Powering LED from 24 volt AC

[ks_physicist]

About a year ago, I replaced the little light bulb in a doorbell button with a white LED. I didn't have the specs on the LED, so I did a little testing and came up with some safe specs for voltage drop and current. I then calculated the necessary resistance based on 24v forward voltage, and let the LED operate at half duty cycle. Actual supply was about 20 volts AC.

I didn't think to protect it against reverse voltage, and eventually it failed (presumably for that reason). I don't have a lot of room to work with, so using four other diodes to achieve a 25 volt reverse voltage tolerance isn't a great option.

Ideas?

 

[Hans Dorn]

Wire 2 white LEDs anti-parallel or put a small diode in series with the LED.

 

[ks_physicist]

The two anti-parallel would work, but the space available is a problem.

Wouldn't the addition of a single diode still suffer the same problem? If the max reverse voltage is 5v for the LED, and 5v (assuming) for the second diode, then I'm at 10 volts total maximum drop--still at least ten to fifteen volts below the actual applied reverse voltage.

 

[snarfer]

If the max reverse voltage is 5v for the LED, and 5v (assuming) for the second diode,

Why would you use a diode with only 5v max reverse voltage? Diodes with max reverse voltage anywhere up to 1000 volts are easily obtained.

 

[ks_physicist]

Yeah, I could use a small power diode. I was still thinking of it as a second LED rather than a discrete non-LED diode.

 

[Oznog]

There are tiny fullwave bridges, esp the SMD ones. Those could probably fit just about anywhere. Can't get 'em from Radio Shack though, you'd need a real supplier.

 

[Morelite]

The two anti-parallel would work, but the space available is a problem.

If space is a problem, you may be able to put the second diode in the chime box on the wall. You should have four wires going into the chime box (six if you have a second button at another door) two wires will be the 24volts from the transformer (usually mounted by the panel box) the other two wires travel to the button.

 

[romulus]

If your project has spacial constraints, make the leap to surface mount components. Digi-key or Mouser usually ship small qtys. of these.

1. Attach an appropriately rated 1/2 Watt surface mount resitor between the anode and +24V.

2. Attach the anode of a surface-mount 1N4001 to the cathode of your LED and attach the cathode of the 1N4001 to ground or common.

If you've got the correct current limiting resistor, it will work.