LED 'afterglow' time?

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,226
how is the project progressing? Have you found the white LEDs to ne slow? Would red LEDs work? Multiple LEDs?
Thank you for your interest!:)

Owing to the 'hurly-burly' of the holidays - I've yet to run any tests:oops: --- My initial experiments will be with a 25 ('ultra-bright white') LED 'head' salvaged from a torch...:cool:

Best regards
HP:)
 

strantor

Joined Oct 3, 2010
5,540
Owing to the 'hurly-burly' of the holidays - I've yet to run any tests:oops: --- My initial experiments will be with a 25 ('ultra-bright white') LED 'head' salvaged from a torch...:cool:

Best regards
HP:)
I think after you run some tests, you'll find that this discussion was mostly unwarranted.

Consider what you're 'scoping; how fast is it? I don't know what a Coolidge RA type rotor is, but does it spin faster than 20,000RPM? Your stroboscope (whether LED or flash tube) would be pointless if it is faster than the intended target. This flash-tube stroboscope goes up to 20,000 flashes per minute. That's 333Hz. When your strobe frequency is 333Hz, what should your turn on and turn off time be?

I say 1.5mS rise and 1.5mS fall is more than sufficient; anything faster than that is excessive. That's just enough time for it to turn all the way on and all the way back off. I say you don't need a blazing fast light source and a cliff-edged square wave of light. Hell, you don't even need to let the [LED/tube] turn all the way off. The persistence of vision in your painfully slow human eyes will iron out all the details.

I've posted Stroboscope stuff on the forum before.


BTW, here's a COTS stroboscope employing LED tech, and it's 15 times faster than the flash tube model I linked to earlier.
http://en-us.fluke.com/products/vib...ols/fluke-820-2-led-stroboscope.html#features

If you need anything faster than that, I think a different technology would be more appropriate.
 

KL7AJ

Joined Nov 4, 2008
2,229
"Somewhat" dim memory was evidently an understatement; turns out I was wrong.

I took a PIN photodiode, put it in series with a 1000Ω load resistor and gave it 10V of reverse bias to reduce its capacitance, then illuminated it with various red, green and white LEDs driven by my pulse generator while observing the voltage across the load resistor with my oscilloscope.

The red and green LEDs gave output rise and fall times of around 100 ns. The white LED (not a high-power illumination-type LED, just an ordinary 5 mm indicator type) showed rise and fall times of about 200-250 ns. But there was no observable afterglow from the white LED on any timescale-- nanoseconds, microseconds or milliseconds.

Santa is bringing me some 3 watt white LEDs to play with, and once we get past Christmas I'll rig up a high-power driver for them and give them a try. I'll also put together a fast transimpedance amplifier for the PIN photodiode which will hopefully take any detector speed limitations (like stray capacitance across that 1000Ω resistor) out of the picture.

But for now, the bottom line is it appears I was wrong about that "tail."
I can't address this question directly, but as an interesting side note, the human eye responds MUCH faster to green LEDs than to red ones. If you feed an LED with a square wave, you can see the green one flicker up to about 60 hertz, while the red one seems to be steadily on at around 15 Hz. This is why stop lights should be green. :)
 
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Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,226
I think after you run some tests, you'll find that this discussion was mostly unwarranted.

Consider what you're 'scoping; how fast is it? I don't know what a Coolidge RA type rotor is, but does it spin faster than 20,000RPM? Your stroboscope (whether LED or flash tube) would be pointless if it is faster than the intended target. This flash-tube stroboscope goes up to 20,000 flashes per minute. That's 333Hz. When your strobe frequency is 333Hz, what should your turn on and turn off time be?

I say 1.5mS rise and 1.5mS fall is more than sufficient; anything faster than that is excessive. That's just enough time for it to turn all the way on and all the way back off. I say you don't need a blazing fast light source and a cliff-edged square wave of light. Hell, you don't even need to let the [LED/tube] turn all the way off. The persistence of vision in your painfully slow human eyes will iron out all the details.

I've posted Stroboscope stuff on the forum before.


BTW, here's a COTS stroboscope employing LED tech, and it's 15 times faster than the flash tube model I linked to earlier.
http://en-us.fluke.com/products/vib...ols/fluke-820-2-led-stroboscope.html#features

If you need anything faster than that, I think a different technology would be more appropriate.
Please see post #40:)

As stated, I was willing to settle for 'sub multiples'! -- Needless to say - your info is good news indeed!:):):)

FWIW - While I recognize the fact that flash tube technology delivers much greater luminous intensity --I have experienced much to much 'agro' with 'jitter' at high flash rates (owed to 'imprecise' triggering) -- that said, I've an open mind on the subject:cool::cool::cool:

Best regards
HP:)
 

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,226
Here's another question for you optics gurus:cool::cool::cool:

1) The port window is fashioned of fused silica.

2) The tube envelope (at the beam bevel) is composed of moderately leaded borosilicate glass (said 'leading' functioning as a high pass filter to energy above 40 keV -- But, in point of fact, constitutes a band-stop filter to energies from ~ 12eV - 35 keV-- and, hence, is quite lucent to visible light [Ca. 2.25 eV]) --- Note that 'solarization' effects (i.e. chemical changes in the glass secondary to roentgen irradiation) result in a distinct 'amber' hue in the glass of used tubes...

3) The port window and tube are intervened by a few millimeters of optically transparent petroleum dielectric oil (Shell Diala)

Any ideas as to the optimum stroboscope spectra pursuant to effective illumination of the interior of the tube?

Best regards
HP:)
 

atferrari

Joined Jan 6, 2004
4,283
If you don't have a photodiode (faster than photo transistor), make your own by simply using a old-school green LED - cathode connected to ground and anode connected to a 1M resistor and other end of resistor connected to ground.

Then connected the resistor cathode node to an amplifier. Always make sure the absorbing LED is longer wavelength than your transmitter LED - but not too much. An IR LED will not detect flashing from a blue or green LED. As mentioned above, lots of newer high-brightness GaN LEDs are emitting blue or UV and red-shifted with phosphors. That means, you cannot use certain green LEDs to detect blue light.

Good luck.
Hola Gopher,

Could you show the schematic? I cannot grasp the actual connections.
 

#12

Joined Nov 30, 2010
18,217
hey, I think I know that one... the resistor is in parallel with the led to accelerate its discharge time when rapid switching is applied... all LED's have a very small amount of capacitance in them, ain't it?
Now you're just teasing me.:p
 

cmartinez

Joined Jan 17, 2007
7,314
You can also reverse bias to make them more sensitive (use in low frequency because of higher capacitance)
Ok, now I'm officially confused.

So LEDs do have some inherent capacitance after all that interferes with their ability of being switched at high frequency... right?
 

GopherT

Joined Nov 23, 2012
8,012
Ok, now I'm officially confused.

So LEDs do have some inherent capacitance after all that interferes with their ability of being switched at high frequency... right?
Yes. All diodes have capacitance and capacitance limits frequency.
Higher drive currents are needed at higher frequencies. And a path to ground is helpful for a sharp turn off.
image.jpg
 
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