Real Analog - Circuits1 Labs: Ch6 Vid1: Physical Inductors & Capacitors

Real Analog - Circuits1 Labs: Ch6 Vid1: Physical Inductors & Capacitors

Inductor and capacitor construction / Nominal capacitance and inductance values from part labels / Electrolytic capacitors.

>> In this video, we'll introduce some of

the practical aspects of capacitors and inductors.

As usual in the lab videos,

I'll assume that you're getting the theoretical information about

these devices from the textbook and the lecture videos.

We'll spend our time in this video then,

looking at some physical capacitors and inductors.

Talking a bit about how these devices store energy,

show you how to identify capacitance and inductance values on physical parts,

and show you how to measure capacitances using your DMM.

Sadly, most affordable DMMs do not have an inductance measurement capability.

So, for this course,

we'll just have to believe the nominal inductance values for any inductors that we use.

First, we'll talk about capacitors;

Typical capacitor construction consists of

two conductive elements separated by a non-conductive material, called a dielectric.

The dielectric prevents current flow from one element

to the other and is characterized by its permitivity,

designated by the Greek letter, epsilon.

If we apply a voltage difference between the two plates,

charges will accumulate on the upper and lower plates.

These will create an electric field between the plates.

The capacitor stores energy in this electric field.

Capacitance is a quantity which tells us how much energy a capacitor can store.

For a capacitor consisting of two parallel plates,

like the one we saw in the previous slide, the capacitance is,

A times epsilon over d. Where A,

is the cross-sectional area of the plates,

d is the spacing between the plates,

and epsilon is the permittivity of the dielectric.

So, we can increase the capacitance by increasing the cross-sectional area,

decreasing the spacing, or increasing the permittivity.

Now, let's take a look at a few physical capacitors,

measure their capacitance to get a feeling for these relationships.

This is a variable parallel plate capacitor.

The two plates actually consist of multiple plates each.

So, these fins all create one plate,

they're all electrically connected.

The other plate is created by these fins.

The two sets of fins are separated by air gaps between them,

so the dielectric is simply air.

If I turn the knob on the side of the capacitor,

I can increase or decrease the capacitance,

by increasing or decreasing the area of overlap of the conductors.

Let's measure the capacitance and verify that,

that's what actually happens.

To measure capacitance using my DMM,

I plug the leads into the COM and Volt/Ohm ports.

I also connect the leads to the two terminals of the capacitor.

To measure capacitance, I turn my knobs so that

the indicator lines up with the little capacitor symbol.

Currently, there's little or no overlapping area between

these fins and I get about 0.1 nanofarads.

By turning the knob,

I can increase the overlapping area and increase the capacitance.

Now, it's at about 0.4 nanofarads.

If the fins are entirely overlapped,

I get about 0.7 nanofarads.

Now, let's take a look at a very simple homemade capacitor.

My capacitor consists of two wires which are approximately parallel.

They make two conductive elements with a dielectric between them which is currently air.

I can use this property to measure water level.

If I change the permitivity between these,

it'll change the capacitance.

If I insert my wires into this tube of water,

then when I change the water level,

I will also change the capacitance between these wires.

Let's connect our DMM,

change the water level and see how it works.

Now, when the water level is low,

our capacitance is about a half a nanofarad.

Adding some water to this,

increases the water level and should cause the capacitance to go up.

Now, let's look at some typical capacitors which we'll use to create electric circuits.

The capacitors I'll show you,

are from the digital analog parts kit.

The general principles are applicable to most capacitors.

Capacitors are generally described by the type of

dielectric material and their overall construction.

Most of the capacitors in our parts kit

are disc-shaped, like this one.

On these types of capacitors,

the nominal capacitance is encoded as three numbers printed on the capacitor.

These numbers provide a capacitance in picofarads in exponential notation.

The first two numbers are the mantissa of

the number and the third number is the exponent.

For example, this capacitance has the digits 103 printed on it.

This means that its capacitance is 10 times 10 to the third picofarads.

A picofarad is one times 10 to the minus 12 farads.

Therefore, the capacitance of this capacitor is 10 times 10 to the third picofarads,

which is times 10 to the minus 12,

which is equal to 10 times 10 to the minus 9, or 10 nanofarads.

Our actual measured capacitance is about 11 and a half nanofarads.

Electrolytic capacitors are also fairly common.

In electrolytic capacitors, one or both of the conductive plates are not metallic.

These capacitors tend to have a relatively high capacitance for their size.

The electrolytic capacitors in your parts kits are can-shaped rather than disc-shaped.

Electrolytic capacitors are polarized,

that means they don't really work the same way if you

switch the polarity of the voltage at their terminals.

The polarity is indicated by the length of the leads on the capacitor.

The anode has a longer wire than the cathode.

It is intended that the anode always be at a higher voltage than the cathode.

If you reversed the polarity,

the capacitor properties will be different.

In fact, if you apply a voltage in which

the cathode is at a higher voltage than the anode,

the capacitor can explode.

If you do this, by the way,

I recommend that you wear eye protection.

I've personally never blown up a capacitor by

reversing its polarity, but there's a first time for everything.

The electrolytic capacitors in our kit,

are physically large enough so that the capacitance is

printed directly on the side of the capacitor.

This capacitor, for example,

is nominally 220 microfarads.

The printing on the side of the capacitor also provides additional information.

The cathode is indicated by a white stripe with a minus sign on

it and the maximum allowable safe voltage is also printed out on the side.

This capacitor's rated voltage is 10 volts.

Now, let's talk a bit about inductors.

Inductors like capacitors store electrical energy.

Unlike capacitors, inductors store energy in a magnetic field.

Typical inductors are constructed by winding a conductive wire around a central core.

When current runs through the coil,

a magnetic field is created.

The inductance of the inductor is indicative

of the amount of energy that can be stored by the inductor.

The inductance is typically related to the number of turns

the coil takes around the central core and the core material.

Ferrite core materials typically result in relatively high inductance.

Here's a very simple homemade inductor,

I've simply wrapped wire around a carriage bolt.

If I connect a voltage source to the wire,

current will flow through the wire and a magnetic field will be created.

To create a fairly large magnetic field,

I'll need a high current.

So, I'll use the six-volt batteries as my power source.

I know I have a magnetic field because I can pick up

screws with my newly created electromagnet.

Inductors come in a wide range of shapes and sizes.

This is a large high current inductor, about 120 millihenries.

The analog parts kit contains two inductors;

a one millihenry inductor and a one microhenry inductor.

The millihenry inductor is encoded with the numerals 102 on

the side, and the microhenry inductor has the numerals 1R0.

These are both ferrite core inductors.