Max flashing rate of LEDs & Luxeons

[Panrock]

I've just posted the same question on Paul Stenning's vintage radio board, but since I can't be sure it won't be removed for some arbitrary reason, I'm asking it here too.

As you know, Steve Anderson is designing a pulse-width driven luxeon modulator. I'm to use this on my upgraded monitor. It's necessary to drive luxeons this way to obtain a linear and full range modulated light response.

But with all this fast switching, does anyone know what the typical maximum practicable operating frequency of these devices (LEDS and Luxeons) is?

A quick look at one LED's rise time data implied that it would probably be good for about 20 MHz or so. But I wonder if this is typical, and like other semiconductor junctions, in fact they vary.

Also, does the light output necessarily faithfully follow the internal electrical changes? Hope it does! Anyone know?

Thanks,

Steve

[Steve Anderson]

But with all this fast switching, does anyone know what the typical maximum practicable operating frequency of these devices (LEDS and Luxeons) is?

A quick look at one LED's rise time data implied that it would probably be good for about 20 MHz or so. But I wonder if this is typical, and like other semiconductor junctions, in fact they vary.

Also, does the light output necessarily faithfully follow the internal electrical changes? Hope it does! Anyone know?

Thanks,

Steve

Steve, don't worry, these devices typically have a switching time of less than 100nS, 100 times faster than our PWM waveform. The light output to current is not quite as linear as a standard LED (over a limited range), but here we're just switching it on and off rapidly, so that's not an issue.

At 100kHz I would not call this fast, somewhat sluggish actually...certain conventional LEDs can switch in less than 100pS (pico-seconds). I'm working on a fibre-optic system where response time is measured in fS (femto-seconds) for HDTV.

Here's a link that should put your mind to rest...

http://www.luxeonstar.com/item.php?id=1 ... 2-PW14-U00

Steve A.

[Panrock]

Thanks... I wasn't really worried - more curious. The K2 luxeon on your link is implied to have a 100nS rise time, so that would give a maximum usable frequency of 5 MHz wouldn't it? Or a lot less than that to produce a good square wave.

A far cry from my earliest experiment with small (low thermal-lag) torch bulbs, when even 1 KHz was really pushing it to get anything at all !

Steve

[Steve Anderson]

A far cry from my earliest experiment with small (low thermal-lag) torch bulbs, when even 1 KHz was really pushing it to get anything at all ! Steve

I think I did some experiments along the same lines when I was in my teens, but that was a long time ago! I think I used 6V/50mA bulbs that were usually used for bicycle tail-lights....but the memory isn't as good as it was.

Steve A.

[Viewmaster]

Using torch bulbs for NBTV is the picture kinda out of focus looking? No HF but good LF?

Might be interesting to try this out on different torch bulbs to see which is the best...and worse! One could have a series of them mounted on a carousel and be able to move in all the various types...including the LED cluster too.
Albert.

[Panrock]

When I was building my first NBTV gear, for the lower sixth science exhibition at school in 1966, the choice then was basically between dim little 90v neons and temptingly bright torch bulbs.

No conceivable equalisation was possible to compensate for the deficiencies of these little bulbs. Though their rise time could be 'forced' with equalisation to some extent, the problem was their natural thermal decay time.

Steve

[Steve Anderson]

Sometime when I was in my late teens I did establish a voice link using neons, simply modulating them with the audio. I got up to about 30m with suprizingly good quality.

In more recent times I have got a 3mW Laser diode to stretch over 7km into a single photodiode, but Bangkok being as flat as it is it's hard to get a line-of-sight any greater. Plus the pollution tends to diffuse and attenuate the signal.

Might justify another trip to Laos or Cambodia!

Steve A.

[Viewmaster]

No conceivable equalisation was possible to compensate for the deficiencies of these little bulbs. Though their rise time could be 'forced' with equalisation to some extent, the problem was their natural thermal decay time.
Steve

What is needed is a blast of cold air inside the bulb's vacuum to cool the hot filament very rapidly.
Albert.