I was working on a battery charger which has a output dc voltage of about 9 V and 700 mA , i am connecting a white LED which has a operating voltage of 3.2 volts and 20mA, so how do you cut down the voltage of the source AND the current has also to be reduced to 20mA.. please tell me how to do it,, and the wattage of the resistor also ?

 

You connect a resistor in series with the LED to limit the current.
The value of the resistor is calculated by using Ohm's Law:
1) The voltage across the resistor is 9V - 3.2V= 5.8V.
2) The value of the resistor is 5.8V/20mA= 290 ohms. 300 ohms is the nearest standard value then the current will be 19.3mA.

The power dissipated by the resistor is the voltage across it multiplied by the current through it. 5.8V x 20mA= only 116mW so even a 1/4W resistor is fine.

The voltage across the LED will always be 3.2V while it is lighted.
LEDs have a wide range of forward voltage. Yours might be 3.0V, 3.2V or 3.5V.

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Uncle $crooge

 

Make sure to place the LED in the input side or output side of the regulator by separating from a diode with the incoming voltage from the battery, otherwise when AC current falls & the LED will light up from your charged battery.

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but audioguru you did not solve 700 mA current by the source ,you have worked on the voltage, Will the current from the source be reduced to 20mA ?

 

Audioguru is sleeping this time don't wake him.

The LED takes it own required current/Voltage not the whole 700mA if so it will burn out without the resister.

& the battery takes the calculated charging current which is required if not then it will draw all the 700mA & causing to overcharge the battery.

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audioguru has it right. It will limit the current to 20mA, it is a current limiting resistor.

He calculated a the value for a resistor that would limit the current to 20 mA given the supply voltage and the forward voltage of the LED.

Ohms law is
V = IR
but you want to solve for R
R = V/I (step 2 below)

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You can connect an LED in series with a suitable current-limiting resistor to a big car battery. The big car battery can supply up to 300A (!) but the LED with its current-limiting resistor will draw only 20mA or whatever current you calculated.

If you didn't use a current-limiting resistor then the LED would try to draw 300A and it would explode.

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Hi there Getbuggs,
I remember when you first got to the forum and were asking about how to learn to understand circuits properly, one of the commenters gave the good advice that Ohm's Law and Kirchoff's Law(s) would be a good start. Unfortunately you hadn't been very clear about what things you already understood...

But these sorts of questions here do suggest that either you didn't already know these laws or you don't understand them quite well enough yet. If you study them well, so they're not just words on a page, you will probably find that understanding things like the current limiting etc in this project, make a lot more sense. In fact, there are probably not so many circuits that you will understand at all if you don't properly understand Kirchoff's Laws, because they apply basically everywhere. Ohm's Law doesn't quite apply to everything but you still absolutely need to understand it well too.

There's "Kirchoff's Current Law" and "Kirchoff's Voltage Law", which are different but quite similar to each other. Like Ohm's Law, there are somewhat more consequences to these laws than just the obvious meaning, and so it's a good idea to learn as much as you can about how they apply. There should be lots of stuff about them (and many other electronic theories) on the internet- try Google and Wikipedia, and also the "Electronic Theory" part of this site!! But also remember the warning: any idiot can make a webpage! (I should know, having done so )- I've seen a few inaccurate claims on website tutorials, and so it's probably a good idea to use several different sources and see where they seem to agree and disagree. If you also have a book to work from, this will probably help even more. If you still have uncertainties, there should at least be fewer of them to ask about, and you'd be better able to understand answers to questions too!

If you manage to learn these laws well, you can then use them to learn (or better understand) concepts such as the rules for resistances in series and parallel, "potential dividers", "input and output impedance", "thevenin's theorem", transistors, and probably also "current sources" etc. These will in turn, let you understand even more complicated circuits. Experimenting with such things using breadboard and a multimeter is also helpful. But start with learning those fundamental things first! After that, other things start making a lot more sense...

 

What do you mean?

Are you trying to repair of modify an existing battery charger or are you desinging your own?

Where are you connecting the LED?

Don't connect it to the output of the charger, it will prevent it from working properly.

Please read this thread.
http://www.electro-tech-online.com/g...-high-amp.html

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No i am not REPAIRING the charger nor modify an existing battery charger.I am drawing current from the charger which says that its a 9 V and 700 mA charger................,
I am connecting LED to the charger ...................

Dont know why you said "Don't connect it to the output of the charger, it will prevent it from working properly."
I am using the charger as a source to light the LED

 

What happens to 700mA.can i still connect the LED

 

If you have a water tank that holds 700 gallons of water, and you want a drink - does that means you have to drink 700 gallons?.

No it obviously doesn't, so your charger 'holds' 700mA, and your LED will only 'drink' what it wants - governed by the series current limiting resistor mentioned above.

Hope this makes it clearer for you?.

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The LED will become drunk then it will start flashing??

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I don't want to know about that, I'll probably pass out if I see an LED's private parts.

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I'm not sure which is worse, the joke itself, or the fact that I laughed at it =|

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