ELECTRONICS question
- Thread starter kingbee
- Start date
Mike V. Florida
Well-known member
I'm not mikek but 750 ohm 1/4 watt should work. The output of the Alt is usually over 14 volts and the 6 v is actually closer to 7 so you have to drop 7+ volts. Allowing for some fluctuation and not driving the led at max brightness I think the above will work just fine.
MikeK
Well-known member
The significant factors are the forward voltage drop of the LED and the maximum design current. The forward voltage drop is different for various colors. If the LED's are red, it is about 2.0V. If green, about 2.2V. If the LED is a 3mm or 5mm descrete epoxy device (domed plastic top cylinder) the forward current is likely about 20 milliamps. (If you have surface mount or a different color, PM me.)
You SUBTRACT the forward voltage rating from the supply voltage and then use Ohm's law to find the resistance that will pass the design current.
What you presently have-
Using 7.2V (gen or alt running) the exact ballast resistance for a red LED should be 260 Ohms, for green 250 Ohms. There are no resistors of that exact value, so the unit you have probably has 270 Ohm resistors. The resistor wattage rating is determined by voltage x current x duty cycle. With 50% on & off duty cycle, 1/8 watt would be sufficient, but 1/4W resistors are cheaper.
NOW, if you want to use it on 12V (14.5V running)-
You need to decide whether you wish to open it and replace the existing resistors or add one into each input. The total resistance required is 615 for green, 625 for red. Again, no such exact standard 20% resistor. Either replace them with 680 Ohm 1/4 watt, or add about 360 Ohms. To add 360 you will need two standard 180 20% in series in each input, or use a single 330 or 390 in each input if you can find them.
Clear as mud? Resistors have such odd-ball values because their manufacturing progression is based on a geometric progression: N = 10((n-1)/k) where: N is the nominal resistance value at n position and k is a coefficient related to tolerance. More expensive 1%, 5%, 10% resistors have different available values that you probably won't find in the parts drawers at Radio Shack.
Just my distant guess, proceed at your own peril.
You SUBTRACT the forward voltage rating from the supply voltage and then use Ohm's law to find the resistance that will pass the design current.
What you presently have-
Using 7.2V (gen or alt running) the exact ballast resistance for a red LED should be 260 Ohms, for green 250 Ohms. There are no resistors of that exact value, so the unit you have probably has 270 Ohm resistors. The resistor wattage rating is determined by voltage x current x duty cycle. With 50% on & off duty cycle, 1/8 watt would be sufficient, but 1/4W resistors are cheaper.
NOW, if you want to use it on 12V (14.5V running)-
You need to decide whether you wish to open it and replace the existing resistors or add one into each input. The total resistance required is 615 for green, 625 for red. Again, no such exact standard 20% resistor. Either replace them with 680 Ohm 1/4 watt, or add about 360 Ohms. To add 360 you will need two standard 180 20% in series in each input, or use a single 330 or 390 in each input if you can find them.
Clear as mud? Resistors have such odd-ball values because their manufacturing progression is based on a geometric progression: N = 10((n-1)/k) where: N is the nominal resistance value at n position and k is a coefficient related to tolerance. More expensive 1%, 5%, 10% resistors have different available values that you probably won't find in the parts drawers at Radio Shack.
Just my distant guess, proceed at your own peril.