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T7D

PRACTICAL CIRCUITS

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

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T7D011 of 11

Which instrument would you use to measure electric potential?

Why Electric potential, or potential difference, is measured in volts, and the instrument that reads volts is a voltmeter. It is connected in parallel across the two points whose difference in potential you want to know, and it has very high internal resistance so it draws almost no current from the circuit.
Watch out A potentiometer sounds like the right word, but it is a variable resistor used to adjust a voltage, not an instrument that displays a reading. An ammeter measures current in amperes and goes in series; an ohmmeter measures resistance with the power off.
Potential = volts = voltmeter, connected across (parallel). Ammeter goes in line (series).
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D022 of 11

How is a voltmeter connected to a component to measure applied voltage?

Why Voltage is a difference in potential between two points, so a voltmeter must touch both ends of the component at once, which means connecting it across (in parallel with) the part. A voltmeter has very high internal resistance, so bridging it across a circuit draws almost no current and barely disturbs what you are measuring. Ammeters are the opposite: low resistance, inserted in series so all the current flows through them.
Watch out The series choice describes how an ammeter is connected; wiring a high-resistance voltmeter in series would block current and give a meaningless reading. Quadrature and in phase refer to signal phase relationships, not meter hookups.
Volts across, amps through. Voltmeter goes in parallel, ammeter breaks the circuit.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D033 of 11

When configured to measure current, how is a multimeter connected to a component?

Why Current is the flow of charge through a path, so to measure it the meter must become part of that path. You break the circuit and insert the meter so all the current under test flows through it. An ammeter is designed with very low internal resistance so it drops almost no voltage and does not disturb the circuit.
Watch out Connecting across a component in parallel is how you measure voltage; doing that with the meter set to current puts a near short circuit across the part and typically blows the meter's fuse.
Amps go THROUGH (series), volts go ACROSS (parallel).
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D044 of 11

Which instrument is used to measure electric current?

Why Current, the flow of charge measured in amperes, is measured with an ammeter. An ammeter is connected in series with the circuit so all the current being measured flows through the meter, and it has very low internal resistance so it does not disturb the circuit. The name matches the unit: amperes are read on an ammeter.
Watch out A voltmeter measures potential difference and connects in parallel across a component; an ohmmeter measures resistance and must be used on an unpowered circuit.
Amps -> ammeter, in series. Volts -> voltmeter, in parallel. Ohms -> ohmmeter, power off.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D055 of 11

How does an ohmmeter measure the resistance of a circuit or component?

Why An ohmmeter contains its own battery or current source. It pushes a small, known current through the component under test and measures the voltage that develops across it, then uses Ohm's law (R = V/I) to display resistance. Because the meter supplies its own current, the circuit being measured must have power removed first, or the reading will be wrong and the meter may be damaged.
Watch out The choice about applying a variable voltage and watching current change describes a sweep or curve-tracer style measurement, not how a simple ohmmeter works; the variable-resistor choices describe a bridge or rheostat arrangement, not a modern ohmmeter.
Ohmmeter = Ohm's law in a box: it injects known current I, reads V, divides to get R. Always power off first.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D066 of 11

Which of the following can damage a multimeter?

Why On the resistance range a multimeter supplies its own small test current from an internal battery and expects the circuit to be unpowered, so its input is a low-impedance path rather than the high-impedance input used for volts. Connecting that low-impedance ohms input across a live voltage source pushes far more current through the meter than the ohms circuit was designed for, which can burn the input, the range resistors, or the fuse. Always de-energize the circuit and switch the meter to a voltage range before touching live terminals.
Watch out Measuring resistance while set to volts simply gives a meaningless or zero reading; nothing is harmed because the voltage input is high impedance and passive.
Ohms mode is for dead circuits only: ohms plus live volts equals smoke.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D077 of 11

Which of the following measurements are made using a multimeter?

Why A multimeter is a combination instrument that measures voltage (as a voltmeter), current (as an ammeter) and resistance (as an ohmmeter). Those DC/AC electrical quantities are what the basic analog or digital multimeter is built to read. Resistance is measured with the circuit powered off, using the meter's internal battery to push a small test current through the component.
Watch out Impedance and reactance are frequency-dependent AC quantities that need an impedance bridge, antenna analyzer or vector network analyzer, not an ordinary multimeter; signal strength and noise are measured with receivers, S-meters or spectrum analyzers.
Multi = volts, ohms, amps. If the quantity depends on frequency (Z or X), a multimeter can't see it.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D088 of 11

Which of the following types of solder should not be used for radio and electronic applications?

Why Solder contains a flux core that cleans the metal so the joint wets properly. Acid flux is designed for plumbing and sheet metal, and the acid residue keeps corroding copper traces, wires and component leads long after the joint cools, eventually causing high resistance or open connections. Electronics work uses rosin flux, which leaves a non-corrosive residue that is safe to leave on the board.
Watch out Lead-tin and tin-copper describe the metal alloy, not the flux, and both are common electronics solders; rosin-core is exactly what you should be using.
Acid is for pipes, rosin is for radios.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D099 of 11

What is the characteristic appearance of a cold tin-lead solder joint?

Why A good tin-lead solder joint flows smoothly and solidifies into a bright, shiny, smooth fillet. If the joint is not heated enough, or if it moves while cooling, the solder freezes in a disturbed, grainy state and the surface looks dull, rough, or lumpy. Such a cold joint is mechanically weak and often makes an intermittent or high resistance electrical connection, so it should be reheated and reflowed.
Watch out A bright or shiny surface is exactly what a properly made tin-lead joint looks like, so that choice describes a good joint, not a cold one. Greenish tinge suggests corrosion or flux residue rather than a cold joint.
Shiny and smooth means good; dull, rough and lumpy means cold. Heat it again.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D1010 of 11

What reading indicates that an ohmmeter is connected across a large, discharged capacitor?

Why An ohmmeter works by pushing a small current from its internal battery through the component and measuring the voltage. A discharged capacitor initially looks like a short, so a lot of current flows and the meter reads near zero ohms. As the capacitor charges up toward the meter's battery voltage, the current tapers off and the indicated resistance climbs toward infinity (open circuit). That rising reading is the classic sign you are looking at a large capacitor.
Watch out The choice saying resistance decreases with time has the charging curve backwards; current starts high and falls, so resistance starts low and rises. A steady full-scale reading would mean an already charged capacitor or a simple open circuit.
Discharged cap starts as a short and ends as an open, so the ohms reading only goes up.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7D1111 of 11

Which of the following precautions should be taken when measuring in-circuit resistance with an ohmmeter?

Why An ohmmeter works by applying its own small internal voltage and measuring the resulting current, then computing R = V/I. Any external voltage in the circuit adds to or fights that internal source, giving meaningless readings and possibly damaging the meter. So always remove power, and discharge any large capacitors, before touching an ohmmeter to a circuit.
Watch out The choices about correct applied voltage or correct operating frequency describe checks you would make on a live circuit with a voltmeter or frequency counter, which is exactly the opposite of what resistance measurement requires.
Ohmmeter brings its own battery: power off, caps discharged, then measure.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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