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

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#467

From: Mr. Land
Date: Thu, 16 Mar 2006 16:06

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You should calculate everything, the shunt resistor too!

I assume the current to the batteries is constant enough to do this.

Ideally, you would have a wire (0 ohms) feeding the current to the
batteries.  But you want to tap off just enough current to light the
LED, while sending the rest to the batteries as before.  There's
probably a range of values that will work for you.

Measure the (max) current to the batteries first, to see what you're
working with.

Take the voltage drop of the LED, and add just a bit of headroom (for
instance, if the LED drops 1.0V during conduction, use 1.5V.)  Then use
Ohm's law to figure out what series resistor you'll need to get the mA
current you need (the LED's rated operating current) through the LED
with 1.5V across the whole thing.

Finally, take the current you measured going to the batteries, and use
Ohm's law again to determine what size shunt resistor you'll need to
get your 1.5V voltage drop.  Use Ohm's law again to make sure the shunt
resistor is large enough in terms of wattage.

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From: "Mr. Land" <graftonfot@yahoo.com>
Newsgroups: sci.electronics.repair,aus.electronics
Subject: Re: I am about to add an LED to a charger but need further opinions.
Date: 16 Mar 2006 08:06:11 -0800
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