Monday, July 29, 2019

introduction to electronics part 4 - capacitor response to DC

We know a 1 farad capacitor will charge to 1 volt with 1 coulomb of electrons on its plates. How long will it take to place this high number of electrons on the plates?
In this circuit I assume NO resistance. As you can see it would take 1000's of amps to charge the capacitor instantly. In the real world this cannot happen we will have some resistance in the circuit.
With 1 ohm resistance the charge is limited too 1 amp. Notice the instant the circuit is closed the charge starts dropping. As the capacitor charges it acts as a battery with an opposing charge being inserted in the circuit. This decay in signal is at a log rate. More on that later.
Changing R1 to 10 ohms limits the current to 100ma and the charge time is extended. Note the curve has the same shape. This time is prodictable.
I changed the values to more realistic values. It follows the same curve but is not easy to see with the time base of the scan.
The time constant (TC) is found with the formulas.
TC = R*C
TC = 100ms
We assume the capacitor is fully charged in 5 TCs. At .5 seconds it is charged for our analysis purposes. In reality as it continues to charge beyond 5 TCs but we won't look at that for now.

The main point is the capacitor will accept a charge in a period of time defined by its value and circuit resistance. It passes current as it charges and then blocks DC.

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