Re: [CAD_CAM_EDM_DRO] On LED's and Gecko Tunings

AbbyKatt wrote:

>
> Two quick, and perhaps silly questions:
>
> Will a 6K resistor be enough with my 50-60V power supply, to light an
> LED and not fry it?
> I'd like to have a neat light telling me if my power-supply is on, and
> the old external HD case I'm using already has one built in... So..
>
> R=V/I
> R=(60/0.01)=6K?
>
> Goodmath? Wrongmath?

As others have suggested, using a source voltage that is much greater
than the LED forward voltage drop means that you can pretty much ignore
the LED forward voltage drop. So the math is good enough. Use any value
that is close since the actual current is not critical. I would probably
use a 4.7k resistor cuz I like the colors ;).

20ma is usually the max acceptable for a LED and is twice as bright as
10ma. This would require a 3k resitor for you. I would probably choose a
3.3k resistor so that the current is close to but less than 20ma.

Nobody discussed power rating for the resistor and a 1/4 watt resistor
is not going to handle the heat. It will need to dissipate nearly a watt
of power and will probably be too hot to touch.

Power Dissipated = V*V/R = I*I*R = V*I

where V (volts) is the voltage across the resistor. You can play it safe
and use the source voltage. I (amps) is the current through the resistor
and through the LED. R (ohms) is the resistor value.

For a 5.6kohm resistor I would use a 1 watt resistor since

approx resistor power = (60volts)*(60volts)/5600ohms = 0.643watts

For a 3.3kohm resistor I might use a 1 watt resistor if I happened to
have one and replacement is easy but I would use a 1.5 watt resistor if
I wanted high reliability.

approx resistor power = (60volts)*(60volts)/3300ohms = 1.0909watts

If you are tired of thinking about this, just use any resistor between
3900 and 5600 ohms that is rated for 1 watt or greater.

Tom Hubin
thubin@...