Passive Devices

Question 1

What are film resistors made of?

Answer 1

Constructed by depositing a thin layer of carbon metal to an insulating substrate

Question 2

In a film resistor, what do we engrave onto the film and why?

Answer 2

A helix with a laser to create much lower tolerances

Question 3

What are wirewound resistors made of?

Answer 3

Made by winding a thin wound of metal alloy wire in form a helix spiral on an insulator.

Question 4

What are the properties of wire wound resistors

Answer 4

They tend to have high parasitic inductance. Used in high power, high current applications

Question 5

What is the definition of capacitance (equation)?

Answer 5

C = Epsilon * A / D
Epsilon is the dielectric constant of the material
A is the surface area of the plates
D is the distance between plates

Question 6

What is an electrolytic capacitor made of?

Answer 6

Aluminium foil, one side of which is covered with an insulating aluminium oxide. The cathode will consist of an electrolyte.

Question 7

What are the properties of an electrolytic capacitor?

Answer 7

Very high voltage rating, upto 630V
A large equivalent resistance compared to ceramic or film capacitors.
Not suitable for high frequency applications

Question 8

What are film capacitors made of?

Answer 8

Winding a large number of layers of metal film that are separated by a di electric material.

Question 9

What are some properties of film capacitors

Answer 9

Typical value around 100 nano farad.
Very high voltage rating - up to 10’s of KV’s.
Good high frequency properties, with a very low ESR, where ESR is their equivalent resistance that allows us to treat them as ideal components.

Question 10

What are Ceramic Capacitors made of?

What is the common application of Ceramic capacitors??

Answer 10

Much like film capacitors, with layers of wound material but they now alternate layers of metal and ceramic.
Commonly used in decoupling applications.

Question 11

What is the charge on a Capacitor (equation)

Answer 11

Q = C*V

Question 12

What is the current through a capacitor?(equation)

Answer 12

Since current is the rate of the flow of charge:
I = Dq / Dt —-> D(CV) / Dt —-> C * Dv/Dt
The capacitance should remain constant which is why we can bring it to the front of the equation.

Question 13

What is the energy stored in a capacitor? (equation)

Answer 13

E = o.5 * C * V^2

Question 14

What happens to a capacitor when it is fully charged? (At steady state?)

Answer 14

At steady state, no more current will flow into the capacitor, therefore it behaves like an open circuit.

Question 15

When a capacitor is charging, at what time is the voltage max and the current min?
What is the value of current and voltage at 1timeconstant

Answer 15

At 5 x the time constat, where the time constant = R * C

At 1 time constant, the voltage is at 63% of its max value while the current is at 37% of its max value

Question 16

When the capacitor is discharging, what is the value of current and voltage at tau?

Answer 16

37% of their initial value

Question 17

How do we add capacitors in series or parallel?

Answer 17

In series, treat them like parallel resistors and the opposite for when they are in parallel.

Question 18

Why do we use Decoupling capacitors?

Answer 18

We use them close to important electronic components like microcontrollers. It will act to protect against fluctuations in voltage.

Question 19

In a digital circuit, what capacitors do we use?

Answer 19

An electrolytic capacitor to store most of the energy and to decouple lower frequencies, and then a smaller ceramic capacitor to handle the high frequencies.

Question 20

What do we use coupling capacitors for?

Answer 20

We place them in series with the signal path to filter out the DC component of the signal.
In analog amplifiers, they will prevent the AC signal from interfering with the bias voltage of the transistor

Question 21

How do we construct inductors?

Answer 21

By coiling a piece of wire around a solid core. The current flowing the wire will create a magnetic field that links the coils and stores energy.

Question 22

What will happen when we vary the current through the inductor?

Answer 22

A varying magnetic flux will be produced that is proportional to the current. This will induce a voltage that will try to oppose the change in current.

Question 23

What is the equation for voltage through an inductor?

Answer 23

V = L * di/dt

Question 24

What happens to an inductor once it reaches steady state?

Answer 24

It will become a short circuit so no voltage will flow across it.