Unit 2ElectronicsObjective Electronics10 min read

Passive Components, Filters, and Reading Schematics

Unit 2: Fundamentals of Electricity and Electronics. Lesson 10 of 60 in CBET Academy.

Passive components store or dissipate energy; they cannot amplify. Knowing what each does, and how it looks on a schematic, is how you find a fault on a board.

Resistors

Limit current and divide voltage. Value is printed as color bands: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, gray 8, white 9; the third band is the multiplier and the fourth is tolerance (gold 5 percent, silver 10 percent). Red-violet-orange is 27,000 ohms. Failed resistors usually open and often show heat discoloration.

Capacitors

Store charge between two plates. They block DC and pass AC, and they smooth ripple in power supplies, couple signals between stages, and store the energy in a defibrillator. Capacitance in farads (usually microfarads). Electrolytic capacitors have polarity and are the most common failure in old power supplies: they dry out, bulge, and leak, causing ripple, hum, and resets. Energy stored is one half x C x V squared.

Inductors and transformers

A coil stores energy in a magnetic field, passes DC, and opposes changes in current, so it blocks high-frequency noise. Two coupled coils form a transformer.

Reactance and filters

Capacitors and inductors resist AC by an amount that depends on frequency, called reactance. Capacitive reactance falls as frequency rises: Xc = 1 / (2 x pi x f x C). Inductive reactance rises with frequency: XL = 2 x pi x f x L. Combining them with resistors makes filters:

Low-pass keeps slow signals and removes fast noise.
High-pass removes slow drift, such as ECG baseline wander.
Band-pass keeps a range, such as the ECG monitoring band of 0.5 to 40 Hz.
Notch removes one frequency, almost always 60 Hz interference.

The RC time constant

A capacitor charging through a resistor reaches 63 percent of its final voltage in one time constant, tau = R x C, and about 99 percent after five. A 10 kilohm resistor and a 100 microfarad capacitor give 1 second. Time constants set filter cutoffs, timing circuits, and defibrillator charge times.

Reading a schematic

Schematics are the map you troubleshoot with.

Symbols: a zigzag is a resistor, two parallel lines a capacitor (one curved if polarized), a coil an inductor, a triangle with a bar a diode, a circle with legs a transistor, a stack of lines ground, a small box or S-curve a fuse.
Signal flows left to right; power rails run along the top and bottom.
Test points are labeled with expected voltages or waveforms. Measure, compare to the printed value, and the first point that disagrees tells you the stage that failed.
Half-splitting: check the middle of the signal path; if it is good, the fault is downstream, if bad, upstream. Repeat.

Exam tip: capacitors block DC and pass AC; inductors do the opposite. A bulging electrolytic capacitor means ripple and resets. One time constant is 63 percent. On a schematic, the first test point that differs from the printed value points to the failed stage.

Remember

Resistor color code black to white 0 to 9. Capacitors block DC, pass AC, store energy (half C V squared), fail by drying out. Inductors block fast changes. Low-pass, high-pass, band-pass, notch at 60 Hz. tau = RC, 63 percent. Half-split with the schematic.

Deeper explanation

Passive Components, Filters, and Reading Schematics, explained slowly

Passive means it cannot make anything bigger

Resistors, capacitors, inductors, and transformers only dissipate, store, or move energy. They set the timing and filtering of every circuit, and on a board they are what most often fails visibly. Then the schematic tells you where to look.

Resistors and their code

Colored bands read from the end nearest the first band.

digit bands:  black 0  brown 1  red 2  orange 3  yellow 4  green 5  blue 6  violet 7  gray 8  white 9
third band:   multiplier (number of zeros)     fourth band: tolerance gold 5%, silver 10%
example:      red violet orange gold  =  2 7 000  =  27 kilohms, 5%
example:      brown black red         =  1 0 00   =  1 kilohm

A resistor fails by opening (infinite resistance), often after browning from heat. Measure resistance with the circuit unpowered and, if it is in parallel with other parts, lift one end.

Capacitors: buckets for charge

Two plates separated by an insulator. Apply a voltage and charge collects; remove the source and the charge stays. Consequences:

Blocks DC, passes AC. Once charged, no more DC current flows; but AC keeps reversing, so it keeps charging and discharging, which looks like conduction.
Smooths ripple: after a rectifier, the capacitor fills during peaks and supplies current during valleys.
Stores energy: E = one half x C x V squared. A defibrillator charges a large capacitor to a couple of thousand volts and dumps it through the patient.

Electrolytic capacitors are polarized and contain liquid that dries out with age and heat. A bulging or leaking top is the single most common visible fault in a power supply, and its symptoms are ripple, hum, resets, and low output under load.

Inductors: flywheels for current

A coil resists changes in current the way a flywheel resists changes in speed. It passes steady DC freely and fights fast changes, so it blocks high-frequency noise. Two coupled coils are a transformer.

Reactance: resistance that depends on frequency

Capacitor   Xc = 1 / (2 pi f C)     high at low frequency, low at high frequency
Inductor    XL = 2 pi f L           low at low frequency, high at high frequency

Put these together with resistors and you choose which frequencies get through.

low-pass    keeps slow, removes fast      smooths noise off a temperature signal
high-pass   removes slow, keeps fast      strips baseline wander (breathing) from an ECG
band-pass   keeps a window                ECG monitoring band 0.5 to 40 Hz
notch       removes one frequency         the 60 Hz mains hum on every physiological signal

The time constant

A capacitor charging through a resistor does not fill instantly. It reaches 63 percent of the final voltage after one time constant, tau = R x C, 86 percent after two, 95 percent after three, and about 99 percent after five.

voltage
 100% |                   . . . . . . .
  95% |            . '
  86% |        . '
  63% |    .'
      |  .'
      |.'
      +----|----|----|----|----|-----> time
          1t   2t   3t   4t   5t         tau = R x C

Worked: 10 kilohms and 100 microfarads: tau = 10,000 x 0.0001 = 1 second. Full charge in about 5 seconds. A defibrillator's charge time and a filter's cutoff are both time constants in disguise.

Reading the schematic

The schematic is the troubleshooting map, and the outline says so explicitly.

symbols:  -/\/\/\-  resistor      --| |--  capacitor (curved plate = negative side if polarized)
          -^^^^-    inductor      ->|-     diode (arrow points the way current flows)
          -o/ o-    switch        --[ ]--  fuse        ===  ground
layout:   inputs on the left, outputs on the right, positive rail on top, ground on the bottom
labels:   test points (TP1, TP2) with expected voltages or waveforms printed beside them

Method: power up, measure each test point, compare to the printed value. Work from the input toward the output; the first point that disagrees marks the failed stage. When the path is long, half-split: test the middle. Good there means the fault is downstream; bad means upstream. Each split halves the search.

How the exam asks it

"Which component blocks DC but passes AC?" A capacitor.
"A power supply shows excessive ripple; which component is most likely at fault?" The filter capacitor.
"How much of the final voltage does a capacitor reach after one time constant?" About 63 percent.
"Which filter removes 60 Hz interference?" A notch filter.
"Red violet orange gold?" 27 kilohms, 5 percent.

What to memorize

Color code 0 to 9; gold 5%, silver 10%.
Capacitors block DC, pass AC, store half C V squared, and dry out. Inductors oppose change.
Low-pass, high-pass, band-pass, notch. tau = RC, 63 percent, five tau to full.
Schematic: compare test points to printed values; half-split the path.