Canadian Basic Qualification pool, section B-003-017

Subelement B-003

B-003

Section B-003-017

B-003-017-001

You construct a simple DC power supply using a transformer, rectifier and filter capacitor. If you use the supply to power a CW transmitter, what problem with signal quality could it cause?

A.
Overmodulation
B.
Key clicks
C.
Harmonics
Correct Answer
D.
Chirp

B-003-017-001

Without a regulator, the output voltage of the power supply could vary significantly when the transmitter switches from idle into transmit mode (due to sudden increased current draw). Such voltage variations could lead to changes in the transmitter frequency.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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B-003-017-002

What device converts 120 volts AC to 12 volts DC?

A.
Inverter
Correct Answer
B.
Power supply
C.
Power conditioner
D.
Low-pass filter

B-003-017-002

A large percentage of modern transceivers are designed to work off 12 volts DC, which is readily available from a car battery. To use a rig in the home, a 'Power Supply' is required to convert 120 volts AC down to 12 volts DC.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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B-003-017-003

When selecting a 13.8 V DC power supply for a transceiver, what design specification is most important?

A.
Output connection compatibility
B.
Undervoltage protection
Correct Answer
C.
Output current capability
D.
Voltage and current metering

B-003-017-003

Key word: TRANSCEIVER. The maximum current that a power supply can safely deliver is a key parameter of power supplies. Receivers rarely draw more than 1 ampere at 12 V DC. A 100-watt transceiver (while on transmit) can draw 20 amperes at 12 V DC.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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B-003-017-004

Compared to a switching (switch mode) power supply, why may a linear power supply be preferred?

A.
Better regulation for FM equipment
B.
Higher efficiency
Correct Answer
C.
Lower risk of radio frequency noise
D.
Reduced physical dimensions and weight

B-003-017-004

Unless designed with great care, a switching power supply is more likely to present radio frequency noise on its DC output or towards its AC input. However, it is lighter, more compact and typically more efficient.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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B-003-017-005

In a mobile installation, why should the fuse in the DC line to the transceiver be located as near to the battery as possible?

A.
To better absorb voltage transients
B.
To prevent the vehicle's electronic systems causing noise
C.
To reduce the voltage drop in the radio's DC supply
Correct Answer
D.
To protect the entire circuit

B-003-017-005

The purpose of a fuse is to protect a circuit from overcurrent. It is wise to install it as close as possible to the source of power. That way, regardless of where a short-circuit occurs, protection is available.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

B-003-017-006

Apart from efficiency, what is one advantage of a switching (switch mode) power supply over a linear power supply?

A.
Lower risk of radio frequency noise
B.
Different simultaneous output voltages
C.
Simpler to repair
Correct Answer
D.
Reduced physical dimensions and weight

B-003-017-006

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

B-003-017-007

Why are heavy-gauge wires used for a 100-watt transceiver's DC power connection?

A.
To minimize ripple
Correct Answer
B.
To minimize the voltage drop
C.
To prevent an electrical shock
D.
To avoid RF interference

B-003-017-007

Transceivers in that class typically operate on 13.8 volts and require 20 amperes on transmit. Any conductor has a small resistance. Imagine a 2-metre power cord purposely undersized, for example, #16 gauge (1,3 mm, 13 milliohms/metre). The voltage drop would be 1.04 volts (4 * 0.013 ohm * 20 amperes). The power dissipated in the cord alone would be 20 watts (1 volt * 20 amperes).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

B-003-017-008

What are the nominal power-line voltages supplied to homes?

Correct Answer
A.
120 volts and 240 volts
B.
110 volts and 220 volts
C.
100 volts and 200 volts
D.
130 volts and 260 volts

B-003-017-008

Nominal household voltages have slowly come up since the early 20th century from 110 V, to 115 V, to 117 V, to 120 V. The current standard is 120 V and 240 V. 240 V is used for energy-hungry devices like water heaters, clothes dryers, electric ovens AND high-power linear amplifiers.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

B-003-017-009

Your transceiver's user guide suggests limiting the voltage drop to 0.5 volts and the vehicle battery is 3 metres away. Given the losses listed below at the required current of 22 amperes, which minimum wire gauge must you use?

A.
Number 14, 0.19 V per metre
B.
Number 8, 0.05 V per metre
C.
Number 12, 0.11 V per metre
Correct Answer
D.
Number 10, 0.07 V per metre

B-003-017-009

Understand that DC power is brought to the radio over a pair of wires. Each wire must not drop more than 0.25 volts (half the given value) over 3 metres. Thus, the loss per metre must be below 0.08 volts. The run must be at least number 10 gauge. Voltage drops (E = R * I) at that current were computed for you from resistance value per unit length available from wire tables. [ # 14 = 1.63 mm, # 12 = 2.05 mm, # 10 = 2.59 mm, # 8 = 3.26 mm ]

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

B-003-017-010

Why must the positive lead from the vehicle battery to your transceiver be fused?

A.
To prevent interference to the vehicle's electronic systems
B.
To protect the radio from transient voltages
C.
To reduce the voltage drop in the radio's DC supply
Correct Answer
D.
To prevent an overcurrent situation from starting a fire

B-003-017-010

A car battery can deliver a hundred amperes or more into a short-circuit; the voltage drop in any current-carrying wire and such large currents produce heat (P = E * I), enough heat to melt wire insulation and other plastics which abound in cars. Fuses close to the battery ensure excessive current is interrupted regardless of where the fault occurs over the DC power line to the radio.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

B-003-017-011

You have a very loud low-frequency hum appearing on your transmission. In what part of the transmitter would you first look for the trouble?

A.
The driver circuit
Correct Answer
B.
The power supply
C.
The variable-frequency oscillator
D.
The power amplifier circuit

B-003-017-011

Key word: HUM. Remember the 'Power Supply' block diagram: a 'Rectifier' (diode) converts AC into 'pulsating DC'. A 'Filter' then turns the 'pulsating DC' into pure DC. If the 'Filter' is deficient, hum or buzzing will appear on the transmitted signal.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

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