ES Lesson 8: Capacitors• What are capacitors? How are they used? • How are they made? What are their sizes?• What is an RC time constant?

© 2012 C. Rightmyer, Licensed under The MIT OSI License, 20 July 2012

Peter van Musschenbroek’s “Leyden Jar”(an early version of a capacitor)

Metal Ball

ES project 8-1. Make a capacitor

http://sci-toys.com/scitoys/scitoys/radio/homemade_radio.html

demo

Rolling our capacitor

http://sci-toys.com/scitoys/scitoys/radio/homemade_radio.html

The finished capacitor

http://sci-toys.com/scitoys/scitoys/radio/homemade_radio.html

What do capacitors used in electronic circuits look like? Here are some of them.

Capacitor come in a large range of sizes.

1 micro-Farad = 1 F = 10^-6 farad = (1/1,000,000) Farad

1 nano-Farad = 1 nF = 10^-9 farad = (1/1,000) F

1 pico-Farad = 1 pF = 10^-12 farad = (1/1,000) nF

How capacitors work?

VsVc = 0

+Q = 0

-Q = 0

Ic = 0

C = capacitance (farads)

Q = charge on + plate (colombs)

Ic = dislacement current (amps)

Vc = voltage across capacitor (volts)

Vs++++++

- - - - - - -

VcElectric Field

+Q

-Q

electrons

Vs++++++

- - - - - - -

VsElectric Field

+Q

-Q

Ic

C = Q / Vs

Q = C VsIc = C dVc / dt

Vc = (1/ C) Ic dt

ES ckt 8-2. Experiment with a capacitor

+

+

4700 uf

volts

LED

220

Adapted from Discover Electronics: Mr Circuit One, Electronic Kourseware Interactive, 2002 (www.EKII.com)

Hookup diagram for ES ckt 8-2

push button switch

+

4700 uf

220

LED

+

9.0

volts

ckt

Hookup diagram for ES ckt 8-3(Same as ckt 8-2 except for the 1K resistor.)

push button switch

+

4700 uf

1K

LED

+

9.0

volts

ES demo 8-4. The “Blinking” Projectckt

ES demo 8-5. A variable tone oscillator

+

volts

1 Meg pot

NPN 3904

B

C

E

PNP 3906B

C

E

+

47

10

0.1 F

ckt

What if we need a special size capacitor? How to calculate size if in series or parallel.

Backup slides

What is an RC time constant ( sec?

After one time-constant (), the capacitor is ~ 63% discharged. After 5 time-constants (5), the capacitoris ~ 99% discharged. This chart can be used to help choose the size of capacitors and resistors that youwill need for projects you will do if you take Beginning Electronics later on.

ES demo 8-4. Capacitor discharge after one time constant.

++ 4700 uf

VOM9 volts

Resistor value (ohms)Expected voltage at 1

Time ConstantExpected discharge time at 1 TC (sec)

Measured time until meter reads 3.4 volts

1,000 3.33 4.7

2,000 3.33 9.4

4700 3.33 22

Hookup diagram for ES demo 8-4

push button switch

+4700 uf

R+

9.0

volts

:10 vdc

Volt

Meter

ckt