24 Aug
2014
24 Aug
'14
5:03 p.m.
Thanks for that, Eric! Very illuminating info.. err, better say
informative. I had no idea of the order-of-magnitude difference between
source & sink currents in TTL stuff. Now I see why all those LEDs are
driven off the low side as you say. Makes perfect sense to me.
The difference in LED efficiency / intensity is truly stunning, though..
the new LEDs required 20X the resistance of the old 70s units - two of
which had "just failed". When was the last time +I+ ever saw an LED go bad?
That would be second to never, pretty much..
But yeah, 3K9 (call it 4K) vs. 220R (call it 200).. a nominal factor of
twenty, and just as bright if not more so off-axis! I just kept cranking
that sub-box knob going "wow, really?".
On Sun, Aug 24, 2014 at 6:07 PM, Eric Smith <spacewar at gmail.com> wrote:
On Sun, Aug 24, 2014 at 2:06 PM, Tony Duell <ard at
p850ug1.demon.co.uk>
wrote:
This is quite a nroaml way to do it. A TTL output
can sink a larger
current tahn it can source,
By which Tony means "can sink a **MUCH** larger current than it can
source".
If you look at the spec for the TI SN7404, the difference is a factor of
40:
max -0.4 mA source (high), vs. max 16 mA sink (low). Generally speaking,
0.4mA is nowhere near enough to be usable for an LED, so driving the low
side of the LED was the only practical approach with normal TTL.
Even the max sink spec of 16mA wasn't enough to drive a typical mid-1970s
red LED very brightly. For example, a Monsanto (later GI) MV5020 red LED
at 20 mA would only give you 2.0 mcd typical (and 0.6 mcd minimum!).
That very low source current spec (-0.4 mA) is about the same for all true
TTL families, excepting some buffer parts that are spec'd able to source
more
current.
Modern CMOS parts tend to have closer or even identical source and sink
limits, so it has become somewhat more common to see high-side drive
for LEDs, though many designers have stuck with low-side drive out of
tradition.
Many modern LEDs are also much more efficient than those from the 1970s,
which is partially because different chemistries producing lower
wavelengths
are commonly used for red, e.g., 620-340 nm vs. 660 nm, and the human
eye is significantly more sensitive to the lower wavelength. However, the
modern chemistries are also more efficient in terms of absolute light
output
per current. A representative modern T1 3/4 LED with comparable spatial
distribution to the MV5020 is the Cree C503B-RCS-CW0Z0AA1, which has a
typical wavelength of 624 nm, and typical luminous intensity of 5100 mcd
(minimum 3000 mcd) at 20 mA.
When comparing LED specs, it is important to look at the spatial
distribution.
A narrow viewing angle LED will give higher luminous intensity but only
on-axis.
Eric