Identifying and Testing Components, and Using the Instruments

By the end you should be able to: Identify electronic components from their markings, read resistor colour codes and capacitor codes, test components with a multimeter, and operate the function generator, power supply and CRO.

Reading a resistor

Four-band colour code:

Colour	Digit	Multiplier
Black	0	×1
Brown	1	×10
Red	2	×100
Orange	3	×1 k
Yellow	4	×10 k
Green	5	×100 k
Blue	6	×1 M
Violet	7	×10 M
Grey	8	—
White	9	—

Tolerance band: gold ±5%, silver ±10%, brown ±1%, red ±2%.

Read: first two bands are digits, third is the multiplier, fourth is tolerance. Brown-black-red-gold = 1, 0, ×100 = 1 kΩ ±5%.

Which end to start from is the usual difficulty. The tolerance band is gold or silver — colours that never appear as digits — so it goes on the right. Hold the resistor with gold or silver at the right and read leftward.

Reading a capacitor

Ceramic disc, three digits: first two digits, third is the number of zeros, result in picofarads.

104 = 10 followed by 4 zeros = 100 000 pF = 100 nF = 0.1 µF
223 = 22 000 pF = 22 nF

Electrolytic capacitors are marked directly in µF and are polarised — the negative lead is marked with a stripe, and the longer lead is positive.

Connecting an electrolytic backwards is the classic first-year mistake. Reversed polarity causes the electrolyte to gas, and the capacitor heats, bulges and can burst. Check the stripe every time.

Testing components with a multimeter

Resistor — resistance range. Compare with the colour code. An open resistor reads infinite (OL).

Capacitor — capacitance range if available. Otherwise use the resistance range: a good capacitor shows a rising resistance as it charges, settling at a high value. A steady low reading means it is shorted; instant OL on a large capacitor suggests open. Always discharge a capacitor before testing.

Diode — diode-test mode. Forward: 0.6–0.7 V for silicon, about 0.3 V for germanium. Reverse: OL. A good diode conducts one way only. Shorted: near zero both ways. Open: OL both ways.

Identifying an unknown diode's cathode: the banded end is the cathode, and in diode-test mode the meter reads forward when the red lead is on the anode.

Transistor — treat it as two diodes back to back.

For an npn: with the red lead on the base, both base-emitter and base-collector read forward (~0.7 V); with the black lead on the base, both read OL. For a pnp it is reversed.

This test identifies the base and the type. The terminal that reads forward to both others is the base; if it does so with the red lead positive, the transistor is npn.

Distinguishing collector from emitter: the base-emitter reading is usually slightly higher than base-collector, and collector-emitter reads OL both ways in a good device.

JFET — gate-to-channel behaves as a diode; with no gate voltage the channel conducts between drain and source.

The instruments

Regulated DC power supply. Set the voltage with the load disconnected. Note the current limit control: set it to a modest value before connecting a new circuit, so that a wiring error limits the current instead of destroying the components. This single habit saves more components than any other.

Function generator. Produces sine, square and triangular waves with adjustable frequency and amplitude, and usually a DC offset. Check the output amplitude on the CRO rather than trusting the dial, and note that many generators are calibrated assuming a 50 Ω load.

CRO / DSO. Displays voltage against time.

Getting a trace when the screen is blank: set the timebase to about 1 ms/div, the vertical to 1 V/div, coupling to DC, trigger source to the channel in use, and trigger mode to auto. Auto mode sweeps even without a trigger, so a flat line appears and can then be adjusted — whereas in normal mode a missing trigger leaves the screen genuinely blank, which beginners read as a dead instrument.

Measuring: $V_{p-p} = \text{divisions} \times \text{volts/div}$ ; $T = \text{divisions} \times \text{time/div}$ ; $f = 1/T$ .

AC and DC coupling matters: DC coupling shows the whole signal including its offset; AC coupling blocks the DC and shows only the variation. To measure a small ripple sitting on a large DC level — the rectifier experiment — use AC coupling and increase the sensitivity.

Probe attenuation. A ×10 probe divides the signal by ten. If the scope is not told, every reading is out by a factor of ten. Check the probe setting matches the channel setting.

Tools

Soldering iron (25–40 W for electronics), desoldering pump, pliers, side cutters, wire strippers, screwdrivers, tweezers, crimping tool, and a hot air station for surface mount work.

Care of the iron: tin the tip when new and before putting it down; keep it clean on a damp sponge or brass wool; never file the tip; return it to the stand every time.