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How do resistor-capacitor circuits work? Answered

I know this is a basic question, but it has bugged me, and is in two parts. The first: Are resistors like reduction valves for electricity? i.e. Do they simply absorb and dissipate excess electricity, or do they restrict its flow, as in making a large capacitor discharge its energy through a resistor to prolong output from the capacitor? Also, do capacitors store electricity until it is full, then release it, or is that only when they fail? Thanks!



Best Answer 9 years ago

Don't go too far with the water analogy. It's only useful up to a point. A resistor is simply not as conductive as wire. It resists current and causes a forward voltage drop. It's helpful to think of capacitors like rubber bands, rather than any vessel for water. When used to charge a capacitor, a resistor will increase the amount of time it takes to charge a capacitor. The larger the resistor, the longer the capacitor will take to charge. The forward voltage drop caused by the resistor reduces the amount of "push" stretching the rubber band. When a capacitor discharges (when the forward voltage disappears, and the "rubber band" snaps back to its original shape), if there is no resistance, it will discharge almost immediately. However, when discharging through a resistor, the resistor reduces the amount of "push" from the "rubber band" itself. The capacitor will discharge more slowly when the resistor is larger. A capacitor will store energy as long as there is voltage to hold it there. For example, I've applied 5v to a capacitor. After some time, it will become fully charged. The force of the 5v supply has pushed a number of electrons to the far plate. As long as the voltage remains, those electrons will be held on the one plate. As soon as that voltage drops, there won't be enough force left to hold all the electrons to that plate, and some will drift back to the other plate, discharging some of its stored energy. This is how capacitors work in power supply filtering, AC filtering and DC power decoupling. Any drops in voltage let loose some of the stored energy, which "picks up the slack." If the voltage holding the electrons to a single plate disappears, one of two things will happen. If there's a path to ground, or a short circuit between the two plates, all of the energy will be released in a very short period of time. If there's a resistor involved, as described above, the discharge will happen more slowly.

All the resistor does is "resist' or slow the amount of electrons traveling across the wire.  The capacitor stores the electricity like a reservoir before releasing it back into the main circuit.  This is expressed as the RC or tau constant.  By simply multiplying Ohms * Farads, you can calculate how it will take the RC combo to charge and discharge in milliseconds.

A fluid analogy works like this A resistor can be thought of as a pipe, where the diamter is inversely proportional to the value off the resistor. Taht is, a larger value is a smaller pipe. The capacitor is a bucket The capacitance is the size of the bucket, the voltage rating is the height of the bucket, or how much force you can deliver during discharge ESR (equivalent series resistance( is a measure of how fast you can fill or empty the bucket. a smaller value of resistance will fill the bucket faster, and a larger value will fill the bucket slower. ESR limits how fast you can charge or discharge the capacitor. If you try to charge it too fast, you stand the chance of damaging the capacitor, if you try to drain it too fast, it will self-limit or, again, be damaged...