# color changing led problem

Discussion in 'Electronic Basics' started by Patrick, Oct 17, 2004.

1. ### PatrickGuest

I bought some color changing led's. It works well with 9v batteries
but when I got home and tried it with 12v (planning to use it on my
car) and a 750ohm resistor it just lit red and didnt change color.
Tried again with lower voltage starting at 1.5 gradualy moving up
without resistor... it changed color at 6v - and then burned! What am
I doing wrong? It worked well with the 9v battery so why did it burn
at 6v? Any ideas on how to get it to work?

Thank you very much and good day

2. ### BlakeGuest

If there is any possibility of getting a data sheet on the LEDs, or at least
the recommended operating voltage and current, that would help you design
the bias curcuit properly. Without a data sheet on these LEDs, all I can
offer is a bit of guesswork.

My guess is that the LEDs don't have an internal resistor, so an external
resistor is necessary to limit current. Without a current limiting resistor,
diode current rises very fast once the turn-on-voltage has been passed, and
a burned-out LED is the usual result.

Your 750 ohm resistor with a 12v source may have been a bit large. It would
limit current to under 15ma. Maybe you need a more current to get into color
changing mode. Try reducing that resistor to something smaller; maybe 600
ohms or 450 ohms.

As for why it worked with a nine volt battery even without a resistor, the
battery may have been weak enough that it self-limited the current to a safe
value. I'll bet that the LED would have burned out quickly enough when
powered by a freshly charged 9v battery.

Just guess work, but I hope it helps.

dont do that

4. ### JohnGuest

LEDs are current-operated devices and need a limiting resistor to
prevent excessive current. Connecting the LED directly to a power
source is generally a recipe for failure...

The reason the LED worked at "9 volts" is that the battery is limited
in the amount of current it can deliver and the battery voltage
dropped when the LED was connected.

You need to limit the LED current to something around 20 ma
(milli-amps) for testing.