Re: ATM A little electronic help on my Foucault tester

[Michael Lindner <mikell@optonline.net>]

On Mon, 03 Dec 2001 17:01:41 -0600, James Taylor said:

> 
>  All I need to know is how do I wire it so that I don't burn it out.  I want to run
>  it off batteries and put a pot on it so I can dim it if needed. 
>  
>  The Orange one:  Normal ops is 2V and 40mA.  300mcd
>  
>  Green: normal ops 2.2V, 40mA. 350mcd
>  
>  Super Bright White: 1.82V +/- .03V and 20mA.  This baby puts out 5000mcd at
>  660nm wavelength.

Take the battery voltage, subtract the LED voltage, and divide by the nominal
current to find the minimum resistor you need in series. For instance, the
orange one with a 9v battery yields (9-2)/0.040 = 175 ohms, the green one with
2 AA batteries yields (3-2.2)/0.040 = 20 ohms, white with 9v battery yields
(9-1.82)/0.020 = 359 ohms. Use the next higher standard value resistor. put it
in series with the LED and the pot. any pot you have (like 1K or higher) will
be fine for dimming. One last thing to consider is the power through the
resistor. That's battery voltage - LED voltage x current, so for the orange one
it would be (9-2)x0.040 or 0.28 watts, so you'll want to get a reisstor that's
rated more than that (like a 1/2 watt). Only other thing to remember is to make
sure of the polarity of the leads when you install the LED or you'll have to
disassemble it again and turn it around.

Hope this helps.

Clear skies.
-- 
Michael Lindner
http://home.att.net/~mikel
http://www.starastronomy.org
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