# Current through a capacitor

Discussion in 'General Electronics Chat' started by niner710, Jan 25, 2009.

1. ### niner710 Thread Starter New Member

Jan 25, 2009
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Hi,

I have a current mirror that applies 100uA to a specific capacitive node in a semiconductor circuit. It charges up that node to say 5V. My question is when the capacitive node discharges will it also supply 10uA of current assuming that the resistance is negligible. Thanks.

2. ### mik3 Senior Member

Feb 4, 2008
4,846
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If the capacitor discharges through a resistor then the current through the resistor will decrease exponentially (like the function of exp(-x)). Provide a schematic to see what is going on.

3. ### DedeHai Active Member

Jan 22, 2009
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Assuming that the resistance is negligible, meaning close to zero, the current will be much higher. as mik3 mentioned, it will decrease exponentially, starting at a value of I = U/R which, with a negligible resistance, can go up to a 1000-times the value you want (10Amps or more) depending on the type of capacitor used.

4. ### niner710 Thread Starter New Member

Jan 25, 2009
4
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I attached a schematic. So if the current source(10uA) is charging up a capacitive node to say 5V what would be the current coming out of the capacitive node and going through the resistor and the forward biased diode. It would not be 10uA right? What happens if the resistor becomes very small or very big?

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5. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Yes, the current through the resistor and the diode is 10uA too because they are all connected in series.
If you assume an ideal current source, the higher the resistance the less time it will the node to reach 5V and vice versa.

6. ### niner710 Thread Starter New Member

Jan 25, 2009
4
0
Ok, thanks. So basically the same current that charges the capacitor would be the same as the current that discharges the capacitor right?

In other words, if 10uA of current charged up a 10pf capacitive node to 5V, then 10uA would be the initial amount of current that discharged that node to 0V right?

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
It depends how you will discharge the capacitor. How you will discharge the capacitor and bring the node to 0V?