Thread: Light for DX7
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Old 03-07-2010, 06:45 AM   #49 (permalink)
N017RW
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I didn't follow every link in this thread... sorry if this was posted before, but for the future Freaks out there, there are several LED resistor calculators on the web. Such as this one: http://led.linear1.org/1led.wiz

You need to know 3 things:
The supply voltage- DX7 would be safe to use 11.0v.
The LEDs forward voltage and forward current- This is usually on the back of the LED package or supplied data sheet. (2.1v and 20 ma. respectively are typical.)

Plug these into the calculator and you get 470 ohms!

For those who wish to learn more or may want to use more than one LED...

Subtract the total of the LED(s) forward voltage(s) from the supply voltage,
Then divide that number by the forward current. This equals the resistance needed.
(Ohm's Law)

Using the example above with TWO LEDs of the same type:
11.0V - (2.1V + 2.1V) =6.8V
6.8V / 20ma (or 0.02A)= 340 ohms.
However resistors or only available in standard or preset values so choose the next higher preset which is 390 ohms.
Depending on the resistance value obtained you can use a combination of lower standard resistor values to get closer to the 340 if you desire.

As others have said the LED can be driven harder (more current) at the expense of lifetime. This may increase brightness. But that is usually a function of viewing angle.

A list of these standard resistor values can also be found on the web such as this one for example:
http://www.swarthmore.edu/NatSci/ech...ardValues.html
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