General Electric StayBright LEDs

Date: December 2009
Store: K-Mart
Price: $10 (10 cents per LED)
LED Color: cool white (bluish white)
LED Size: 5mm
LED Lens: inverted wedge
LED Count: 100 leds. 3 series strings of 33-34 lights each
Current limiting resistor: 1 for each string of 33 leds, 3 total. 900 ohms each. Pack includes a reverse current protection diode.
Rectification: half wave using the LEDs as the rectification diode.
Bulb lead material: steel, subject to rust
Power: 6.5 watts

Notes:

As you can see in the oscilloscope reading voltage across one of the LEDs, this is a half-wave set. It only uses one half of the AC sine wave. Two troubling things here. First, we're getting some reverse voltage. That means the LEDs are breaking down and allowing current to flow in the wrong direction. Almost no current is flowing then but over time it could still damage the LEDs and burn them out.

Note how the time that voltage flows over the resistor (right picture) peaking at 44 volts is much shorter than the AC half-wave visible across the LED. You'll only see a voltage drop across the resistor when a non-trivial amount of current is flowing through the circuit. In essence, as the sine wave rises it applies to the LEDs first. Once they have enough juice to light up, the resistor clips the top part of the sine wave so that the LEDs don't get too much current.