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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