Capacitor in an RC Filter: Wiring and a Live Circuit

At a glance

The Capacitor in brief

What it is, and every setting the properties panel gives it.

A capacitor is the one part on the bench that genuinely integrates over time rather than answering instantly. It joins whatever else is on its nets into a shared analog solve, and the matrix works out the whole island together, which is what lets an RC network land on the true exponential curve rather than an approximation of one. Charging or discharging toward a fixed voltage through a fixed resistance, the result is exact for any time step, however large; what changes with the step size is only how finely a moving source is tracked in between. That is the part behind a 555’s timing, an RC debounce filter, and any circuit on this site where something changes gradually instead of switching instantly. It is still a simple model: there is no leakage, no equivalent series inductance, and no dielectric behavior beyond a small series resistance, only the charge and discharge curve itself, done properly.

Family: Basics
Pins: 2
value: 10; capacitance, in the unit the parts bin uses (microfarads). 10 is a typical decoupling or timing value; a 555 astable commonly uses 1 to 100.

Pins

2 pins

Named as the editor names them, so a wire lands where you expect it.

What it does

1

Either plate.

2

The other plate.

How to use it

How to use a capacitor in an RC filter

A capacitor stores charge, and through a resistor it charges at a rate set by the two together: the time constant, R times C. That makes a resistor and a capacitor a filter. In series with the signal and with the capacitor to ground, they pass slow changes and smooth fast ones away, a low pass with its corner at 1 / (2 pi R C).

The circuit here is that low pass on the bench: a function generator into 16 k and 0.1 uF, a corner near 100 Hz, with a scope on both sides of the resistor and a multimeter across the capacitor. Sweep the generator’s frequency and watch the output shrink past the corner. An electrolytic capacitor has a polarity: its stripe marks the negative leg, which goes to the lower voltage.

Part pin

Board pin

Resistor, 16 k

Generator output to the capacitor

Capacitor, 0.1 uF

Between the resistor and GND

Scope CH1 and CH2

The input, and the output across the capacitor

Next to a microcontroller, a 0.1 uF capacitor across each chip’s supply pins is the one every circuit needs, and an RC filter on a PWM pin turns it into a slowly changing voltage.

RC filter on the benchlive0.000 s 0.00x

Click to open it in the editor

This is the simulator itself, running here. Click anything to open it in the editor.

Open this project in the editor Read the full tutorial

How it is modeled

What is true about the Capacitor, here

In its help article’s own words: what the simulation gets right, and where it stops.

Modeled

The capacitor declares itself to the kernel as analog and then does nothing else at all. It reads its own voltage, V(1) - V(2), straight back off the solved nets for the probe. That is what lets an RC timing network land on the exponential to a tenth of a percent, and what puts a 555 timer's astable within 0.05% of the textbook formula, because the timing genuinely comes from the RC network and not from a rounded-off approximation of it.

Not modeled

It is not an ideal capacitor: there is 0.1 ohm of series resistance in it, which the analog island needs to stay well-posed. That is smaller than any real electrolytic's ESR and smaller than a wire, so it changes nothing a lesson measures, but it is there and it is not zero.

Every capacitor starts discharged. The operating point solved at power-on puts 0 V across every capacitor in the circuit, so a circuit is never resumed part-way charged.

No leakage, no dielectric absorption, no voltage or temperature coefficient, no self-resonance, and no polarity enforcement: an electrolytic-style value wired backwards behaves exactly as it does the right way around, where a real one would vent.

From Capacitor, in full.

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