Re: Running LED off 9.3 Volts AC

In article
<55d3b349-17d1-4a1f-8849-ef8f7aff3a88@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Dave.H" <the1930s@xxxxxxxxxxxxxx> wrote:

I have an upcoming project involving running three LEDs off a single 9
volt battery, what resistors would I need, and would I also need a
capacitor, as in this circuit?


The specifications for the LED's I am planning to use, (the same one's
I used for the radio dial) are

5mm (T1 1/2) round with flange
16,000mcd (typical) output
Water Clear Lens
High Brightness
620 - 625nm Wavelength (Orange-Red)
15o Viewing Angle
Forward voltage, Vf = 2.0V (min), 2.3V (max)
Forward current, If = 20mA

Two simple options:

9V battery, 3x 2V (min), 3V to be dropped across a resistor X at 20mA -
V=IR, R=V/I R=3/0.020, R=150 ohms.

9V battery, 3x 2.3V (max) 9-6.9=2.1V headroom - you could use an LM317
set up as a current source. That would allow driving reliably with 20mA
(actual voltage not that big of an issue for LEDs, and not a very
reliable number). The LM317 acts as whatever size resistor is needed to
run 20mA (when it's set up to act as a current source). There are also
newer devices that do that as a unit, with no external parts.

--
Cats, coffee, chocolate...vices to live by
.

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