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why this led blown out? Answered

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i have connected  aled of 3ma with 9v dc battery and with resistance of 10k and lateron i connected a polarity capacitor4.7UF in parllel to resistance then led glowed brightly and blown out why? and also how the capacitor can be used as relay here

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oldmanbeefjerky (author)2012-11-03

full current is passing through the LED at 9v which blows up most 1-3v leds, get something like a 200-1000ohm resistor and put that between battery positive and the LED to limit current. Alternatively get an LED that can handle 9v and requires under 100ma to work properly

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user

He has a resistor (R) between the LED and ground BUT has put a capacitor across the resistor.

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user

i know, however he should limit the current that can go through the LED so it doesnt blow

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user

Assuming the fwd voltage of the LED to be around 2 volts 0 then to get 30mA from the 9 volt battery the resistor should be 270 ohms at 2 watts.

HOWEVER this isn't why the LED failed when the capacitor was attached - See my other post.

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rickharris (author)2012-11-03

When you attach the voltage to the capacitor initially the capacitor has to charge up.

It will do this very fast at first hence draw a LOT of current this goes through the LED and blows it.

As the capacitor is charges the current falls off until at fully charged there is almost no current flow.

(Electronics people please forgive my simple explanation!)

Depending on what you wanted to do - You can restrict the current flow into the capacitor by putting a resistor in series with it.

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user
avinash reddy (author)rickharris2012-11-03

ok sir thank for this and how can i use capacitor for blinking led

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rickharris (author)avinash reddy2012-11-03

Try putting the capacitor in series with the existing resistor. A lot depends on the value of the resistor and the capacitor.

The Time constant( charging time) of the Capacitor resistor is C X R Seconds

You should be able to work out how long the capacitor will tale to charge.

In real world it's more complicated than this and if you need to blink the LED I would buy a 555 timer ic and learn how it works.

In a way I like you thought process though.

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