Re: Running LED off 9.3 Volts AC

Dave.H wrote:
I need to replace an incandescent dial lamp in a modern radio, and I
would love to replace it with a high brightness LED, to save having to
replace it in the near future. Problem is, the source voltage is 9.3
volts AC, not suitable for running an LED off directly. What value
resistor would I need for this?

The specifications of the LED I had in mind are as follows:

# Emitted Colour: Sunset Red
# Forward Voltage = 2.0V Typical @ IF = 20mA
# Reverse Voltage = 5V
# Maximum Voltage = 2.6V @ IF = 20mA
# Luminous Intensity = 6,000mcd Minimum, 8,000mcd Typical
# Peak Emission Wavelength = 640nm
# Power Dissipation = 80mW
# Continuous Forward Current = 50mA

If you can use these LEDs in pairs, paralleled, one goingh each way, one will run on one half cycle of the AC, and one on the other. A single resistor in series with the pair will limit the current from the AC source. I would run them at a peak current around 20 mA, for long life.

9.3 volts AC hits peaks of about +-13.2 volts. Since each way, there will be one LED using up voltage of about 2.6 volts (in each direction, the resistor must use up the rest, while letting through 20 mA. (13.2-2.6)/0.02=530. a 470 ohm 1/4 watt would be close enough. If you could use 4 LEDs, put another pair in series with the first, and maybe lower the resistance a bit, maybe to 390 ohm.

Since each of the LEDs is hitting a peak of only about 20 mA and conducting only half the time, if this isn't bright enough, you could go as low as half these values to double the light output (that is a just noticeable increase), but use 1/2 watt resistors.
.

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