Introduction to CapacitorsChapter 20

Topics in this PowerpointHow it’s made

8 important capacitive ideas

How it works

Calculating Capacitance given physical dimensions

Units of Capacitance

Calculating current

Types of capacitors

Capacitor Codes

Calculating Total Capacitance

Capacitance Vocab.Capacitance – the property of a dielectric to

store electric charge.Dielectric – an insulator used to store electric

charge in a capacitorElectrode - A collector or emitter of electric

charge (not necessarily metallic)A capacitor is made up of 2 electrodes and a

dielectric.Permittivity - how well a dielectric material can

establish electrostatic lines of force. (Like permeability but for capacitors)

Inside a capacitor

Electrodes or

Do not use the property of magnetism like inductors do!Capacitors store energy in an electrostatic field insteadThe number of electrons it can store in this field determines its

capacitance sizeThe units of Capacitance are measured in Farads [F] and the

symbol for Capacitance is C.Capacitors act like springsOpposes DC but passes high frequency AC

Easy to flow during change but is open when DC

Some capacitors are polarity sensitive (For DC circuits) Signal to ground

Current doesn’t actually flow through a capacitor, rather it accumulates charge.

Top 8 Capacitor Concepts

Capacitor Schematic Symbol

In the presence of an electric field, electrons are attracted towards the more positive side.

Explain Variable Capacitor and show this video

Factors affecting size of capacitanceLaser analogyLooking at the capacitor

on the left, which facts do you think affect the size of the capacitor?

1. Area of the plates

2. Distance between the plates

3. Dielectric constant – the material the dielectric is made of.

The equation given for capacitance size:

 

K = Dielectric Constant (or Permittivity)From Table 20.1

A = Area of plate (measured in meters2)D = Distance between plates (measured in meters)

Everything is referenced to vacuum as K = 1, because a vacuum is a very weak dielectric

Determining Capacitance Size 

Dielectric Material - AirA = .004m2

D = .001m

Find the size of the Capacitance given the above info.

C = 35.4pF

Another example 

Dielectric Material – Aluminum Oxide = k = 10A = 4cm x 6cmD = 5mm

Find the size of the Capacitance given the above info.

C = 4.25nF

Another example 

Dielectric Material – Tantalum Oxide = k = 26A = 3cm x 3cmD = 4um

Find the size of the Capacitance given the above info.

C = 51.8nF

The typical range for a capacitor is from .5F to 1 pF.

More generally for our class, between 1mF and 10nF

RVOTD

http://www.youtube.com/watch?v=EjOvI0TOx98&NR=1

All these examples beg the question:What is a Farad?

For any given capacitor, the ratio of the charge on one plate to the potential difference across the plates is a constant. This constant is the property called capacitance.

     

1 Farad = 1 coulomb/1 volt

A capacitor has a capacitance of 1 Farad if 1 Coulomb of charge must be deposited to raise the potential difference by 1V across the capacitor.

When a capacitor is uncharged, the same amount of electrons are on both sides of the dielectric.

When a capacitor is charged to this voltage, it does not mean the capacitor is fully charged to its capacity. Its just fully charged for the given voltage source. If a larger voltage source was applied, the capacitor may retain this larger voltage depending on its capacitance size. Remember, Capacitance equals charge per unit volt. So for every volt that’s placed across it, the more charge Q the capacitor can retain.

When a DC source is applied across the capacitor, electrons flow from one side onto the other until the voltage across the capacitor reaches its source voltage.

Determining capacitance

Determine the capacitance when 50uC of charge deposited on the plates raises the potential difference by 2V.

 

 

Determine the Charge required to raise a 10nF capacitor by 15V.

 

So how does current, voltage and capacitance relate? 

 

If you recall, what does Current equal in terms of Charge?

   

 

 

A constant current of 5mA charges a 20uF capacitor in 80ms. What is the voltage across the capacitor?

… but t is always a change in time, not just a particular instant. It’s also a change in charge Q, which causes there to be a change in voltage. (See 20.1.5)

 

 

This equation should look very similar to one we’ve already worked with.

 

 

Determine the capacitor current when the voltage across a 4uF capacitor changes from 16V to 24V in 2ms.

 

 

 

Determine the capacitor current when the voltage across a 100nF capacitor changes from 0V to 5V in 2us.

 

Another Example

Another RVOTD

Capacitors Blowing UP! http://www.youtube.com/watch?v=3b7mjukhTyQ

Read page 143 on your own.

The starting up of voltage across a capacitor is similar to the starting up of current across an inductor.

The starting up of current across a capacitor is similar to the starting up of voltage across an inductor.

There are many types of capacitors, each one useful for different applications.

Mica Capacitor (See page 155 for typical applications and pro’s/con’s)

Notice the Voltage Rating.

Is this capacitor polar sensitive?

Ceramic Capacitor

Plastic Film Capacitor

Aluminum Oxide Capacitor

This is an Electrolytic Capacitor which means it is polar sensitive. This kind is typically used in DC circuits since AC circuits are not polarized.

When installing this type of capacitor, you must put the side with the negative dashes on the most negative side of the circuit.

Tantalum Oxide Capacitor

Another RVOTD

https://www.youtube.com/watch?v=rr7bPmGTQUk

Capacitor CodesWhat do all the letters and numbers mean on a capacitor?

How do you determine the size of a capacitor?

Determining capacitance size in some cases is not as straight forward as determining resistance size.

Them symbols on a capacitor refer to both the capacitance size and the temperature affect on the capacitance size.

The capacitors in your kit have a tolerance of +/-5%

Temperature Coefficient Codes

There are 3 classes of capacitors – Class I, II, and III

Class I dielectrics display the most stable characteristics.

The most common Class I dielectric is the COG designation, which is 0ppm/°C ±30ppm/°C, which is equivalent to the NPO (zero temperature coefficients) code defined by MIL specifications

Table 20.4; Temperature Coefficient of Class I Dielectrics

Determine the temp. coefficient of a capacitance with the marking R2G.

-220 ppm/ºC ±30ppm/ºC

Class I dielectrics display the most stable characteristics.

The most common Class I dielectric is the COG designation, which is 0ppm/°C ±30ppm/°C, which is equivalent to the NPO (zero temperature coefficients) code defined by MIL specifications

Skip the next 3 slides

Code Practice!

Using the chart in section 20.4.1, table 20.4 determine the temperature coefficient of a ceramic capacitor marked with the following:

U4M, P6G, T0G, R7K-75000ppm/°C ±1000ppm/°C15ppm/°C ±30ppm/°C-47ppm/°C ±30ppm/°C220ppm/°C ±250ppm/°C

Class II

Offer much higher constants than Class I, but with less stable properties to changes in temperature, and voltage.

They have a maximum capacitance change of ±15% or less over an operating temperature range of -55°C to +125°C.

They are called general-purpose capacitors.

Table 20.5 Max Change in Capacitance

Determine:Y5VX7R

Y5V= +22%, -82% maximum change from +25°C, over a -30°C to +85°C temperature range.

Determining Capacitance Size

Capacitance Size – Through HoleIf the capacitor is physically large enough, it will say the

capacitor size right on it.

Otherwise, whole number values on capacitors usually indicate a value in picofarads.

Ie: 102 = 1 0 with 2 more 0’s. So 1000 picofarads or 1nF.

See table 20.6 for value interpretations

What is the value and tolerance of a cap that says 223J?

22nF+/-5% = .022uF (See next slide for “J”)

What is the value and tolerance of a cap that says 334K?

330nF +/-10% = .330uF

What is the value of a cap that says 229?

2.2pF

What size is this capacitor?(This is the types in our parts kit)

Capacitance Size – Through Hole

If the value is 4 numbers long, then just read it directly in pF

Capacitance Size – Surface MountCapacitance is always given in picoFarads

The first digit is typically a letter. This letter represents a number. (See table 20.7) ie: U = 5.6

The second digit is the multiplier

Ex: What capacitance is T2?

T = 5.1 & 2 = x100…… So 5.1x100 = 510pF

What capacitance is X4?

75000pF = 75nF = 0.075uF

What capacitance is 39?

39pF (If there is no letter, the number is read directly in pF.)

How to add capacitors in parallel and in series.

 

So why is this true? What is the reasoning behind this?

 

What is the total capacitance CT of the circuit below?

What is the total capacitance CT of the circuit below?

What is the total capacitance CT of the circuit below?

What is Stray Capacitance?

Stray capacitance is more noticeable at higher frequenciesWays to reduce stray capacitance1. Use low frequencies (not really an option)2. Keep lead lengths short3. Use lots of space between wires and traces4. Mount components high off chassis

What would happen if you measured resistance of a capacitor using a VOM?

How to could you tell if a cap is shorted?

How to could you tell if a cap has an open?

https://www.youtube.com/watch?v=jhvrfyjtOgU&feature=youtu.be (probing eye resistance)

Finish going over Capacitor/Inductor Comparison Handout