"Ned Simmons" wrote in message
...
On Tue, 25 Sep 2018 18:31:29 -0400, "Jim Wilkins"
wrote:
"Ned Simmons" wrote in message
. ..
On Tue, 25 Sep 2018 09:15:10 -0400, "Jim Wilkins"
wrote:
"Ned Simmons" wrote in message
m...
On Mon, 24 Sep 2018 19:30:37 -0700 (PDT), whit3rd
wrote:
On Monday, September 24, 2018 at 6:12:23 PM UTC-7, Ned Simmons
wrote:
On Sat, 22 Sep 2018 19:44:42 -0700 (PDT), whit3rd
wrote:
A constant current source is ideal for powering LEDs; with
the
right
size LED (or the right multiplicity of paralleled LEDs)
that's
useful power.
Sure, but you can't just connect parallel strings of LED's to
a
constant current source and expect the current in the
individual
strings will be equal.
Yet, it's done all the time. A 'nine-LED' flashlight with
three
AAA cells
has nine strings of one LED each, in parallel.
Desk lamps with three or four LED strings in parallel are
common,
also.
Sometimes resistors are added to each string, but not always.
So how do you insure that the current is divided equally among
the
strings?
--
Ned Simmons
They are somewhat self-ballasting, as shown by the I-V curves.
https://www.electronics-tutorials.ws/diode/diode_8.html
Unless I'm misintrepeting the curves, or looking in the wrong
place,
the graph shows a potential for runaway -- the slope of the curves
increases with increasing voltage.
Here's a similar curve for a tungsten filament, which is
self-ballasting.
https://www.quora.com/What-does-the-...lamp-look-like
This article talks about the hazards of paralleling strings of
LEDs.
https://www.ledsmagazine.com/article...-magazine.html
--
Ned Simmons
Try this:
https://www.cree.com/led-components/...LA1BWKWMKW.pdf
The slope isn't straight like a resistor voltage divider, the
reason
I qualified them as "somewhat". It also shows their limited
tolerance
for reverse bias.
-jsw
Yeah, it's sinking in. Slowly.
--
Ned Simmons
In the 1980's I designed and built a delicately balanced constant
current string of LEDs, thermistors and Zeners that sensed and
indicated four liquid ink levels in a rotating print head. It was a
finely tuned version of the Low Fuel light in my car. At the time I
understood the components' I-V curves well, but I haven't dealt as
seriously with LEDs since.
The constant current limiter was a depletion mode JFET with its gate
connected to the source like example 7. They were available as
2-terminal devices that looked like diodes.
https://www.quora.com/What-are-Fet-application
-jsw