Re: AC or DC Power for LEDs


John Bishop
 

Is there a concern if the LED is powered thru a lighting feature of a decoder?

John Bishop


--------------------------------------------

On Sat, 4/25/15, dvollrath@magnetek.com [WiringForDCC] <WiringForDCC@yahoogroups.com> wrote:

Subject: Re: [WiringForDCC] Re: AC or DC Power for LEDs
To: WiringForDCC@yahoogroups.com
Date: Saturday, April 25, 2015, 8:07 AM


 









LEDs do function somewhat like normal
diodes...EXCEPT that the reverse breakdown voltage is much
lower than you would expect. The manufacture's spec for
an LED stipulates a reverse voltage tolerance (Vbr) of only
about 5 volts. This does not mean that all parts will fail
if/when exposed to 5.01 volts or greater of reverse voltage.
But what it does mean that if/when the LED is exposed to
reverse voltage greater than the manufacturers Vbr spec, it
may indeed conduct current in that direction. When that
happens, the semiconductor junction inside will not emit
light but will create heat according the reverse current
times the actual breakdown voltage. This heat may indeed be
greater than the part is designed to take, especially when
the actual breakdown voltage might be 8 or 10 or even 12
volts. What is worse is that the manufacturer spends efforts
to improve the efficiency of LED light emissions rather than
making sure that reverse breakdown will occur uniformly
across the junction area, like for a zener diode. This means
that reverse breakdown may be concentrated at a weak point
in the junction structure causing spot heating and long or
even short term component degradation.

The best way to prevent that is to use a a DC
power source to operate LEDs. If you must use AC (including
DCC), add a conventional diode in series with the LED, or
another diode in anti-parallel to prevent excessive reverse
voltage and current through the LED.

DonV









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