Early LEDs had even lower ratings iirc.
I can remember flicking a design back where two LEDs were in parallel but
opposite ways around for ac operation. The fwd volts of the illuminated one
exceeded the specs for expecting long life of the other one.
John K
----- Original Message -----
From: "David Forbes" <dfor...@dakotacom.net>
To: <neonixie-l@googlegroups.com>
Sent: Friday, March 25, 2016 4:24 AM
Subject: Re: [neonixie-l] Re: Off topic, but it has an orange display..
Heathkit ID-1590 anemometer
Bill,
Thanks for the long rant, but typical LEDs have an absolute maximum
reverse voltage rating of 5V. Exceeding this is not guaranteed to
destroy the LED, but The LED is not guaranteed to work properly after
such use.
It is irresponsible to recommend that people ignore the datasheet
ratings of components.
On 3/24/16 10:35 AM, Bill van Dijk wrote:
From time to time I read dire warnings about the death of a LED by reverse
polarity. It does not happen. A LED is a diode, and is used normally in
forward bias, with a resistor (or some other circuit) to limit the
current. If the LED is reversed, it simply blocks the current, and
precisely nothing happens. You can make a cool bridge rectifier with 4
LEDs, but although technically OK, that is not very practical in most
applications due to their high forward drop and low current tolerance. Two
wire, bi-color LEDS use this principle where two different color dies are
connected back to back, depending on the polarity, one color or the other
lights. A red and green LED back to back make therefore a great polarity
indicator. Interestingly, if fed an AC current, a third color can be
produced, i.e. a green / red combination will shine yellow on AC. RGB LEDs
have 3 dies, but can display many colors by lighting one or more dies at
varying intensity. This is done through PWM, somethin
g a LED i
s very much suited for. Dimming of LEDs, as well as multiplexing of LEDs is
predicated on PWM. Many of the newer Ultra-bright LEDs will still light
pleasantly on 5V with series resistors as high as 1.5 kOhm.
The series diode does nothing to protect the LED, but does indeed drop the
voltage slightly due to its forward drop. From an efficiency perspective
it makes no difference, the diode also dissipates the drop Voltage times
the current in heat, just as the resistor does. Since the circuit has a
diode it is probably fine to leave it there, but adding one in a new
design has no use at all.
--
David Forbes, Tucson AZ
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