doorbell - is LED avail

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On 4/24/2013 11:38 PM, gregz wrote:

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. ^_^
TDD
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A diode prevents the reverse entirely. I should have said limits to only forward direction.
Going into reverse could be fatal IF current was not limited. There is usually a current limiting resistor in series.
Greg
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On 04/24/2013 11:34 PM, gregz wrote:
[snip]

Of course thats just as true for forward. A diode is like a resistor that's infinite at voltages below the threshold, and 0 at the threshold (there's no above, since the diode is a voltage regulator).
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Mark Lloyd
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On 04/24/2013 10:07 PM, snipped-for-privacy@attt.bizz wrote:
[snip]

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).
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On 04/24/2013 07:35 PM, gregz wrote:
[snip]

Nice for what? The resistor limits current in both directions, to a safe level.
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So you can get maximum brightness without overheating. Eliminating reverse current. When the led pulses, there is a new maximum current capability, instead of average capability.
Greg
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On 04/25/2013 06:57 PM, gregz wrote:

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).
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On 04/24/2013 06:06 PM, snipped-for-privacy@internet.com wrote:
[snip]

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 looks white).

off-topic:
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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On 4/23/2013 5:35 PM, ps56k wrote:

Go to Amazon.com and search for led doorbell button
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On 4/23/2013 5:35 PM, ps56k wrote:

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