LEDs - Expanding Batteries - CR2032 to 2 C or D size

MarlyManning

Many, MANY THANKS for your EXCELLENT article on choosing resistors for LEDs - http://www.evilmadscientist.com/2012/resistors-for-leds/ We learned a great deal from it and from the comments too.
 
How would we use that formula to calculate the resistors needed to: 1) Replace one CR2032 with 2 C or D batteries to extend battery life for existing LED lights used with model trains; and 2) Replace three LR44 button batteries with three C or D batteries for an existing LED fixture we want in a dollhouse?  We are trying to get the equivalent current using larger, longer life batteries and don't want to tear the lights apart to see if we can find the specs on the LEDs they use.

Is this a straight forward application of Ohm's Law using the mAh we find for these batteries?  Is there anything else we should consider?  I don't want to blow up the little LEDs they are currently using, just extend the battery life.

Is there a procedure we could use with a multimeter to determine the correct resistor? Or can you suggest a topic we could Google?

Many thanks! 


 

Windell Oskay

It's straightforward, but you should use the method described in the article: Start with the current and voltage, and work forward to the resistor that you need.  

The mAh rating of a battery should only come in as a secondary consideration.  That rating simply means how much energy is stored.   If you are using two C cells in series, each rated for 8000 mAh, then you have 3 V total, with a capacity of 8000 mAh.  Thus, if you were drawing 1 mA through the batteries to your LED, then it would run for 8000 hours.  

MarlyManning

WOW!  8,000 hours would be great!  Many thanks!  Sorry I am really new at this.  So long as I don't change the voltage, do I need additional resistors or can I just simply use:

(2)  C or D batteries in series => ~
3.0 volts    to replace one   CR2032 => ~ 3.0 volts, 3 mA
(3)  C or D batteries in series => ~
4.5 volts    to replace one   LR44 => ~ 4.5 volts

1)  Without additional resistors?  Or, 2)  Do I need to measure the current on the existing circuit (A1) with a multimeter and then protect the LED by using the formula V = I x R, for the first example 3.0 = A1 * R1

Many thanks!!!

Windell Oskay

You do need to use the V= I * R formula, in almost all cases.   The exception-- the only case where you do not --is when the forward voltage of the LED is greater than the voltage source you are providing (for example, a blue or white LED driven from 3 V).

Also, an LR44 is a 1.5 V alkaline cell.  Three C cells can replace three LR44s.