# active device

Discussion in 'Homework Help' started by amaresh92, Nov 6, 2010.

1. ### amaresh92 Thread Starter New Member

Oct 31, 2010
23
0
why capacitor can not consider as active device as it has also ability to control the flow of electron (as charging and discharging)?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Capacitors and inductors are energy storage elements. They can only ever return the same stored energy with which they are supplied from an external source - and that's an ideal situation. Real capacitors and inductors have losses and cyclically varying energy in a capacitor or inductor will eventually decay to zero without further stimulus from an external energy source.

Nor can capacitors or inductors really control anything - they simply respond to an input stimulus. They can of course be used to control energy flow in a circuit which is energized by an external source. But the same is true of a resistor which is not an active device.

amaresh92 likes this.
3. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
The flow of electrons into a capacitor is a constant due to the characteristics of a capacitor.

In order for a capacitor to "control" that flow (be an active device), the rate would need to be a variable instead of a constant.

amaresh92 likes this.
4. ### amaresh92 Thread Starter New Member

Oct 31, 2010
23
0
would you please explain the first paragraph in more simple words.
thanks

5. ### amaresh92 Thread Starter New Member

Oct 31, 2010
23
0
i think you are indicating the rate as of rate of charging and discharging of capacitor.
but the rate of charging and discharging keeps on varying as time passes due to accumulation of charges in capacitor.then how it can be constant?
thanks.

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
The result of the charging formula is constant for any given value of a capacitor at any given time. It my not respond linearly, but it cannot change it's behavior, either.

7. ### amaresh92 Thread Starter New Member

Oct 31, 2010
23
0
i hope you would not irritated' but i am not getting your point.

8. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
This type of question begs for a definition of "Active Device." So, I searched on it.

Here is the definition by an agency of the US government:
http://www.its.bldrdoc.gov/fs-1037/dir-001/_0139.htm

Doesn't sound bad, but I was surprised to see LED's included.

So here is another definition:

And finally, from the mother source itself:

We can see that capacitors do not control current by means of another electrical signal.

Now I ask, how does one reconcile the US governmental agency and the AAC definitions? Must the controlling signal be electrical? Can it be mechanical, heat, light, etc.?

John

amaresh92 likes this.
9. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Naming a capacitor an active device due to its abilitiy to affect the electro flow is rather superficial. From the same perspective, you could name a resistance an active device, as it regulates the current trough it according to the voltage applied to it.

However, a real active device can regulate a single quantity (voltage or current) related to it, independently of all the other elements the circuit it is in has.

amaresh92 likes this.