color changing led problem

P

Patrick

Guest
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
 
"Patrick" <jpatricks1@yahoo.com> wrote in message
news:fc07bb89.0410171200.77714cf1@posting.google.com...
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
Click to expand...
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.
 
jpatricks1@yahoo.com (Patrick) wrote in message news:<fc07bb89.0410171200.77714cf1@posting.google.com>...
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...
dont do that
Click to expand...
 
All,
Instead of guessing, trial and error, and smoking those leds which
stink badly there is a formula for this:
R = (Vcc-Vled)/Iled where where
R = resistor to put in series. Vcc = supply voltage Vled = Led
voltage across the Led Iled = led current

Michael
http://groups.yahoo.com/group/microcontrollerworld/
http://www.geocities.com/microcontrollerworld/

jpatricks1@yahoo.com (Patrick) wrote in message news:<fc07bb89.0410171200.77714cf1@posting.google.com>...
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
Click to expand...
 
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