T7D: Test instruments and soldering · Technician Exam Practice

T7D Test instruments and soldering

Using basic test instruments: voltmeter, ammeter, and ohmmeter; Soldering

This group covers basic bench instruments, namely the voltmeter, ammeter and ohmmeter, and good soldering practice.

The three basic meters

Instrument	Measures	Connected
Voltmeter	Voltage (electric potential)	In parallel with the component
Ammeter	Current	In series with the circuit
Ohmmeter	Resistance	Across the unpowered component

A multimeter combines these functions and measures voltage, current and resistance. It does not measure signal strength, noise, impedance or reactance.

To measure voltage across a part, put the meter in parallel. To measure current, break the circuit and put the meter in series so all the current flows through it. Putting a current-measuring meter across a component in parallel can short it out and damage the meter.

How an ohmmeter works

An ohmmeter applies a small current to the component and measures the resulting voltage, then shows resistance using Ohm’s law.

Always make sure the circuit is not powered when measuring in-circuit resistance. Outside voltages give false readings and can damage the meter.
Attempting to measure voltage while the meter is set to resistance can damage a multimeter. The ohmmeter function is not designed to be connected to a live circuit.
Across a large, discharged capacitor, the reading starts low and shows increasing resistance with time as the meter’s current charges the capacitor.

Soldering

Do not use acid-core solder for electronics. It is for plumbing, and its corrosive flux damages circuits.
Use rosin-core lead-tin solder, the standard for electronics. Lead-free tin-copper solders designed for electronics are also acceptable.
A good tin-lead joint is smooth and bright or shiny. A cold joint, where the metal was not hot enough or moved while cooling, has a rough or lumpy look and can be unreliable.

Key facts to memorize

Voltmeter: parallel. Ammeter: series.
Ohmmeter: small current in, voltage measured; circuit must be unpowered.
Multimeter: voltage, current and resistance.
Never use acid-core solder on electronics; use rosin-core.
Cold solder joint: rough or lumpy.
Ohmmeter across a discharged capacitor: resistance increases with time.

Memory hooks

Volts across, amps through. A voltmeter compares two points, so it bridges across the part (parallel). Current must flow through an ammeter, so it goes in line (series).
Ohms with the power off. An ohmmeter supplies its own current; a live circuit corrupts the reading and can damage the meter.
Acid is for pipes, rosin is for radios.
Shiny is fine, lumpy is cold.

Questions in this group (11): answers shown

T7D01

Which instrument would you use to measure electric potential?
- A. An ammeter
- B. A voltmeter ✓ Correct answer
- C. A potentiometer
- D. An ohmmeter
Electric potential is voltage, and a voltmeter measures it. An ammeter measures current, an ohmmeter measures resistance, and a potentiometer is a variable resistor rather than a meter.
T7D02

How is a voltmeter connected to a component to measure applied voltage?
- A. In series
- B. In parallel ✓ Correct answer
- C. In quadrature
- D. In phase
A voltmeter connects in parallel, across the component, so it measures the potential difference between its two terminals. A series connection is for measuring current.
T7D03

When configured to measure current, how is a multimeter connected to a component?
- A. In series ✓ Correct answer
- B. In parallel
- C. In quadrature
- D. In phase
To measure current, the meter must be in series so that all the current flows through it. Connecting it in parallel would effectively short the component through the low-resistance current-measuring meter.
T7D04

Which instrument is used to measure electric current?
- A. An ohmmeter
- B. An electrometer
- C. A voltmeter
- D. An ammeter ✓ Correct answer
An ammeter measures electric current in amperes. A voltmeter measures voltage, an ohmmeter measures resistance, and an electrometer measures very small charges or voltages.
T7D05

How does an ohmmeter measure the resistance of a circuit or component?
- A. By applying a small current and measuring the resulting voltage ✓ Correct answer
- B. By placing a variable resistor in parallel with the circuit
- C. By placing a variable resistor in series with the circuit
- D. By applying a variable voltage and measuring the resulting current change
An ohmmeter passes a small known current through the component and measures the resulting voltage, then displays the resistance using Ohm's law. It does not work by placing a variable resistor in series or parallel with the circuit under test.
T7D06

Which of the following can damage a multimeter?
- A. Attempting to measure resistance using the voltage setting
- B. Failing to connect one of the probes to ground
- C. Attempting to measure voltage when using the resistance setting ✓ Correct answer
- D. Not allowing it to warm up properly
Using the resistance setting on a live circuit applies the circuit's voltage to the ohmmeter's sensitive circuitry and can damage the meter. Measuring resistance with the voltage setting, or not grounding a probe, is not harmful in this way.
T7D07

Which of the following measurements are made using a multimeter?
- A. Signal strength and noise
- B. Impedance and reactance
- C. Voltage and resistance ✓ Correct answer
- D. All these choices are correct
A multimeter measures voltage, current and resistance. Signal strength, noise, impedance and reactance need specialized instruments such as analyzers or signal meters.
T7D08

Which of the following types of solder should not be used for radio and electronic applications?
- A. Acid-core solder ✓ Correct answer
- B. Lead-tin solder
- C. Rosin-core solder
- D. Tin-copper solder
Acid-core solder, used for plumbing, leaves corrosive residue that damages electronics. Rosin-core lead-tin solder is the standard choice for radio and electronic work.
T7D09

What is the characteristic appearance of a cold tin-lead solder joint?
- A. Dark black spots
- B. A bright or shiny surface
- C. A rough or lumpy surface ✓ Correct answer
- D. A greenish tinge
A cold joint, which did not get hot enough or moved while cooling, looks rough, dull or lumpy. A good joint is smooth and shiny, and cold joints can cause unreliable connections.
T7D10

What reading indicates that an ohmmeter is connected across a large, discharged capacitor?
- A. Increasing resistance with time ✓ Correct answer
- B. Decreasing resistance with time
- C. Steady full-scale reading
- D. Alternating between open and short circuit
A discharged capacitor draws current from the ohmmeter as it charges, so the reading starts low and climbs with time as the capacitor fills. A steady reading would indicate an ordinary resistance, not a charging capacitor.
T7D11

Which of the following precautions should be taken when measuring in-circuit resistance with an ohmmeter?
- A. Ensure that the applied voltages are correct
- B. Ensure that the circuit is not powered ✓ Correct answer
- C. Ensure that the circuit is grounded
- D. Ensure that the circuit is operating at the correct frequency
Remove power from the circuit before measuring in-circuit resistance. The ohmmeter supplies its own test current, and external voltages give wrong readings and can damage the meter.