Section 5

Question 1

How does a capacitor in an AC circuit react?

Answer 1

It constantly charges and discharges
Current lags behind voltage by 90 degrees

When Voltage is 0 current is max
when volage is max, current is 0

In a circuit with a resistor current follows voltage and are “in-phase”

REMEMBER: When VOLTAGE INCREASES the current will increase but if it’s steady there is no current

Question 2

What is the equation for the phase shift that happens between current and voltage in a capacitor?

Answer 2

i = C dv/dt

Question 3

What is Xc

What is its formula?

Answer 3

Capacitor Reactance - Measure of a capacitor’s opposition to AC

Units measure in Ohms

Xc = 1/2pifC

f = frequency
C = capaitance
pi = 3.142

The higher Xc the lower the current

Question 4

The current flow through a capacitor is _________ to the frequency

Answer 4

Inversely proportional

The higher the freq of the AC signal, the lower the Xc (capacity reactance)

Question 5

If the frequency is 0Hz (basically this is DC)
What is the Xc of the capacitor?

Answer 5

Capacitor Reactance = infinite

Basically this means an open circuit

I = 0

Question 6

If the frequency is infinite, what is the Xc of the capacitor?

Answer 6

Capacitor Reactance = Normal wire
Like the capacitor isn’t even there
I = MAX

Question 7

Calculate the capacitive reactance value of:
220nF capacitor
1kHz
and again at:
20kHz

Answer 7

Xc = 1/2pifC

Xc = 1/ (2pi) * 1000 * (220 * 10 to the negative 9th power) (just put 9
0’s after the one to do powers)

2*pi = 6.28 (pie squared)
1/1000000000 = .000000001 (10 to neg 9 power)
1000 * 6.28 = 6280
.000000001 * 220 = .00000022
.0000002 * 6280 = .0013816
1/.0013816 = 723.798

20kHz would make 36.2ohms which is less resistance
So, the lower the freq the higher the ohms

Question 8

What is Irms?

Answer 8

Equivalent steady DC current which gives the same effect on the load that it’s passing through

ROOT MEAN SQUARE CURRENT

Question 9

What is the equation to find the IRMS?

Answer 9

IRMS = ROOT MEAN SQUARE CURRENT

Irms = Vrms/Z

Question 10

What is Z and how do you calculate it

Answer 10

Total Impedance - opposition to current flow in AC circuit. It’s kind of like resistance

Z = R to the second power + Xc to the second power, then you square all that.

Question 11

How do you calculate Vrms?

Answer 11

Vrms = 0.7Vpeak
Vpeak = 1.4Vrms

If you are showing 12V just use that as the Vrms

Question 12

How do you calculate Irms?

Answer 12

Irms = Vrms/Z(total impedance)

Question 13

The higher the capacitance the _______ the impedance

Answer 13

lower

Question 14

What is capacitor coupling?

Answer 14

AC signal can pass through but DC can’t

This makes it to where you can only complete a circuit with AC

EXAMPLE = DC powers on microphone but capacitor keeps it from going to other side’s ground, AC can pass through the capacitor though

Remember, you can find what current goes through this with your Irms equation

Question 15

What are decoupling capcitors?

Answer 15

Removes AC to only get DC

This cleans up noise

Capacitor is in parallel iwth power supply
_________
P C LOAD
|___|___|

This is due to reactance of capacitor
It doesn’t allow DC to traverse it.
AC CAN pass through though allowing them to go to ground, plus it’s closer to ground which is more desirable to AC

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Question 16

What are Bypass Capacitors?