Years ago I was building remote trouble annunciators for the Generac
power systems I was installing and I was using the new (back then) 10mm
super bright LED's I picked up from Radio Shack. I put the big LED in
series with a 5 volt DC 5mm red blinking LED and in series, the current
draw was within limits at 12 volts DC. You can see the light from the
big red LED flashing on the opposite wall of a garage. The annunciator
goes off when the genset trouble output goes active. You can push the
silence button to stop the noise but the big red flashing LED is
obnoxious enough that you don't forget to call for service. I actually
got a call a few years ago from a service guy who was working on one I
installed 15 years earlier. ^_^
A diode prevents the reverse entirely. I should have said limits to only
Going into reverse could be fatal IF current was not limited. There is
usually a current limiting resistor in series.
On 04/24/2013 10:07 PM, firstname.lastname@example.org wrote:
IIRC, I made that assumption once. That's when I knew very little about
diodes. A diode (and LEDs are diodes) is a voltage regulator. The
voltage across a diode will never be higher than its threshold voltage.
This is true both in the forward and reverse directions. It would try to
draw infinite current without a series resistor. As long as you use a
suitable resistor, it WON'T be toast.
For one LED I have:
forward voltage: 2V
reverse voltage: 4V
maximum current: 30mA
I have this on a 16V supply (doorbell), with a 500-ohm resistor.
When LED is forward-biased, current is 28mA (16V - 2V / 500 ohm). It lights.
When LED is reverse-biased, current is 24mA (16V - 4V / 500 ohm). No
light this way.
The anti-parallel diode actually does (minor) harm here by causing the
circuit to draw even more current when the LED is off (by replacing the
4V LED voltage with the .7V drop of an ordinary diode).
That sounds like a full-wave rectifier, which is 4 diodes and would
increase the LED duty cycle. The post I was replying to seemed to
indicate using a single diode.
Note that the duty cycle is not 100%, considering the time voltage is
below about 3.4V (forward LED drop plus 2 Si diode drops).
On 04/24/2013 06:06 PM, email@example.com wrote:
LEDs are diodes. Why do you need another? If you're thinking of the LED
reverse breakdown voltage, the LED won't be damaged unless there's too
much current through it (the resistor prevents that).
It should be safe to design for 20mA current through the LED. A 1K (or a
little lower) .5W resistor should work. Current also needs to be too low
to operate the bell.
BTW, there are no REAL white LEDs (there's no single "white" frequency).
What's there is often a combination of blue and yellow (the combination
Did you ever see a LED light that worked without a series resistor?
There's one is some very small flashlights. It just has a LED and a
coin-cell. The "switch" pushes the LED leads together over the battery.
The battery's "resistance" limits current.
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