Quote:
Originally Posted by JesseK
I didn't understand the results of my testing -- since the door light's source was 9VDC I needed to be able to use 9V. I accomplished this with a small-value resistor (100-300ohms, trial and error to find the right one) and no less than 6 LEDs in series. Any less and I'd get a nice POP POP POP out of the first LED within seconds. At most I could use 8 LEDs before all of the LEDs became too dim to see, and at no point could I used LEDs in parallel with each other. I had my 6 LED strip setup in my car for 2 years before I sold it, and none ever burned out.
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Sorry for posting twice in a row, but I should have addressed this point as well.
LEDs are non-ohmic devices. In other words, they don't follow Ohm's law. In fact you will find that most LEDs appear to have an extremely high resistance when they see less than about 1.5 volts. This means that if you have 8 LEDs in series, you probably won't have any significant current flow with anything less than 12 volts.
When that Voltage is reached, current flow tends to increase exponentially... thus a small fluctuation in voltage can cause a huge change in current. This increase in current can quickly burn out an LED. That is why it is important to have a resistor in series with the LED... it provides an ohmic (linear) increase in current with voltage and keeps LEDs from becoming DEDs. (See above)
Wikipedia has some
interesting information on LEDs that explains this in much more detail, and also mentions that many LEDs have a maximum reverse voltage of about 5v... meaning that the application suggested by the original poster where the diodes are hooked up to a 6.5VAC supply may be over stressing the LEDs in the reverse direction. This is not something I have observed, but then again, I tend to use my LEDs in DC circuits and thus would not expect to have observed it.
Jason