doorbell - is LED avail

[generic]

I didn't know that. I thought it was needed on AC power. Now that you mention it, I worked on a set of xmas LED lights and never saw a diode either.

[gregz]

The Xmas lights might not ever go reverse current because I'm pretty sure it's higher than forward voltage to conduct. I don't like the look of blinkys. A bridge rectifier would double the frequency.

Greg

[gregz]

I would not just add a bridge without testing. The LEDs might get too hot.

Greg

[krw]

How does a diode "limit current"? Without an anti-parallel diode, an LED will be toast. They won't take more than five or ten volts reverse.

[The Daring Dufas]

I thought a regular 5mm red LED used 2 volts DC at around 30ma. A small switching diode rated at around 50 volts DC and a suitable resistor to limit the current with everything in series with the LED should work across the doorbell button since it's working as a simple continuity indicator across an open switch but in series with the doorbell and power transformer. ^_^

TDD

[gregz]

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

[gregz]

transformer. ^_^

Right, but 30 ma can be too bright with current LEDs. The very large LEDs which can take amps, are surprisingly bright at less than 50 ma. These have higher forward voltage levels,2-4 volts.

Greg

[Clifford Clavin]

Go to Amazon.com and search for led doorbell button

[The Daring Dufas]

transformer. ^_^

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

[Mark Lloyd]

[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.

[Mark Lloyd]

[snip]

Nice for what? The resistor limits current in both directions, to a safe level.

[Mark Lloyd]

For xmas LED lights, the series I've seen have 25, 30, or 35 LEDs. Longer strings have multiple series, in different polarities. These have diodes. That's what LEDs are.

However, some LED strings seem to have full-wave rectifiers (4 diodes) in them, in addition to the LEDs.

[Mark Lloyd]

[snip]

Maybe not.

AFAIK, the reverse threshold (voltage that must be reached for a diode to conduct) is usually higher (AFAIK) than the forward voltage.

If there is ever any reverse current, it'll be too small to be concerned about.

[Mark Lloyd]

A bridge would increase the efficiency a little. Probable too little to be concerned about here. If you do use one, consider that it adds an about 1.4V voltage drop.

I wouldn't use a bridge here, because of the increased complexity and reduced reliability.

Unlikely unless current was excessive (resistor too small).

BTW, I have (accidentally) connected 12V (1.5A supply) to a LED without a resistor. It popped and split in half immediately.

[The Daring Dufas]

Terrorist! LED's everywhere are in fear for their lives. ^_^

TDD

[Mark Lloyd]

[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).

[Mark Lloyd]

[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).

[gregz]

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

[Congoleum Breckenridge]

[hrhofmann]

They put a diode and/or a resistor in a little lump somewhere in the power cord or in the string of lites itself.