Re: I am about to add an LED to a charger but need further opinions.

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#469

From: James Sweet
Date: Wed, 15 Mar 2006 03:00

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alitonto wrote:
> I am trying to add an LED to a NiMh charger for 5 AA battery pack
> (total voltage about 6.93) to be able to see that is actually turned
> on.
> 
> I have seen some chargers that have the dropping resistor and LED in
> series  with each other and then this combination is in parallel with
> another resistor of lower ohms which goes to the battery pack: examples
> 
> (82 ohm and LED) in parallel with (a resistor of 24 ohms) - used for
> charging 4 AA batteries
> 
> 
> (47 ohm and LED) in parallel with (a resistor of 18 ohms) - used for
> charging 2 AA batteries
> 
> 
> I can calculate the resistor for the LED but any thoughts on what the
> other resistor should be? or what things to take into account in
> choosing this resistor ?
> 
> Thanks
> Ali
> 

Is there a resistor in the charger already which is in series with the 
battery being charged? If so then measure the voltage across it, 
subtract the LED's forward voltage from that and then use ohms law to 
determine the resistor you need for the correct current across the LED, 
5-10 mA ought to do the trick. In a pinch you can just experiment and 
see what works.

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