[SOLVED] diode connected in parallel with an LED

can anyone help to answer this
Why is a diode connected in parallel with an LED?

p.mahen said:

can anyone help to answer this
Why is a diode connected in parallel with an LED?

Click to expand...

Its for rectification..


see the diagram........

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p.mahen said:

can anyone help to answer this
Why is a diode connected in parallel with an LED?

Click to expand...

you can get more Idea on this link : http://research.ncku.edu.tw/re/articles/e/20080530/5.html

Thanks Manoj

connected in reverse to protect from reverse voltage. see section 7.4
http://www.talkingelectronics.com/Download eBooks/Principles of electronics/CH-07.pdf

It can be conected in paralel in basic charger circuit to indicate a fully charged battery

The correct answer is in post #4, ignore the others.

Brian.

Thanks brian
is it to protect the LED from AC?

Re: diode connected in parallel with an LED
Thanks brian
is it to protect the LED from AC?

Click to expand...

yes,it is used to protect the diode from breaking down due to reverse polarity applied across the anode and the cathode of the LED.

expanding on that...

When one polarity is applied, current flows through the LED and it lights up. In this state it will drop voltage across the resistor to maintain a steady voltage across the LED. For indicator LEDs, this is typically around 1.5V and for high brightness types, about 3.5V.

When the reverse polarity is applied, it doesn't conduct or light up. Because it doesn't conduct, no voltage is dropped across the resistor and if it was possible, full voltage would appear across the LED. However, in reverse direction most LEDs have a very low breakdown voltage, it's typically only about 5V. If you exceed the breakdown voltage the LED will conduct but is also liable to being damaged. When a reversed normal diode is connected across the LED in such a way that it conducts when the LED voltage is reversed, it draws current and drops the voltage across itself to around 0.7V, well below the danger level that could damage the LED.

So on one half AC cycle the LED lights and on the other half cycle the diode conducts and protects the LED.

Brian.