I have a number of blue LEDs, that work when connected to a resistor
and a 9V battery (don't know much else about them!)
How would I go about making a wall socket to I could plug in a group
of LEDs?
If I got an old 6V or 9V AC/DC adapter from say an old walkman or
something, Could I safely connect that somehow to the LEDs?
Should I wire the LEDs in parallel?
Where can I find info to do this?
http://www.misty.com/~don/ledd.html
among other places.
I recommend against paraleling LEDs.
With a 6-volt DC "wall wart", I would assume 7 volts with a light load.
Blue LEDs usually have a voltage drop near 3.5 volts and usually want 20
mA typical, 30 mA max, and I would recommend somewhat less (15 mA) if you
want to get the usually-advertised "100,000 hours" and you do not know
that the thermal situation is better than that in the LED manufacturer's
test lab. But you can probably get away with a little over 30 mA.
Back to the 7-volt "6-volt DC wall wart": Put each LED in series
with a dropping resistor, and then put all LED-resistor "strings"
in parallel with each other. Subtract the 3.5V LED voltage from the 7V
supply voltage, and that leaves 3.5 volts across the dropping resistor.
3.5 volts (resistor voltage, that is) divided by .02 amp 20 milliamps)
yields 175 ohms, and the nearest common value is 180 ohms. I recommend
220 ohms to play safe if you want a good expectation of really long life.
You will probably get away with 150 ohms and maybe with 100 ohms.
Do not try matching supply voltage to LED voltage. The current through
the LED will be of unreliable magnitude. The current through an LED as
voltage across it varies greatly with small changes in voltage, and the
voltage required to push a given amount of current through an LED varies
enough with temperature and manufacturing tolerances to make current as a
function of voltage unreliable.
If you have a 12-volt DC "wall wart", assume 13.5 volts for a light
load. You can put two LEDs and a resistor in series, and a few of these
"strings" in parallel.
With two 3.5 volt LEDs subtracted from 13.5 volts, you get 6.5 volts
across the dropping resistor. Divide by .02 amp (20 milliamps), and this
gives 325 ohms for a dropping resistor. The nearest common value is 330
ohms. I would use 470 ohms if I needed to count on a few 10,000's of
hours of LED life expectancy, although I forsee probably no quick failure
with as low as 220 ohms.
As for resistor wattage: Multiply 6.5 volts resistor voltage by .02
amp, and that means .13 watt. That sounds like a 1/4 watt resistor is
enough, but I recommend a half watt one if you want good reliability.
Resistor reliability decreases enough at over half rated power for
military 1/4 watt resistors to resemble commercial grade 1/2 watt ones,
and that 12 volt DC wall wart might just provide 16 volts to a really
light load.
Although you can probably put 3 blue LEDs and a resistor in series with
each other and run this from a 12 volt DC "wall wart", the LED current can
easily be greatly different from what you expect and may vary excessively
with temperature and presence of other loads on the "wall wart".
- Don Klipstein (
[email protected])