# using capacitor as lowpass(bypass) filter

#### donut

Joined May 23, 2012
51
Having a problem explaining to my 10 year old son how a capacitor works as a low pass filter? I mean how do I explain to him how the high pass signal gets filtered out and the low signal passes to the output?

So lets say that the Capacitors Xc(reactance) is small; why would the high pass frequency want to travel through the capacitor and not through the output?

#### BillB3857

Joined Feb 28, 2009
2,561
The basic capacitive reactance formula explains it. As frequency goes up, reactance goes down. Lower reactance to ground puts more of the higher frequency to ground.

#### MrChips

Joined Oct 2, 2009
26,088
Sometimes we use the analogy of fluids, water or air.
An electrical circuit is like water or air flowing in a pipe.
The analogy of a capacitor is a large tank connected to the circuit.
Imagine you have air flowing through pipes and the air pressure (voltage) is fluctuating.
When the air reaches the tank or reservoir (capacitor), the tank buffers and dampens the fluctuations. What comes out after the tank is air pressure with less fluctuation, i.e. the high frequencies have been dampened or filtered out - a low-pass filter.

#### JMac3108

Joined Aug 16, 2010
348
Donut,

Remember, the impedance of a capacitor (capacitive reactance) depends on frequency. As the frequency increases, the impedance decreases. So in the case of the low pass filter, the higher frequencies see a lower impedance to ground than the lower frequencies. Make sense?