calculations are correct.

I just don't want to burn out a ton of LEDs cuz I could have done something different,
research first, then try it . Carpenters measure twice, cut once.



--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19

 

[CWatters]

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
news:QKXfc.35110$vF3.1996335@news20.bellglobal.com...

Actually you need a current regulator not a voltage regulator but that's
being picky.

Curious, lets say I have 10 LEDs that draw 20mA each, so 200mA total at
lets say the
breakover voltage is 2.0V, wouldn't a 7802 (if they even have regulators
that small, 7805
common enough) be able to regulate it?

I did say I was being picky....

LED's act like a "constant voltage sink" so if you want to connect them to a
voltage source you need to put something in series to limit/regulate the
current. You can use a resistor in series to limit the current.

Using your examples...

R = (Vs-Vled)/I
R = (5-2)/0.02
= 150 R.

and the power dissipated in the resistor is

P = 3 * 0.02
= 60mW

so 1/8th W resistors are OK.

You could try putting two LED in series with each resistor and recalculate
the value needed as...

R = (5-4)/0.02
= 50 R

and then

P = 20mW

Basic maths says that if you want an accurate answer to an equation (eg
accurate control of the current) you should avoid subtracting one number
from another similar one (eg avoid Vs-Vled approaching 0!).

On the other hand the larger the voltage drop across the resistor the less
sensitive the curcuit is to resistor and LED tollerance - so you may not
need a voltage regulator at all (but watch the power dissipation in the
resistor if it's very large).

Overall you need to balance these issues.

What is the source voltage? (eg input to the regulator).

Colin

 

[Myron Samila]

"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:SI5gc.74364$oW3.4949041@phobos.telenet-ops.be...

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
news:QKXfc.35110$vF3.1996335@news20.bellglobal.com...

Actually you need a current regulator not a voltage regulator but that's
being picky.

Curious, lets say I have 10 LEDs that draw 20mA each, so 200mA total at
lets say the
breakover voltage is 2.0V, wouldn't a 7802 (if they even have regulators
that small, 7805
common enough) be able to regulate it?

I did say I was being picky....

LED's act like a "constant voltage sink" so if you want to connect them to a
voltage source you need to put something in series to limit/regulate the
current. You can use a resistor in series to limit the current.

Using your examples...

R = (Vs-Vled)/I
R = (5-2)/0.02
= 150 R.

and the power dissipated in the resistor is

P = 3 * 0.02
= 60mW

so 1/8th W resistors are OK.

You could try putting two LED in series with each resistor and recalculate
the value needed as...

R = (5-4)/0.02
= 50 R

and then

P = 20mW

Basic maths says that if you want an accurate answer to an equation (eg
accurate control of the current) you should avoid subtracting one number
from another similar one (eg avoid Vs-Vled approaching 0!).

On the other hand the larger the voltage drop across the resistor the less
sensitive the curcuit is to resistor and LED tollerance - so you may not
need a voltage regulator at all (but watch the power dissipation in the
resistor if it's very large).

Overall you need to balance these issues.

What is the source voltage? (eg input to the regulator).

Colin


Essentially, Vs can be just about anything at this point, I will design around the load

requirements. It will be stepped down from AC, rectified and filtered.

Thanks for the info, I'm trying to source out a local supplier for some white LEDs of good
quality and of the output I require (the more mcd the better).

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19