Flash frequency possible for LED

[Charlie+]

I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547. This is quite effective and will strobe at just
over 60 Hz. Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?
I do notice that this strobe smears the disk sector much more than a flash
strobe so that would perhaps point to a not very sharp switching of the LED
but I dont know if this is a shortcoming of the driving circuit or the LED
characteristics, which might in turn show a definite limit to practical RPM
possible to detect even if the oscillator ran faster. Would HiBright single
colour LED be better in this application, perhaps the phosphors in white LED
have a slow rise/fall character?
Charlie

 

[Robert Roland]

On Thu, 02 Sep 2010 08:19:16 +0100, Charlie+ <charlie@xxx.net> wrote:

I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547. This is quite effective and will strobe at just
over 60 Hz. Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?

As always, the data sheet has the final word.

Remote controls for TVs etc. usually operate at 38kHz. These are IR
LEDs, but the principle should be the same. I have seen music
transmitted through a red LED, and I have transmitted 9600bps
unmodulated data through a red LED, so 60Hz is probably well within a
LEDs capabilities.

I do notice that this strobe smears the disk sector much more than a flash
strobe so that would perhaps point to a not very sharp switching of the LED

Smearing is probably caused by the LEDs on-time being too long. You
need shorter blinks. Of course, this will result in less visual
brightness, so you will have to compensate by using a brighter LED.
--
RoRo

 

[Jon Kirwan]

On Thu, 02 Sep 2010 08:19:16 +0100, Charlie+
<charlie@xxx.net> wrote:

I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547. This is quite effective and will strobe at just
over 60 Hz. Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?

Very much faster than 60Hz. I've got a small circuit nearby
operating them around 80kHz, easily.

I do notice that this strobe smears the disk sector much more than a flash
strobe so that would perhaps point to a not very sharp switching of the LED
but I dont know if this is a shortcoming of the driving circuit or the LED
characteristics, which might in turn show a definite limit to practical RPM
possible to detect even if the oscillator ran faster. Would HiBright single
colour LED be better in this application, perhaps the phosphors in white LED
have a slow rise/fall character?

It's really hard to say. A strobe is often a capacitive
discharge triggered by an SCR mechanism and may have a very
small duty cycle. I've no idea what your LED duty cycle is
or the rates you are using for each during the above
observation. But your issue likely is about duty cycle here,
my guess.

I don't know much about phosphors used with white LEDs. I'm
kind of familiar with some phosphors that I use, though,
which are used for temperature measurement and are selected
for longer tau periods and single exponential decay modes. I
think Don knows quite a bit about it, though. Perhaps he
will correct some of what I'll write, now.

The term 'phosphor' is old and well predates modern knowledge
about mechanisms. In broad usage, the term covers several
different mechanisms that happen to have some similarities to
an observer. Mostly, just about anything illustrating a
Stokes Shift qualifies (shift to lower wavelength.) Just to
make it more confusing, anti-Stokes Shifts are probably
included. So 'phosphor' means just about anything than can
cause any noticeable change in wavelength.

LEDs probably use faster phosphors that depend on
fluorescence. Fluorescence is caused by absorption (which is
VERY FAST -- on the order of 10^-14 second, or so) -- and
then often some vibrational relaxation (thermal relaxation),
which can take place on the order of 10^-12 seconds, or
perhaps an order of magnitude slower or faster in solids and
liquids. Fluorescence involves both, plus some time for
emission probability to play out (which is also fast), so I
usually just keep in mind that fluorescence is a roughly
nanosecond phenomenon. (Singlet state, all totalled, works
out to on order of 10^-8 second.) About 20ns covers most
cases likely to be seen, though some are ten times longer.

Phosphorescence is on the order of 1/10th of a millisecond to
tens of milliseconds... and rarely into seconds. It's much
slower, because it depends on intersystem crossing between
singlet and triplet states, which are very much less probable
and therefore appear to be slower when they do happen.
Triplet state lifetimes are on the order of 10^-4 second, or
so. Because of how long it takes, in liquids these states
usually get de-energized as heat through internal conversion
and aren't visible. But in solids, like ceramics, they can
be quite visible. I just don't think LEDs depend on
phosphorescent modes, though.

There's another method, delayed fluorescence, with times that
fall between the two, perhaps on the order of 10's to 100's
of microseconds, though I think various dopants greatly
affect all that and widen the range quite a bit. Anyway, I
don't know much about it, except that it seems to involve two
triplet states (biphotonic.)

If I'm right that LEDs probably depend on fast fluorescence,
the tens of nanoseconds required by them shouldn't be an
issue for you.

It's probably about the LED duty cycle.

Jon

 

[Charlie+]

On Thu, 02 Sep 2010 01:34:40 -0700, Jon Kirwan <jonk@infinitefactors.org>
wrote as underneath my scribble :

My thanks to both yourself and Robert for your thaughts - I am now informed
enough to go searching for a better circuit for high speeds!
Charlie

On Thu, 02 Sep 2010 08:19:16 +0100, Charlie+
charlie@xxx.net> wrote:

I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547. This is quite effective and will strobe at just
over 60 Hz. Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?

Very much faster than 60Hz. I've got a small circuit nearby
operating them around 80kHz, easily.

I do notice that this strobe smears the disk sector much more than a flash
strobe so that would perhaps point to a not very sharp switching of the LED
but I dont know if this is a shortcoming of the driving circuit or the LED
characteristics, which might in turn show a definite limit to practical RPM
possible to detect even if the oscillator ran faster. Would HiBright single
colour LED be better in this application, perhaps the phosphors in white LED
have a slow rise/fall character?

It's really hard to say. A strobe is often a capacitive
discharge triggered by an SCR mechanism and may have a very
small duty cycle. I've no idea what your LED duty cycle is
or the rates you are using for each during the above
observation. But your issue likely is about duty cycle here,
my guess.

I don't know much about phosphors used with white LEDs. I'm
kind of familiar with some phosphors that I use, though,
which are used for temperature measurement and are selected
for longer tau periods and single exponential decay modes. I
think Don knows quite a bit about it, though. Perhaps he
will correct some of what I'll write, now.

The term 'phosphor' is old and well predates modern knowledge
about mechanisms. In broad usage, the term covers several
different mechanisms that happen to have some similarities to
an observer. Mostly, just about anything illustrating a
Stokes Shift qualifies (shift to lower wavelength.) Just to
make it more confusing, anti-Stokes Shifts are probably
included. So 'phosphor' means just about anything than can
cause any noticeable change in wavelength.

LEDs probably use faster phosphors that depend on
fluorescence. Fluorescence is caused by absorption (which is
VERY FAST -- on the order of 10^-14 second, or so) -- and
then often some vibrational relaxation (thermal relaxation),
which can take place on the order of 10^-12 seconds, or
perhaps an order of magnitude slower or faster in solids and
liquids. Fluorescence involves both, plus some time for
emission probability to play out (which is also fast), so I
usually just keep in mind that fluorescence is a roughly
nanosecond phenomenon. (Singlet state, all totalled, works
out to on order of 10^-8 second.) About 20ns covers most
cases likely to be seen, though some are ten times longer.

Phosphorescence is on the order of 1/10th of a millisecond to
tens of milliseconds... and rarely into seconds. It's much
slower, because it depends on intersystem crossing between
singlet and triplet states, which are very much less probable
and therefore appear to be slower when they do happen.
Triplet state lifetimes are on the order of 10^-4 second, or
so. Because of how long it takes, in liquids these states
usually get de-energized as heat through internal conversion
and aren't visible. But in solids, like ceramics, they can
be quite visible. I just don't think LEDs depend on
phosphorescent modes, though.

There's another method, delayed fluorescence, with times that
fall between the two, perhaps on the order of 10's to 100's
of microseconds, though I think various dopants greatly
affect all that and widen the range quite a bit. Anyway, I
don't know much about it, except that it seems to involve two
triplet states (biphotonic.)

If I'm right that LEDs probably depend on fast fluorescence,
the tens of nanoseconds required by them shouldn't be an
issue for you.

It's probably about the LED duty cycle.

Jon

 

[Jasen Betts]

On 2010-09-02, Charlie+ <charlie@xxx.net> wrote:

I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547. This is quite effective and will strobe at just
over 60 Hz. Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?

some number of megahertz last time I checked.

I do notice that this strobe smears the disk sector much more than a flash
strobe so that would perhaps point to a not very sharp switching of the LED
but I dont know if this is a shortcoming of the driving circuit or the LED
characteristics, which might in turn show a definite limit to practical RPM
possible to detect even if the oscillator ran faster.

LEDs (especially the cheap ones) are a lot dimmer than xenon flash tubes, to
compensate for this the LEDs in your strobe unit are driven with a longer
on-time than the flash tube would have

as a result the disk moves further whilst the led is lit.

Would HiBright single
colour LED be better in this application, perhaps the phosphors in white LED
have a slow rise/fall character?

that could be a factor too, but if present would show up as blue/yellow fringing
on the disk. I think flourescent dyes are used instead of phosphors, and I've
never noticed a phosphor effect with them.

--
¡spuɐɥ ou 'ɐꟽ ʞooꞀ

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

 

[George Herold]

On Sep 2, 7:59 am, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-09-02, Charlie+ <char...@xxx.net> wrote:

I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547.  This is quite effective and will strobe at just
over 60 Hz.  Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?

some number of megahertz last time I checked.

Yup, I use red LED's to check the speed of ~MHz photodiode TIA's.

I hit the LED with a step and look at the PD response. The IR LEDs
are a lot slower. There must be some physics/ solid state reason for
this but I don't know what it is.

Next time I'm testing PD's I'll look at them with some white LEDs and
see if there is any sign of a longer turn-off time.

To the OP how much current are you sending through the LEDs? As
others have said it sounds like you have a long pulse time. More
current for a shorter time may help.

George H.

I do notice that this strobe smears the disk sector much more than a flash
strobe so that would perhaps  point to a not very sharp switching of the LED
but I dont know if this is a shortcoming of the driving circuit or the LED
characteristics, which might in turn show a definite limit to practical RPM
possible to detect even if the oscillator ran faster.

LEDs (especially the cheap ones) are a lot dimmer than xenon flash tubes, to
compensate for this the LEDs in your strobe unit are driven with a longer
on-time than the flash tube would have

as a result the disk moves further whilst the led is lit.

Would HiBright single
colour LED be better in this application, perhaps the phosphors in white LED
have a slow rise/fall character?

that could be a factor too, but if present would show up as blue/yellow fringing
on the disk.  I think flourescent dyes are used instead of phosphors, and I've
never noticed a phosphor effect with them.

--
¡spuɐɥ ou 'ɐꟽ ʞooꞀ

--- news://freenews.netfront.net/ - complaints: n...@netfront.net ---

 

[Jamie]

Charlie+ wrote:

I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547. This is quite effective and will strobe at just
over 60 Hz. Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?
I do notice that this strobe smears the disk sector much more than a flash
strobe so that would perhaps point to a not very sharp switching of the LED
but I dont know if this is a shortcoming of the driving circuit or the LED
characteristics, which might in turn show a definite limit to practical RPM
possible to detect even if the oscillator ran faster. Would HiBright single
colour LED be better in this application, perhaps the phosphors in white LED
have a slow rise/fall character?
Charlie
You most likely have a saturation problem in the LED from your driving

circuit, the turn off time is more noticeable at higher rates. Also, you
maybe not be sinking the off cycle which will cause a little off delay.

I suppose one could use a feed back PD (Photo Diode) to throttle the
current feed when the LED is actually generating light. This will result
in a faster turn off because you won't have the LED in or over saturation.

Not that i've ever had a need to resolve an issue of this type, it
does sound logical.

Jamie..

 

[Grant]

On Thu, 02 Sep 2010 19:33:53 -0400, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

Charlie+ wrote:
I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547. This is quite effective and will strobe at just
over 60 Hz. Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?
I do notice that this strobe smears the disk sector much more than a flash
strobe so that would perhaps point to a not very sharp switching of the LED
but I dont know if this is a shortcoming of the driving circuit or the LED
characteristics, which might in turn show a definite limit to practical RPM
possible to detect even if the oscillator ran faster. Would HiBright single
colour LED be better in this application, perhaps the phosphors in white LED
have a slow rise/fall character?
Charlie
You most likely have a saturation problem in the LED from your driving
circuit, the turn off time is more noticeable at higher rates. Also, you
maybe not be sinking the off cycle which will cause a little off delay.

I suppose one could use a feed back PD (Photo Diode) to throttle the
current feed when the LED is actually generating light. This will result
in a faster turn off because you won't have the LED in or over saturation.

Saturation? What an odd notion that seems to me? I think you might have
a slower turn off in white LEDs due to the phosphor, the answer then would
be to go monochrome LEDs. No phosphor lifetime or afterglow or whatever
the effect is called, to cause a smear. Use RGB LED if you need white?

Grant.

 

On Sep 2, 12:19 am, Charlie+ <char...@xxx.net> wrote:

I have a variable strobe which uses two bright leds in series
driven by an
oscillator and a BC547.  This is quite effective and will strobe at
just
over 60 Hz.  Could anyone give me an idea of the maximum on/off
strobe
frequency possible from LEDs?
I do notice that this strobe smears the disk sector much more than
a flash
strobe so that would perhaps  point to a not very sharp switching
of the LED
but I dont know if this is a shortcoming of the driving circuit or
the LED
characteristics, which might in turn show a definite limit to
practical RPM
possible to detect even if the oscillator ran faster.  Would
HiBright single
colour LED be better in this application, perhaps the phosphors in
white LED
have a slow rise/fall character?
Charlie

When I was designing the photo sensor preamps for an HDTV telecine, I
built a driver to work with HP super bright LEDs that had 10 nS turn
on / off times used as a preamp test for the photo sensors. The LED
light source driven by a Burr Brown transconductance amp was a dB down
at 30 MHz and quite linear from off to 10 mA drive. A 30 kHz (line
rate) linearity ramp was in fact linear. Garden variety LEDs from
Radio Shack aren't anywhere near as fast but could do audio.

G˛

 

[Eeyore]

Charlie+ wrote:

I have a variable strobe which uses two bright leds in series driven by an
oscillator and a BC547. This is quite effective and will strobe at just
over 60 Hz. Could anyone give me an idea of the maximum on/off strobe
frequency possible from LEDs?

Faster than you can imagine. Certainly well beyond flicker rates.

Graham

 

[Wim Lewis]

In article <97lu76hoi067sn4b6duqs56ebpav523op6@4ax.com>,
Jon Kirwan <jonk@infinitefactors.org> wrote:

Very much faster than 60Hz. I've got a small circuit nearby
operating them around 80kHz, easily.

Is it in your remote control, by any chance?


(Thanks for posting the info on LED phosphors, BTW; I found it
interesting.)

--
Wim Lewis <wiml@hhhh.org>, Seattle, WA, USA. PGP keyID 27F772C1
"We learn from history that we do not learn from history." -Hegel

 

[Jon Kirwan]

On Wed, 8 Sep 2010 02:40:15 +0000 (UTC),
wiml@underhill.hhhh.org (Wim Lewis) wrote:

In article <97lu76hoi067sn4b6duqs56ebpav523op6@4ax.com>,
Jon Kirwan <jonk@infinitefactors.org> wrote:
Very much faster than 60Hz. I've got a small circuit nearby
operating them around 80kHz, easily.

Is it in your remote control, by any chance?

(Thanks for posting the info on LED phosphors, BTW; I found it
interesting.)

No problem, at all. Thanks for the kind comment.

Jon