Capacitance 3


Introduction to Capacitance

Capacitors oppose a change in voltage.     The unit of Capacitance is called a “Farad”.  We use capacitance to filter the “ripple” effect when converting AC to DC.    We will also use capacitors in resonator circuits, such as a tuner for an AM or FM radio.

Let’s look at a pure AC signal.  These images are generated using tinkercad.

Pure AC

Now, let’s rectify this signal. The voltage is not going negative, but we do have a little bit of “Ripple” effect that we need to smooth out.

DC Ripple

We will place a 10 MicroFarad capacitor on the output of the rectifier, and you will see that we have a nearly pure DC output now, even though our source is 60Hz AC.

Filtered

 

Calculations

Sometimes, it becomes necessary to make a certain capacitance value from various capacitors that you may have laying around your shop.     Let’s look at how we would calculate a certain capacitance value from multiple capacitors that are connected together in series or in parallel.     Calculating capacitance is different than calculating resistance.

When calculating capacitors in parallel, we just add the values together.    Here is an example:

Parallel Capacitors

 

Here, there are three capacitors in parallel.    These are 22pf capacitors.    For capacitors in parallel, we simply add the values together to derive the total capacitance.    In this case our total capacitance is 66pf.

When using capacitors in series, things are a little bit more complicated.   Let’s look at another example using the same 22pf capacitors.

Series Capacitors

The formula for calculating the capacitors in series is:   1/C1 + 1/C2 + 1/C3 ………   = 1/Ct

Therefore 1/22pf + 1/22pf + 1/22pf = 3/22pf

This means that the total capacitance is 22pf/3 or 7.3333 pf

Be aware in this example, we are using ceramic capacitors.   Ceramic capacitors are not polarized.   This means that it does not matter how we place the capacitors into the circuit.

You are also going to come across electrolytic capacitors.    Electrolytic capacitors are polarized, and MUST be placed in the circuit with the correct polarity!!!     The capacitor can (and likely will) explode if the polarity is reversed.     Electrolytic capacitors are clearly marked as to which lead is the – (negative) lead.

Electrolytic Capacitor

— Ricky Bryce


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