New Zealand NZART pool, section ZLF22

Subelement ZLF

Power Supplies

Section ZLF22

Regulated Power Supplies

ZLF2201

The block marked 'Filter' in the diagram is to

A.
filter RF radiation from the output of the power supply
Correct Answer
B.
smooth the rectified waveform from the rectifier
C.
act as a 50 Hz tuned circuit
D.
restore voltage variations

ZLF2201

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ZLF2202

The block marked 'Regulator' in the diagram is to

A.
regulate the incoming mains voltage to a constant value
B.
ensure that the output voltage never exceeds a dangerous value
C.
keep the incoming frequency constant at 50 Hz
Correct Answer
D.
keep the output voltage at a constant value

ZLF2202

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ZLF2203

The block marked 'Transformer' in the diagram is to

A.
transform the incoming mains AC voltage to a DC voltage
B.
ensure that any RF radiation cannot get into the power supply
Correct Answer
C.
transform the mains AC voltage to a more convenient AC voltage
D.
transform the mains AC waveform into a higher frequency waveform

ZLF2203

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ZLF2204

The block marked 'Rectifier' in the diagram is to

Correct Answer
A.
turn the AC voltage from the transformer into a fluctuating DC voltage
B.
rectify any waveform errors introduced by the transformer
C.
turn the sinewave output of the rectifier into a square wave
D.
smooth the DC waveform

ZLF2204

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ZLF2205

The block marked 'Regulator' in the diagram could consist of

A.
four silicon power diodes in a regulator configuration
B.
two silicon power diodes and a centre-tapped transformer
Correct Answer
C.
a three-terminal regulator chip
D.
a single silicon power diode connected as a half-wave rectifier

ZLF2205

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ZLF2206

In the block marked regulator in the diagram below, a reversed diode may be present across the regulator. Its job is to

A.
Block negative voltages from appearing at the output
B.
Blow a fuse if high voltages occur at the output
C.
Blow a fuse if negative currents occur at the output
Correct Answer
D.
Bypass the regulator for higher voltage at its output compared to its input

ZLF2206

Correct answer: bypass the regulator for higher voltage at its output compared to its input

A diode placed reverse-biased across a regulator acts as a protection device. If the input voltage suddenly falls (for example during power-off or a short circuit) while the output capacitor remains charged, the output can temporarily be at a higher voltage than the input. This reverse voltage can damage the regulator.

When this happens, the diode becomes forward biased and safely conducts current around the regulator, preventing excessive reverse voltage across it.

block negative voltages from appearing at the output is not the function of this diode, negative voltages are normally prevented by the rectifier and regulator stages.
blow a fuse if high voltages occur at the output is not the intent, the diode protects the regulator rather than acting as an overvoltage fuse trigger.
blow a fuse if negative currents occur at the output is incorrect, the diode allows reverse current to flow safely instead of forcing a fault.

Therefore, the diode’s purpose is to bypass the regulator when the output voltage becomes higher than the input voltage.

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ZLF2207

A power supply is to power a solid-state transceiver. A suitable over-voltage protection device is a

Correct Answer
A.
crowbar across the regulator output
B.
100 uF capacitor across the transformer output
C.
fuse in parallel with the regulator output
D.
zener diode in series with the regulator

ZLF2207

Correct answer: crowbar across the regulator output

Solid-state transceivers are highly sensitive to over-voltage. If a regulator fails and the output voltage rises, rapid protection is required to prevent damage.

A crowbar circuit detects an over-voltage condition and deliberately creates a short circuit across the supply, causing the power supply fuse to blow or the protection device to trip. This quickly removes power and protects the transceiver.

A capacitor across the transformer output smooths ripple but provides no over-voltage protection.
A fuse in parallel with the output does nothing useful; it will not blow unless current flows through it.
A zener diode in series would cause excessive voltage drop and power dissipation and is not a practical protection method.

Therefore, the correct over-voltage protection device is a crowbar across the regulator output.

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ZLF2208

In a regulated power supply, the 'crowbar' is a

A.
means to lever up the output voltage
B.
circuit for testing mains fuses
Correct Answer
C.
last-ditch protection against failure of the regulator in the supply
D.
convenient means to move such a heavy supply unit

ZLF2208

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ZLF2209

In a regulated power supply, 'current limiting' is sometimes used to

A.
prevent transformer core saturation
B.
protect the mains fuse
Correct Answer
C.
minimise short-circuit current passing through the regulator
D.
eliminate earth-leakage effects

ZLF2209

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ZLF2210

The purpose of a series pass transistor in a regulated power supply is to

A.
suppress voltage spikes across the transformer secondary winding
B.
work as a surge multiplier to speed up regulation
C.
amplify output voltage errors to assist regulation
Correct Answer
D.
allow for a higher current to be supplied than the regulator would otherwise allow

ZLF2210

Correct answer: allow for a higher current to be supplied than the regulator would otherwise allow

In a regulated power supply, the series pass transistor is placed in series with the load and is controlled by the regulator. Its purpose is to handle the load current, allowing the supply to deliver much more current than the regulator IC or control circuit could safely provide on its own.

The regulator controls the transistor’s base or gate, while the transistor supplies the bulk of the output current and dissipates the associated power.

Suppressing voltage spikes is the job of snubber circuits, capacitors, or transient suppressors, not the pass transistor.
A “surge multiplier” is not a valid function in regulation.
Error amplification is handled by the error amplifier inside the regulator, not the series pass device itself.

Therefore, the series pass transistor allows the supply to deliver higher output current than the regulator alone could manage.

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