# Help need to discharge capacitor in given circuit

Discussion in 'General Electronics Discussion' started by mguptamel, Jun 8, 2012.

1. ### mguptamel

6
0
Jun 8, 2012
Hi all,

I am a newbie in electronics and trying to get grasp of capacitors. I am unable to discharge capacitor in attached circuit. So far the circuit is doing the following

1) RC network to turn-on LED after a given time, for example, T seconds.

2) Once LED is turned-on I want to discharge capacitor so that cycle begins again. But I am unable to do this at this moment. PLEASE HELP.

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2. ### Harald KappModeratorModerator

11,826
2,757
Nov 17, 2011
A single transistor can't do the job. You need a second transistor which will start a discharge cycle after the charge cycle of the capacitor is complete.

There are different circuits for that. Look up >astable multivibrator< in Google.

Harald

3. ### john monks

693
3
Mar 9, 2012
Have you considered using a diac?

4. ### mguptamel

6
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Jun 8, 2012
Hi Harald,

Thanks for the info. In above circuit I wanted to do this by introducing 2nd transistor as you said. Just saw some diagrams for astable multi-vibrator. Looks confusing to me. However, is that the only way to discharge capacitor in above figure?

11,826
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Nov 17, 2011
6. ### mguptamel

6
0
Jun 8, 2012
Nopes!!! I just a beginner and I want to understand basic components

7. ### Harald KappModeratorModerator

11,826
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Nov 17, 2011
In John's defense: a diac is a basic component.
A diac is normally non-conducting. As soon as the diac's threshold voltage is reached, it starts conducting with a breakdown characteristic. this means that the voltage across the diac, once it starts conducting, is greatly reduced compared to the threshold voltage. It thus can be used, as John suggests, to discharge a capacitor.
As far as I know diacs are mostly used in conjunction with voltages higher tnhan approx. 10 V, therefore it may not be suitable in your application.
A tutorial is here: http://www.americanmicrosemi.com/information/tutorial/index.php?t_id=2