Test Equipment
A technician is only as good as their ability to measure. The Associate CET exam expects you to know what each instrument measures, how to connect it correctly, and its limits. This guide covers the core bench instruments — the multimeter, oscilloscope, signal sources, spectrum analyzer, and logic probe. Sections 2, 5 and 8–14 cover the rest of competency 8.0: how meters work, frequency counters, logic pulsers, substitution boxes, ESR meters, dummy loads, rheostats, variacs and isolation transformers, potentiometers, and the care of equipment and leads. Section 15 is job practice: testing components on the bench.
On the exam from this page
- a 10× probe's main job isn't just reducing voltage — it's reducing the capacitance the scope presents to the circuit
- Voltage = parallel; current = series, circuit broken.
- an ammeter shunt goes in parallel with the movement and carries the current above full scale; a voltmeter multiplier goes in series and drops the voltage above full scale
- high ESR marks a failing capacitor even when its capacitance still reads close to the marking
- a variac does NOT isolate you from the mains
- DC coupling shows the whole signal, AC coupling blocks the DC part so a small signal riding on a large DC level can be enlarged, and GND shows where zero volts is on the screen
- center frequency and span set the analyzer's frequency axis and the reference level sets the top of its amplitude axis; a narrower resolution bandwidth separates closer signals and lowers the noise floor but makes the sweep slower
The digital multimeter (DMM)
The DMM is the everyday workhorse. It measures DC and AC voltage, DC and AC current, resistance, and continuity, and most add diode test, capacitance, and frequency. The display is typically an LCD. A digital meter is generally preferred over an analog VOM for convenience and accuracy — no needle to interpret, no parallax error.
| Connect it… | Rule |
|---|---|
| Voltage | In parallel with the component — across it. A voltmeter has very high input impedance so it draws almost no current. |
| Current | In series — the circuit must be broken and the meter inserted so all current flows through it. A low-resistance shunt. |
| Resistance | On a de-energized circuit only — the meter supplies its own test current. Measuring a live circuit gives wrong readings and can damage the meter. Isolate the part too (lift one lead) so parallel paths don't skew the reading. |
| Continuity | De-energized; the meter beeps when resistance is near zero (a complete path). |
Safety
The classic mistake: leaving the leads in the current jacks (a near-short) and then probing a voltage source — that can blow the meter's fuse or worse. Voltage = parallel; current = series, circuit broken.Think of it
Range & loading: on a manual meter pick a range above the expected value. A good meter has high input impedance (≈10 MΩ) so it doesn't load the circuit and skew the reading. DMMs are generally not used in RF circuits — their frequency response is too limited.Think of it
AC readings — true-RMS vs average-responding: a basic meter measures the average of the rectified signal and scales it to show RMS, and that scaling is correct only for a pure sine wave. On a square wave it reads about 11% high; on distorted, PWM or switching-supply waveforms the error can be much larger. A true-RMS meter reads the real RMS value of any waveform within its bandwidth and crest-factor limits.A meter's own rating matters before any of this. On anything connected to the mains, the meter and its leads must carry a CAT rating for the place you measure (competency 8.3); Safety, section 11 explains the categories and why the voltage and the category both count.
How analog and digital meters work
Competencies 8.1 and 8.2 ask how analog and digital meters operate and what they are built from. An analog meter is built around a moving-coil movement: the D'Arsonval and Weston designs are both permanent-magnet, moving-coil (PMMC) movements. A coil on a pivot sits in a strong magnetic field, and current through it turns the coil and its needle against a spring, so the deflection is proportional to the current. Every movement has two ratings: its full-scale current, the current that drives the needle all the way across (1 mA or 50 µA, for example), and the internal resistance of its coil (500 Ω, for example). The movement is polarity-sensitive, so on AC it would only vibrate; an analog meter rectifies AC with diodes first, and its AC scale assumes a sine wave.
One movement serves every function and range because resistors around it share out the current or the voltage:
| Part | What it does |
|---|---|
| Shunt (ammeter ranges) | A low resistance in parallel with the movement. It carries all the current above the movement's full-scale current, so only the full-scale current flows through the movement at the top of the range. Shunts carry large currents and must be rated for the heat. |
| Multiplier (voltmeter ranges) | A resistor in series with the movement. It drops all the voltage above what the movement itself takes at full scale (its full-scale current times its resistance). |
| Ohmmeter battery and zero adjust | The ohmmeter has its own battery, which drives current through the unknown resistance. Short the leads and turn the ZERO OHMS control until the needle reads zero; if it won't reach zero, the battery needs replacing. The ohms scale runs backwards (zero at the right, infinity at the left) and is crowded at the high end, and half scale equals the meter's internal resistance. |
| Sensitivity (ohms per volt) | A voltmeter's resistance for every volt of range, set by the movement's full-scale current: a 1 mA movement gives 1 kΩ/V, a 50 µA movement 20 kΩ/V. The Simpson 260 is rated 20,000 Ω/V on DC volts, so on its 10 V range it puts 200 kΩ across the circuit. |
where Vm = Im × Rm; IFS and VFS are the new full-scale current and voltage. These are Ohm's law applied to the movement and are not on ETA's formula sheet.
Worked example
Extending a 1 mA, 500 Ω movement: at full scale the movement drops Vm = 1 mA × 500 Ω = 0.5 V. For a 100 mA range the shunt carries the other 100 − 1 = 99 mA, so Rsh = 0.5 V / 99 mA ≈ 5.05 Ω, wired in parallel. For a 10 V range the meter must total 10 V / 1 mA = 10 kΩ, so the multiplier is 10 kΩ − 500 Ω = 9.5 kΩ, wired in series. Its sensitivity is 1 / 1 mA = 1 kΩ/V.On the exam
Shunt or multiplier: an ammeter shunt goes in parallel with the movement and carries the current above full scale; a voltmeter multiplier goes in series and drops the voltage above full scale. A low ohms-per-volt rating means a low-resistance voltmeter, which loads the circuit more.A digital multimeter does the same jobs electronically. The meter scales each quantity to a small voltage (the classic ICL7106 converter reads 200 mV or 2 V full scale), an analog-to-digital converter (ADC) turns that voltage into a number, and the display shows it. Dual-slope (integrating) ADCs are common in DMMs: the converter integrates the input for a fixed time, then times how long a reference voltage takes to bring the integrator back, so noise spikes average out. That 3½-digit converter drives an LCD directly and gives a count of 1000 × Vin / Vref. A meter's counts set its resolution: a 4,000-count meter on its 4 V range resolves 1 mV. On its voltage ranges a DMM presents a fixed input resistance, commonly 10 MΩ, instead of an ohms-per-volt rating. For capacitance it charges the part with a known current and measures the voltage that results.
That difference is the loading story (competency 8.6) in numbers. A divider of two 100 kΩ resistors across 10 V has 5.0 V at its middle. The 20 kΩ/V meter on its 10 V range adds 200 kΩ across the lower resistor and reads 4.0 V; a 10 MΩ DMM reads about 4.98 V.
The oscilloscope
The oscilloscope draws a graph of voltage versus time — it's how you actually see a waveform's shape, amplitude, frequency, and timing. The two axes are the whole story:
Read amplitude by counting vertical divisions × the volts/div setting, and period by counting horizontal divisions × the time/div setting — then frequency is f = 1/T. A dual-trace scope shows two signals at once (two vertical inputs); it displays them by alternating sweeps between channels (ALT, for fast time bases) or by chopping between channels within each sweep (CHOP, for slow time bases). A Digital Storage Oscilloscope (DSO) samples the signal and uses a microprocessor and memory to capture and hold it for analysis — letting you freeze, zoom, and measure single events.
On the exam
The ×10 probe: a 10× probe's main job isn't just reducing voltage — it's reducing the capacitance the scope presents to the circuit, so it doesn't load high-impedance, high-frequency nodes. It also multiplies the usable input range.Competency 8.12.1 asks for the purpose of each front-panel control. Tektronix sorts them into vertical, horizontal and trigger sections, with the display and the input connectors:
| Control | Section | What it does (Tektronix) |
|---|---|---|
| Volts/div | Vertical | Scales the trace vertically: at 5 V/div an 8-division screen shows 40 V from bottom to top. Divide by the probe's attenuation if the scope does not do it for you |
| Position | Vertical | Moves the trace up or down the screen |
| Coupling: DC | Vertical | Shows all of the input signal, its AC and DC parts |
| Coupling: AC | Vertical | Blocks the DC part, so the waveform is centred on zero volts: for a small signal riding on a DC level too large for the volts/div setting |
| Coupling: GND | Vertical | Disconnects the input, so with the auto trigger mode a flat line shows where zero volts is |
| Bandwidth limit | Vertical | Cuts displayed noise, and the signal's high-frequency content with it |
| Time/div | Horizontal | Scales the time axis, the sweep speed |
| Position | Horizontal | On a digital scope, where the trigger falls in the record, so you can see what happened before it (pre-trigger viewing) |
| Source | Trigger | Which signal starts the sweep: any channel, an external input or the power line. Do not trigger on channel 1 while you look at channel 2 by mistake |
| Level and slope | Trigger | Slope picks a rising (positive) or falling (negative) edge; level picks the voltage on that edge where the trigger occurs |
| Mode | Trigger | Auto sweeps even without a trigger, so a trace always shows; normal sweeps only when the signal reaches the trigger point; single captures one sweep |
| Holdoff | Trigger | An adjustable time after each trigger during which the scope cannot trigger again, so it locks onto one point of a complex waveform |
| Coupling | Trigger | AC or DC, and on many scopes high-frequency, low-frequency or noise reject, to stop noise causing false triggers |
| Intensity | Display | The brightness of the trace, the Z axis of an analog scope |
Probe compensation. Tektronix: a passive attenuating probe must be compensated, balanced to the particular scope it is used on, and you should do it every time you set the scope up, because a poorly adjusted probe makes measurements less accurate. Attach the probe to a channel, connect its tip to the scope's square-wave reference terminal on the front panel and its ground clip to ground, and adjust the probe until the corners of the square wave are square.
Worked example
Ripple on a 5 V rail: the rail carries 50 mV of ripple. DC-coupled at 1 V/div, the ripple is 0.05 / 1 = 0.05 division high, too small to see; at 20 mV/div the 5 V level would sit 5 / 0.02 = 250 divisions up, far off the screen. AC-coupled, the 5 V is blocked and at 20 mV/div the ripple fills 0.05 / 0.02 = 2.5 divisions.Trap
AC coupling hides the DC level: a waveform that sits at +5 V looks exactly the same AC-coupled as one that sits at 0 V. To measure a DC voltage, or to see where a signal sits relative to ground, use DC coupling, and switch to GND first to find where zero volts is on the screen.On the exam
Exam point: DC coupling shows the whole signal, AC coupling blocks the DC part so a small signal riding on a large DC level can be enlarged, and GND shows where zero volts is on the screen. The trigger's level and slope set where on the waveform each sweep starts.Signal sources
Function / signal generator
Produces test waveforms — sine, square, triangle — at a set frequency and amplitude. Used to inject a known signal for tracing through a circuit. For audio work, ~1 kHz is the standard test tone.
RF signal generator
A calibrated source of RF at a precise frequency and level, often with modulation, for aligning and testing receivers and RF stages.
Competency 8.5 asks for the purposes and types of signal generators. They all do one job, supplying a known signal so you can see how a circuit responds, but they differ in what they can make:
| Type | What it makes and what it is for |
|---|---|
| Function generator | The common waveforms only: sine, square, ramp (triangle) and pulse, at a set frequency and amplitude. The bench workhorse for injecting a test signal, tracing a signal through stages and checking frequency response. |
| Arbitrary waveform generator (AWG) | Custom waveforms built point by point instead of preset ones, for example to simulate the signals in an anti-lock brake system or to stress-test a wireless link. An arbitrary/function generator (AFG), the most common architecture today, makes the standard waveforms digitally and can switch between frequencies almost instantly. |
| RF signal generator | A calibrated RF signal at a precise frequency and level, usually with AM, FM or phase modulation, for aligning and testing receivers. What matters is its frequency range and setting resolution, maximum output power and phase noise. |
| Vector signal generator | An RF generator that also makes digitally modulated signals (QAM, QPSK, FSK, OFDM and others), used for receiver sensitivity testing on digital radio systems. |
The 50 Ω output. Most function generators have a fixed 50 Ω source impedance in series with the output. The amplitude a generator displays assumes the load you told it to expect: set for a 50 Ω load but connected to a high-impedance input such as a scope, the actual output is twice the displayed value. Into a real 50 Ω load the source and load form a divider, so the load gets half of the generator's open-circuit voltage. Set the generator's load setting to match what it drives, or measure the amplitude with the load connected.
Frequency counters
Competency 8.7 asks what frequency counters are for and what limits them. A counter shows frequency as digits, far more precisely than you can read a period off a scope's graticule. A basic direct counter counts the cycles of the input during a known time, the gate time, which its internal timebase sets. Its resolution is therefore fixed in hertz: with a 1 s gate the smallest step it can show is 1 Hz, and a longer gate gives finer resolution but takes longer.
Worked example
Counting a 1 kHz tone: with a 1 s gate the counter counts 1,000 cycles, so it resolves 1 Hz and the ±1 count uncertainty is 1 in 1,000, or 0.1% of the reading. A 0.1 s gate updates ten times as often but counts only 100 cycles: 10 Hz resolution, and ±1 count is 1%.| Limit | What it means |
|---|---|
| ±1 count | The gate and the input are not synchronized, so the last digit can be off by one count either way. |
| Timebase accuracy | Every reading is only as accurate as the timebase. A plain quartz crystal shifts with temperature; an oven-controlled crystal oscillator (OCXO) is far more stable. |
| Frequency range | Each input has a top frequency. On the Keysight 53220A the standard inputs reach 350 MHz, and microwave frequencies need a separate optional 50 Ω input (to 6 or 15 GHz). |
| Sensitivity and false counts | The signal must cross both of the input's trigger thresholds to count (20 mV peak up to 100 MHz on the 53220A). Too small a signal gives no reading; noise or harmonics can trigger extra counts. Set the trigger level on the signal's steepest slope, and use the low-pass filter if the counter has one. |
| Input impedance and loading | Inputs are typically 1 MΩ or 50 Ω, selectable, with some capacitance (under 25 pF on the 53220A). The 50 Ω setting heavily loads a high-impedance circuit such as an oscillator, and even 25 pF is only about 640 Ω at 10 MHz. Connect at a low-impedance or buffered test point, and respect the input's damage level (1 W at 50 Ω on the 53220A). |
| Low frequencies | A direct counter needs a long gate to resolve a low frequency. A reciprocal counter measures the period and inverts it, so its resolution is a number of digits for each second of gate time, whatever the frequency. |
Spectrum analyzer
Where a scope shows voltage vs. time, a spectrum analyzer shows amplitude vs. frequency — it displays a signal's magnitude and frequency, revealing the carrier, harmonics, and spurious signals you can't pick out on a scope. Essential for RF and EMI work.
Competency 8.13 asks how a spectrum analyzer is used and operated. Rohde & Schwarz: most analyzers are heterodyne receivers that sweep across the span, measuring the signal level as a resolution filter moves through it. The horizontal axis is frequency and the vertical axis is power, read in dBm (decibels relative to 1 mW, section 6 of the Mathematics guide). Five settings do most of the work:
| Setting | What it does |
|---|---|
| Center and span | Set the frequency axis: the center frequency sits mid-screen and the span is the width shown, which together fix the start and stop frequencies (R&S) |
| Reference level | The top edge of the display, the most power expected at the input; set it slightly above the strongest signal. Too high wastes dynamic range. The analyzer also uses it to set its input attenuator (R&S) |
| RBW | The resolution bandwidth, the width of the final filter (Siglent). Two signals separate only if the RBW is smaller than their spacing, and ten times narrower RBW lowers the displayed noise floor by about 10 dB (R&S) |
| Sweep time | Narrow filters take longer to settle, so the sweep slows as the RBW narrows: Siglent gives sweep time as proportional to the span and inversely to the square of the RBW. Sweeping too fast causes amplitude and frequency errors (R&S) |
| VBW | The video bandwidth, a filter that smooths the trace and its noise; unlike the RBW it neither lowers the noise floor nor separates close signals (R&S, Siglent) |
Worked example
Choosing the RBW, and protecting the input: two carriers 20 kHz apart merge into one hump at 30 kHz RBW and separate at 10 kHz (Siglent's example). Going from 100 kHz to 10 kHz RBW lowers the noise floor by about 10 log(100 / 10) = 10 dB, and makes the sweep about 10² = 100 times slower over the same span. A 50 W mobile radio puts out 10 log(50 / 0.001) ≈ 47.0 dBm. Siglent's SSA3000X Plus accepts at most +30 dBm average (33 dBm damages it), so the radio needs at least 47 − 30 = 17 dB of attenuation just to avoid damage; through a 40 dB power attenuator rated for 50 W it arrives at about 7 dBm.Safety
Protect the analyzer's input: never connect a transmitter straight to a spectrum analyzer. Put a power attenuator rated for the transmitter's power between them, check that the level reaching the input is below the analyzer's maximum, and keep DC off the input: the SSA3000X Plus tolerates only ±50 V DC.On the exam
Exam point: center frequency and span set the analyzer's frequency axis and the reference level sets the top of its amplitude axis; a narrower resolution bandwidth separates closer signals and lowers the noise floor but makes the sweep slower.Logic probe & other tools
| Tool | What it does |
|---|---|
| Logic probe | Indicates digital high / low (and pulse) logic conditions at a point — a quick go/no-go for logic levels without a scope. |
| Analog VOM | Volt-Ohm-Milliammeter with a moving needle. Lower input impedance than a DMM; still useful for watching trends a digit display hides. |
| LCR meter | Measures inductance, capacitance, and resistance precisely — for characterizing reactive parts. |
| Frequency counter | Displays an exact frequency by counting cycles over a precise time gate. |
| Bench power supply | Provides adjustable, current-limited DC for powering circuits under test. |
Logic probes and logic pulsers
Competencies 8.10 and 8.11. A logic probe reads the state of a digital node; a logic pulser drives one. Used together they test a circuit by stimulus and response, in circuit, with no unsoldering.
Using the probe. Clip its black lead to the circuit's ground and its red lead to the circuit's supply (Vcc): the probe runs on the circuit's own power, and its CMOS thresholds are set as fractions of that supply. Set the TTL/CMOS switch to the logic family. On a typical probe, TTL reads high above about 2.3 V and low below 0.8 V, while CMOS reads high above 70% and low below 30% of the supply. One indicator shows HIGH, another LOW; neither lit means a bad level or an open, floating node. Pulses blink the lamp, a pulse far too short to see is stretched into a visible blink, and a memory setting latches a single pulse so you can catch an event that happens once.
Using the pulser. Touch the tip to a node and press the button: a high node is pulsed low and a low node pulsed high, and every input and output tied to that node sees the pulse. Between presses the tip is high-impedance, so it does not load the node. To check a gate, pulse its input and watch its output with the probe. With the probe and pulser on the same node, a node that stays low while you pulse it is shorted to ground. On a long chain, pulse one end, then check the middle and keep halving until you find the stage that stops the pulse.
| Precaution | Why |
|---|---|
| Power it from the circuit, within its rating | Both tools take their power from the circuit under test and are rated for logic supplies only (one common probe takes 5–15 V DC; HP's classic pulser 4.5–5.5 V for TTL and 3–18 V for CMOS). The probe's input is limited too (40 V on that probe). They are not for mains or high-voltage circuits. |
| Match the logic family | TTL and CMOS have different thresholds. On the wrong setting a probe can call a valid level bad, or a bad level valid. |
| Short pulses only | To force a node that an IC output is holding, the pulser must source or sink a large current. It is safe for the IC only because each pulse is a fraction of a microsecond long (about 0.3 µs on HP's pulser) and the button gives a low repetition rate, so the energy stays small. |
Trap
Bench trap: a probe with neither HIGH nor LOW lit is not reading a low. It is telling you the node is at a bad level or floating, such as an open trace or an input with nothing driving it.R-C-L substitution boxes
Competency 8.8. A substitution box (decade box) holds precision resistors, capacitors or inductors selected by switches, so one box can stand in for any value in its range. Resistance boxes such as IET's RS series cover 0 to 9,999,999 Ω in 1 Ω steps, and inductance boxes such as its LS-400 cover 0 to 9.999 H in 1 mH steps.
| Use | How |
|---|---|
| Test a suspect part | Unsolder or clip one lead of the suspect part, clip the box in its place and set the same value. If the fault goes away, the part was bad. |
| Find the right value | Dial values in a working circuit to home in on the one that gives the result you want, beyond what the design calculation gave you, then fit a fixed part of that value. |
| Check test equipment | A known, calibrated value verifies a meter before you rely on it. |
Limits. Each step has a power, current or voltage rating: the RS series allows 0.5 W a step (rising to 2.5 W at step 9), the LS-400's current rating falls from 700 mA on its smallest steps to 20 mA on its largest, and capacitor boxes built with small electronic switches may be limited to about 50–100 V. Check that the box is rated for the circuit, and power down for a moment if you are unsure what switching values will do.
ESR meters
Competency 8.9. A real capacitor's ESR (equivalent series resistance) is a single resistance standing for all of its internal ohmic losses. It matters most in switching power supplies, where the filter capacitors carry large, fast ripple currents. Part of the output ripple is simply the ripple current times the ESR, and with electrolytic capacitors that part dominates. The ripple current also heats the capacitor: the power lost is the ripple current squared times the ESR. ESR rises as an aluminum electrolytic ages, which means more heat for the same ripple current. In a switch-mode supply high ESR can cause starting failure, loss of regulation and excessive high-frequency noise on the outputs.
Worked example
Ripple from ESR alone: a buck converter's inductor ripple is 1.2 A peak-to-peak. With a 0.05 Ω output capacitor the ESR part of the ripple is 1.2 × 0.05 = 0.06 V (60 mV). If the capacitor's ESR rises to 0.5 Ω, it is 1.2 × 0.5 = 0.6 V. At 1 A RMS of ripple current, the heat in the capacitor goes from 1² × 0.05 = 0.05 W to 1² × 0.5 = 0.5 W.How an ESR meter works. It applies a small AC test signal, typically at 100 kHz, the frequency makers usually quote ESR at, and reads the resistance. Because the voltage it develops across the capacitor under test is very low (well under 100 mV), it won't forward-bias diodes or transistors, so you can test capacitors in circuit. Two cautions: capacitors in parallel read together, as their ESRs in parallel, and an in-circuit reading is less accurate than one on the bare part. Never connect it to powered equipment, and discharge large capacitors first.
| Standard electrolytic, room temperature | Typical ESR (Peak Electronic Design) |
|---|---|
| 10 µF, 16 V | about 16 Ω |
| 100 µF, 25 V | about 1.4 Ω |
| 1,000 µF, 35 V | about 0.11 Ω |
On the exam
Exam point: high ESR marks a failing capacitor even when its capacitance still reads close to the marking. Capacitance and ESR are separate end-of-life limits (Cornell Dubilier counts a part worn out when its capacitance falls below 80% or its ESR exceeds 200% of the initial requirement), so a capacitance check alone can pass a bad filter capacitor.Dummy loads
Competency 8.14. A dummy load is a resistor that stands in for the real load so equipment can run at full power without it. The common one is the RF dummy load: a 50 Ω termination for testing and adjusting transmitters without radiating. It goes on the transmitter's antenna connector in place of the antenna, so nothing goes on the air to interfere with anyone, and the transmitter sees a known, matched load while you measure its power and adjust it. Audio technicians use dummy loads too: in place of the speaker, one lets an amplifier be tested at full power without the noise, and keeps speaker pickup out of the measurement.
| Requirement | Why |
|---|---|
| The right impedance | 50 Ω for most RF work, with a low SWR across the band; Bird's 500 W oil-cooled load is specified at 1.1:1 maximum up to 1 GHz. A 75 Ω load on a 50 Ω transmitter is a 1.5:1 mismatch. |
| A big enough power rating | The load turns all the power into heat, so its rating must cover the transmitter's output for as long as you transmit. That Bird load dissipates 500 W continuously, cooled by oil and air convection. |
| Non-inductive construction | The resistor must be non-inductive: inductance in it would keep it from looking like a plain 50 Ω at radio frequencies. |
Worked example
Sizing a dummy load: a 100 W transmitter into 50 Ω puts V = √(100 × 50) ≈ 70.7 V RMS across the load and drives I = √(100 / 50) ≈ 1.41 A through it. A 50 W load would be asked to dissipate twice its rating; use one rated for at least 100 W for the length of time you transmit.Rheostats, variacs and isolation transformers
Competency 8.15 asks why technicians use these three, and why their size matters.
| Device | What it is and why you use it | Why size matters |
|---|---|---|
| Rheostat | A variable resistor wired with two terminals, the wiper and one end, placed in series with a load to set its current, such as a motor's speed. It wastes the dropped power as heat. | A rheostat has a maximum current as well as a wattage, and the current in each turn of its winding must stay below it at every setting. Ohmite lists a current for every value: a 50 W, 16 Ω rheostat is rated 1.76 A. |
| Variac (variable autotransformer) | Adjustable AC from one tapped winding. Use it to bring repaired equipment up slowly from 0 V while you watch the current (a current that rises sharply above some voltage points to breakdown or arcing), and to test equipment at low or high line voltage. | Its current rating applies at every setting: even at a low output voltage, the output current must never exceed it. |
| Isolation transformer | A transformer whose primary and secondary are separate windings. Equipment with a two-wire cord should be treated as hot chassis; the isolation transformer separates it from the AC line so test equipment can be connected. It does not make the circuit itself safe: you can still be shocked between two points in it. Connecting a scope's ground lead to a hot chassis without one shorts the hot side of the line, and clipping the chassis to earth ground is not a fix. | Its VA rating must cover the load, and a load's VA is its voltage times its current: a 300 VA unit rated 2 A, for example. |
On the exam
Exam point: a variac does NOT isolate you from the mains. Input and output share one winding, so the output is still tied to the AC line, and its terminals, brush and track are at mains voltage. For hot-chassis work you need an isolation transformer.Safety
Two hot chassis: never plug two pieces of hot-chassis equipment into the same isolated outlet at once. Unless their line-to-chassis connections are identical, there can be a serious shock hazard between the two chassis.Worked example
Why size matters: a 50 W, 16 Ω rheostat carries Imax = √(50 / 16) ≈ 1.77 A at full wattage across its whole winding, which is the 1.76 A Ohmite lists. Turned down to 4 Ω, it dissipates only 1.76² × 4 ≈ 12.4 W at that current, yet the current through every turn still in use is the same, so 1.76 A remains the limit. Working it out from 50 W and the 4 Ω setting (√(50 / 4) ≈ 3.5 A) would overload the winding. For a transformer, a unit that draws 1.5 A at 120 V is a 120 × 1.5 = 180 VA load: within that 300 VA, 2 A isolation transformer, and the 2 A limit still holds when you turn the voltage down.Potentiometers as variable voltage dividers
Competency 8.16. A potentiometer is a resistor with three terminals: the two ends of a resistive strip and a movable wiper that touches the strip. With the ends across a voltage, the wiper taps off a fraction of it, so the pot is a variable voltage divider whose ratio is set by the wiper's position. Wired with only the wiper and one end, the same part is a rheostat.
That formula assumes nothing draws current from the wiper. A load connected from the wiper to ground sits in parallel with the lower part of the strip and pulls the output down, so keep the load's resistance much larger than the pot's.
Worked example
A loaded divider: a 10 kΩ pot across 12 V with its wiper 3 kΩ from the grounded end gives 12 × 3 / 10 = 3.6 V with nothing connected. Add a 10 kΩ load from the wiper to ground: 3 kΩ in parallel with 10 kΩ is about 2.31 kΩ, so the output falls to 12 × 2.31 / (7 + 2.31) ≈ 2.98 V.Calibration & care
Measurements are only trustworthy if the instrument is. Equipment is periodically calibrated against a known standard; an out-of-cal meter gives confident, wrong answers. Respect input limits (voltage and current ratings), use the correct probe and range, and account for the instrument's own effect on the circuit. A reading only means something compared to the expected value from the schematic.
Competency 8.4 asks for the care of equipment and test leads. The meter makers' manuals say much the same thing:
| Care item | What to do |
|---|---|
| Inspect the leads | Before each use, look for nicked or cracked insulation, exposed metal, and gaps or cracks where the wire meets the plug or probe. Insulation dries out and turns brittle with age. Replace damaged leads; don't use a meter or leads that look damaged. |
| Test the leads | Short the probes together on the ohms range: good leads read about 0.5 Ω or less. Flex them while you watch, to catch an intermittent break. Use ohms, not the beeper: a continuity beeper can sound with up to 15 Ω in the leads. For heavily used leads, consider replacing them every year. |
| Rated leads | The leads must be rated for the same category and voltage as the meter, or higher (see Safety, section 11). |
| Prove the meter | Check it on a known voltage before you trust a reading; a meter that fails its checks needs calibration or repair. |
| Fuses | Replace a meter fuse only with one of the amperage, interrupt, voltage and speed ratings the maker specifies, with the leads removed. Check the meter's fuses before you measure current. |
| Battery | Replace it as soon as the low-battery indicator appears. |
| Cleaning | Wipe the case with a damp cloth and mild detergent, never abrasives or solvents, and never with the leads connected to a source. Dirt or moisture in the input terminals can affect readings. |
On the job: bench component testing
Job practice, not exam content. How technicians check parts on the bench with a DMM's resistance, diode-test and capacitance functions and an ESR meter. The readings below are typical; the part's datasheet, the schematic and a known-good part are the final reference.
Safety
Power off, then discharge: turn the equipment off and unplug it, discharge its capacitors through a resistor (Fluke suggests 20 kΩ, 5 W), and confirm with the meter that the voltage is gone before you use any resistance, continuity, diode or capacitance function. Voltage on a resistance range causes reading errors if it is low and damages the meter if it is high.Trap
In-circuit readings mislead: in circuit, the meter measures every path in parallel with the part, not just the part. A good 10 kΩ resistor with another 10 kΩ across it reads 5 kΩ, and semiconductor junctions on the board can change a resistance reading depending on which way round the leads are. Before you condemn a part on a low reading, lift one lead or take it out of the circuit and measure again.| Part | How to test it | Good | Bad |
|---|---|---|---|
| Resistor | Ohms range, power off. Lift one lead if the reading is low. For very low values use the meter's relative (REL) mode, which subtracts the leads' 0.2–0.5 Ω, and keep your fingers off the probe tips: your body is a parallel path. | Within its tolerance: a 4.7 kΩ ±5% resistor reads 4,465 to 4,935 Ω. | Open (OL), or outside its tolerance. |
| Capacitor: capacitance | Discharge it, take it out of circuit, and use the capacitance range: the meter charges it with a known current and measures the voltage. | Close to its marked value, within tolerance. | OL, or a value well off the marking. |
| Capacitor: ESR | ESR meter, in or out of circuit (see section 10), compared with a known-good part or the maker's figure. | Low, near the typical value for its size and voltage. | High ESR, even when the capacitance reads right. |
| Capacitor: leakage | The maker's method for an aluminum electrolytic: apply the rated voltage through a 1,000 Ω protective resistor and measure the current five minutes later, at 25 °C. | At or below the datasheet's leakage limit. | Above it. A part stored for years may need reforming first: rated voltage through 1,000 Ω for 30 minutes. |
| Diode | Diode-test mode, both directions. | Forward (red on the anode): 0.5 to 0.8 V for silicon. Reverse: OL. | The same low reading both ways (shorted), or OL both ways (open). |
| LED | Diode-test mode, forward. Some meters' diode test can't show a voltage that high (the Fluke 177 reads OL above 2.4 V). | A red LED (about 1.85 V typical) gives a reading. A good blue or white LED (about 3.0 to 3.6 V) can read OL on such a meter, so test it on a low-voltage supply through a series resistor (see the LED Resistor calculator). | Conducting in reverse, or dark on a supply that should light it. |
| BJT | Diode test between each pair of leads, both ways: six readings. A BJT tests as two diodes back to back, sharing the base. | Exactly two of the six conduct: base to emitter and base to collector, one way only (red on the base for NPN, black on the base for PNP), each roughly 0.6–0.7 V; base to emitter reads slightly higher. Collector to emitter: OL both ways. | Any collector-to-emitter conduction (unless the datasheet shows a built-in damper or free-wheeling diode, as on some power Darlingtons and horizontal-output transistors), a junction that reads the same both ways, or a junction that is open. |
| MOSFET (N-channel, enhancement) | Handle it as static-sensitive. Diode test drain to source, both ways, and gate to each other lead. Then the gate-charge check: touch red to the gate and black to the source to charge the gate, then read drain to source again with red on the drain and black on the source. Short the gate to the source to discharge it. | The built-in body diode reads one way only (red on the source, black on the drain). The gate is insulated: OL to drain and source both ways. After charging the gate, drain to source reads low; after discharging, it is back to the body diode only. | Drain to source low both ways (shorted), or any reading from the gate (a punctured gate oxide). If the gate-charge check fails, first check that your meter's diode-test voltage exceeds the gate threshold (2 to 4 V on a standard part such as the IRF460). |
| Inductor or transformer | Ohms across each winding, compared with the schematic or the datasheet. | Continuity at about the expected resistance. | An open winding. A shorted turn barely shows: one turn of a 500-turn, 10 Ω coil is 0.02 Ω, and the inductance hardly changes either. A ringing test finds it in a small coil; bring a power transformer up slowly on a variac while you watch the current. |
| Fuse | Out of its holder, ohms or continuity. | About 0 Ω. | OL: blown. A blown fuse is a symptom; find the fault before you replace it (Safety, section 8). |
| Switch | Ohms across the contacts in each position, operating it several times. | Closed: near 0 Ω. Open: OL. A beeper is only a rough check (the Fluke 177 beeps at anything under 25 Ω). | High or changing resistance when closed, or conducting when open. |
| Relay | Coil: ohms across it, against the datasheet. Contacts: energize the coil at its rated voltage from a supply and check each contact set in both states, using REL to subtract the leads. | Coil within tolerance (an Omron G5LE 12 V coil is 360 Ω ±10%). Closed contacts near 0 Ω (that relay's limit is 100 mΩ, less than your leads), open contacts OL. | An open or shorted coil, contacts that don't change over, or high resistance on closed contacts. |
Check yourself
Answer in your head first, then reveal.
How are a voltmeter and an ammeter connected?
Voltage = parallel; current = series, circuit broken.
What is the main job of a ×10 scope probe?
it's reducing the capacitance the scope presents to the circuit
What does a spectrum analyzer display?
a spectrum analyzer shows amplitude vs. frequency
How do you get a signal's frequency from a scope trace?
period by counting horizontal divisions × the time/div setting — then frequency is f = 1/T
When may you measure resistance?
On a de-energized circuit only — the meter supplies its own test current.
What do the oscilloscope's DC, AC and GND input coupling settings do?
DC coupling shows the whole signal, AC coupling blocks the DC part so a small signal riding on a large DC level can be enlarged, and GND shows where zero volts is on the screen
Which spectrum analyzer settings set its axes, and what does narrowing the resolution bandwidth do?
center frequency and span set the analyzer's frequency axis and the reference level sets the top of its amplitude axis; a narrower resolution bandwidth separates closer signals and lowers the noise floor but makes the sweep slower
Practice this topic: Test-equipment questions are in the Study Hub quiz bank, hands-on reading is in the Scope Lab, and key terms are in Flashcards.