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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Explosions in electronics.
Date: Thu, 03 Oct 2002 01:43:42 GMT
NNTP-Posting-Date: Wed, 02 Oct 2002 18:43:42 PDT
Winfield Hill wrote:
> Chuck Simmons wrote...
> > Saw that. Very interesting. I once built a device that pulsed small
> > LEDs to about 3 amps but the pulse duration was maybe 0.5 nsec and
> > the repetition rate so low that the LED seemed hardly to light. The
> > goal was a fast rise time light pulse and the switch was a transistor
> > operated in avalanche mode. The LEDs tended to be more reliable than
> > the transistors in this kind of circuit.
> > That brings up a question. Does anyone use transistors in avalanche
> > mode today?
> To me that brings up a different question, how did you get the LEDs
> to illuminate and extinguish so fast? I have tried a variety of
> visible-light LEDs with 1 to 2ns-long high-current pulses, and found
> that I couldn't get the light to respond as fast as the current, in
> fact having a time constant of more like 10ns. My short ns current
> pulses simply resulted in weak longish 10ns light-output pulses.
> To get 10ns instead of 20ns, I had to reverse the current direction
> after the first few ns. I tried currents of 0.25 to 10A.
I answered this in email because I saw it there first. I suspect that
LED construction has changed significantly since 1970 or so in that the
mrf quoted rise and fall times are much larger today than the rise and
fall times quoted by Monsanto. I did happen to notice that there are
some fiber coupled LEDs rated for 125Mbps communications. Some of these
are less than 1 nanosecond rise and fall. LEDs rated for panel light use
seem to be over 1 microsecond rise and fall.
Back in 1970, we were aware of current spreading in LEDs. The LED pulser
I built produced an emitting spot on the junction about half the size of
the spot produced by a constant current. The pulser probably charged the
juction capacitance quite fast because the applied voltage was on the
order of 20 volts for Fairchild 2N3646 transistors that I selected.
Avalanche started in these transistors at about 60 volts and the voltage
dropped immediately to about 40 volts. The circuit used several parallel
coaxial cables of a few inches length to stop the avalanche. The idea
being that the collector voltage would drop a bit during avalanche so
the reflection from the open transmission lines would drop the voltage
across the transistor sufficiently to stop the avalanche.
If I recall the statistics correctly, about 20% of 2N3646s would work in
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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