Generic LED Lights from Target

Date: December 2009
Store: Target
Price: $7 (12 cents per LED)
LED Color: warm/soft white
LED Size: 5mm
LED Lens: dome
LED Count: 60 leds, 2 series strings of 30 leds each
Current limiting resistor: 2 for each string of 30 leds, 4 total, placed after the first led on either side of the string. 750 ohms each, 1500 ohms per 30-led series string.
Rectification: half wave using the LEDs as the rectification diode.
Bulb lead material: steel, subject to rust
Power: 4.8 watts

Notes:

These are my favorite LEDs for hacking and experimenting. They're simple, they have extra resistors, they give off a nice warm white light and they're cheap at 12 cents per LED.

Putting the current limiting resistor after the first led instead of before seems to have been a bad idea. Which led do you figure burned out first?

The polarity of each string is 30 is reversed, so the first 30 light on one half of the AC sine wave and the second 30 light on the second half.

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. I don't like the reverse voltage I'm seeing at the end of the half wave. 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. Other than that they look fine. Note that the typical reading was 3.3 volts rather than 3.0.

Note how the time that voltage flows over the resistor (right picture) peaking at 30 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.

There are two resistors in each series string, so the other resistor drops a similar amount of voltage. Unlike most of the other sets I tested, the resistors are only mildly warm to the touch.