Re: Running LED off 9.3 Volts AC

Dave.H 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

If the sum of all the LED forward voltage drops is significantly less than 9 volts, just divide the remaining voltage (that must be used up by the resistor) by the current you want the series string of LEDs to pass.

For instance, if the LED forward voltage is 2 volts, 3 of them in series will need 6 volts. So the resistor must burn the remaining 3 volts (9-6=3). If you want those LEDs to see a current of 10 mA (0.01A), you would need 3/0.01=300 ohms in series. No other parts are needed.

By the way, it is generally a good idea to derate the LED current by about half, if you want a long life with a steady light output. Same goes for wattage of resistors, versus their rated wattage.
.

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