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 need just one LED and a fairly pure shade of red (like 632.8 nm
He-Ne laser or half a fine red hair more-pure-red) is OK and light output
maybe a bit on the low side is OK, do this:
1. Get a Radio Shack 276-307 red LED. Use some fine sandpaper on it to
make it more-diffusing. If you need really wide spread light dispersion
pattern, then sand it with sandpaper until it is shortened by about 1-1.25
millimeters (.04-.05 inch), and make it shaped to be slightly less pointy
in shape than it was.
2. Get or make just about any full wave bridge rectifier. I like to get
plenty of 1N4000 series diodes, either of type 1N4004 (400 volt) or 1N4007
(1000 volt) - cost increase with increasing voltage rating is small enough
to get only higher voltage ones for all needs as far as I see things!
3. Feed the LED with the bridge rectifier. The LED drops about 2 volts,
and the bridge rectifier at 20 mA or whatever drops about 1.3 volts. For
a rough-and-dirty first order approximation, subtract 3.3 volts from 9.3
to get 6 volts across the dropping resistor. To get 20 mA, nearest
very-common value is 220 ohms. I see RMS current being about 18 mA and
average current being about 16 mA as guesstimates for now. This makes me
suspect 270 ohms is good for 20 mA average LED current, and 220 ohms
should make the average LED current about 24-25 mA, which the LED should
take quite well.
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For a brighter and slightly orangish red LED: Get them from Digi-Key!
I really like Avago HLMP-ED33-SVOOO, DigiKey catalog number 516-1390-ND.
Do same as above, get about 3 times as much light as with the Rat
Shack LED and the color is a slightly orangish shade of red. In some
situations it will appear plainly red, and this color is more of a
slightly orangish shade of red rather than a reddish shade of orange.
Cost is 80 cents apiece for 1-9 pieces, $4.76 per 10 pieces for 10-90
pieces, plus shipping (just a few dollars for UPS ground), plus a $5
handling charge if your order before tax and shipping is under $25 (IIRC).
Both of these LEDs have MCD under 6,000, but at 20 mA average current
the Rat Shack one produces about half a lumen of red light, and the Avago
one above produces about 1.6 lumens of red light.
5V 60 mA incandescent indicator lamps rated to last 25,000 hours produce
about .63 lumen, and 5V .115 amp incandescent indicator lamps rated to
last 40,000 hours produce about 1.9 lumens, without any color filtering.
Keep in mind that with an incandescent, filtering it to red or even
orange-red will block roughly half to 2/3 of the luminous output. Even if
this is a low wattage low current long life incandescent and the color
temperature is close to 2000 K, filtering it to red will block at least
half the luminous output and filtering it to a borderline red-orange
close to that of red-orange-glowing clear neon sign tubing will remove
about 40% of the luminous output.
The Rat Shack one has peak wavelength around 650-660 nm and dominant
wavelength (color specification that approximately means "hue") of about
637-640 nm. This is half a fine red hair more pure a red than a 632.8
nm He-Ne laser. The Avago one has peak wavelength in the mid-upper 630's
of nm and dominant wavelength in the mid 620's of nm. This is more orange
than a He-Ne laser, but still basically red, slightly less orangish than
most automotive taillights, less orangish than clear red-orange neon
tuning, and slightly less orangish than a display of "pure red stimulus"
on most computer monitors and TV sets.
- Don Klipstein (
[email protected])
