Author Topic: My LED in my RC circuit is glowing after 5t  (Read 1019 times)

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Offline electrowhizTopic starter

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My LED in my RC circuit is glowing after 5t
« on: July 10, 2018, 07:22:15 am »
I learnt about RC circuits. I decided to learn it practically. I created a circuit in which Resistor, LED, Capacitor are in series.LED is in between resistor and cap.I powered it using 9v battery. The value of resistor is 10k and cap is 220uf. The time constant is 2.2 sec. It will take 5t to get fully charged after which no current should flow i.e LED should be off. But led is slightly glowing. Why?

 

Offline tooki

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Re: My LED in my RC circuit is glowing after 5t
« Reply #1 on: July 10, 2018, 07:25:58 am »
I’m no expert, but I assume it’s because components are not perfect. So their might be some leakage. And LEDs need VERY little current to glow slightly.
 
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Offline forrestc

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Re: My LED in my RC circuit is glowing after 5t
« Reply #2 on: July 10, 2018, 08:28:42 am »
I learnt about RC circuits. I decided to learn it practically. I created a circuit in which Resistor, LED, Capacitor are in series.LED is in between resistor and cap.I powered it using 9v battery. The value of resistor is 10k and cap is 220uf. The time constant is 2.2 sec. It will take 5t to get fully charged after which no current should flow i.e LED should be off. But led is slightly glowing. Why?

Most capacitors have a certain amount of leakage.   If they didn't they'd never discharge... 

Even if you've only got a few microamps of leakage, it's enough to have some leds slightly turn on - like you can tell it's glowing, not that it's really illuminated.

 
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Offline CJay

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Re: My LED in my RC circuit is glowing after 5t
« Reply #3 on: July 10, 2018, 08:53:18 am »
Third on what the others have said, LEDs can glow with *tiny* amounts of current so they're not always a good way to demonstrate principles where components aren't perfect.

There was a long running thread (like years) on another forum where someone had taken the LED module from an illuminated ballon complete with the tiny alkaline cells, it was still glowing slightly with sub microamp currents a couple of years later (and we think it was also charging the cells when exposed to light)
 


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