10mm LED's from Radio Shack?

10mm LED\'s from Radio Shack?

I bought 4 10mm leds from Rat Shack and hooked them in series to the 12v line of my computer power supply. I believe the LED's said that they work at 4 volts. However, they only lasted a few minutes before one burned out.

I thought that hooking LED's in series meant you could divide the source voltage by the number of LED's to get the right vf for the leds. If the leds were 4 volts each and the source was 12 volts, shouldn't 4 in series have worked?

Any advice on this would be appreciated. In contrast, I hooked six 5mm LED's in parallel to the 3.5 volt line of my power supply for some accent lighting on my case and they work great. These LED's were bought from superbrightleds.com so maybe they were better quality?

Re: 10mm LED\'s from Radio Shack?

What color were they? Red, yellow, orange, and yellow green take only about 2.2 volts.

Re: 10mm LED\'s from Radio Shack?

No matter how you connect your LEDs, it is always good practice to provide some kind of current limiting or reguation for them. The outputs of a computer power supply are fixed, low-impedance power sources. Unlike DDing an LED from a battery supply which has internal resistance, DDing an LED from a DC power supply can pop them instantaneously if you're not careful.

I would suggest using a ballast resistor to limit the current. The Vf specifications on the LEDs are general values that will vary depending mostly on operating temperature. Perhaps the most widely misunderstood facet of driving LEDs is that they are current-operated devices and not voltage-operated...so some kind of constant-current or current limited source must be provided. For instance, a series resistance of 330 ohms between a white or blue LED and the 12V rail will allow ~25mA through the LED.