# Simple capacitor question

Discussion in 'Homework Help' started by c1rcu1ts, Nov 12, 2013.

1. ### c1rcu1ts Thread Starter Member

Oct 19, 2013
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0
why do we need a resistor when charging a capacitor?

2. ### MrChips Moderator

Oct 2, 2009
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Depends. Your question is not clear. What is your application? Show us a circuit.
One does not usually charge a capacitor just for the sake of charging a capacitor except perhaps in a laboratory experiment.

A capacitor has many other useful functions in an electronic circuit, the least of which is just to put a charge in it.

3. ### c1rcu1ts Thread Starter Member

Oct 19, 2013
62
0
Application is just to charge a capacitor. (for my understanding)

Just wondering why in all of the circuits i have seen, they always put a resistor in series with the capacitor. In the charging equation V = Vo[1-e-(t/RC)] why not just put R = 0.
e-(t/RC) would eventually become 0 and Vo[1-0] = V (100% charged).

Basically why not just a capacitor and voltage source in series rather than a voltage source, resistor and capacitor in series.

Last edited: Nov 12, 2013
4. ### MrChips Moderator

Oct 2, 2009
12,648
3,458
The RC circuits you see in the textbooks are for demonstrating the properties and behaviour of capacitors in an RC circuit.

You can connect the capacitor directly to a voltage source with no external resistor.

The exponential formula still applies but note that there is always a finite amount of resistance R in the circuit.

Hence the value RC still has some finite value. It is never zero. It means that the capacitor will reach full charge in a very short time.

But watch out. Calculate the peak current to the capacitor. You may be surprised how high it can be. A series resistor will lower the current.

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5. ### MrChips Moderator

Oct 2, 2009
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The main reason for having the resistor in an RC experiment is so that you can study the exponential rise and fall of the voltage on a voltmeter or oscilloscope.

Without the resistor the time-constant would be too short to observe even on a fast oscilloscope.

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6. ### c1rcu1ts Thread Starter Member

Oct 19, 2013
62
0
oh okay thaks

7. ### WBahn Moderator

Mar 31, 2012
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To limit the current to an acceptable level.

On paper, the lack of a resistor would result in an infinite current for an infinitesimal amount of time. In a real circuit, there will always be some parasitic resistance, if nothing else, but it may be low enough that the resulting currents are high enough to cause problems. If so, then additional actual resistance needs to be added. If not, then call it good.

Last edited: Nov 12, 2013

Oct 23, 2010
737
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You don't.