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BASIC NEC 2023

Transistors, Thyristors & Solid-State Switching Devices Practice Test

Free online transistors and thyristors practice test. 25 basic electrical questions on BJT basics, SCRs, TRIACs, MOSFETs, and solid-state relays. Good for electrician exam prep.

TOPICS COVERED

Transistor Basics BJT Basics Transistor as Switch Transistor as Amplifier SCR Basics SCR Behavior

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Beyond simple diodes, modern electrical and control systems rely on transistors and thyristors — controllable solid-state devices used for switching and amplification. A bipolar junction transistor (BJT) can act as a switch or amplifier. An SCR (silicon-controlled rectifier) and TRIAC are thyristors used to control AC power, like in dimmer switches and motor soft-starters. Understanding how these devices are triggered and controlled is key to understanding modern control circuits.

25

Questions

38 min

Time

75%

Pass

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Transistors, Thyristors & Solid-State Switching Devices Practice Test — Question List

Question 1: A transistor is a solid-state device primarily used to:

Amplify or switch electronic signals
Store large amounts of electrical energy
Convert AC directly to mechanical motion
Generate voltage from nothing

Question 2: A bipolar junction transistor (BJT) has how many terminals, and what are they called?

Three -- base, collector, and emitter
Two -- anode and cathode
Four -- gate, source, drain, and body
One -- terminal only

Question 3: The two basic types of bipolar junction transistors are:

NPN and PNP
AC and DC
Series and parallel
Fixed and variable

Question 4: On a BJT, the "base" terminal is best described as:

The control terminal -- a small current here controls a much larger current between collector and emitter
The terminal that carries the largest current in the device
A terminal with no function at all
The terminal only used for cooling

Question 5: When a transistor is used purely as a switch, applying enough base current to fully turn it on is called putting it into:

Saturation
Cutoff
Breakdown
Rectification

Question 6: When a transistor used as a switch has zero (or insufficient) base current, so essentially no current flows between collector and emitter, it is said to be in:

Cutoff
Saturation
Avalanche
Forward bias

Question 7: When used as an amplifier (rather than a simple on/off switch), a transistor operates:

In its linear (active) region, between full cutoff and full saturation
Only ever in full saturation
Only ever in full cutoff
Amplifiers cannot use transistors at all

Question 8: SCR stands for:

Silicon-controlled rectifier
Standard capacitor relay
Single-conductor resistor
Series capacitor regulator

Question 9: An SCR has how many terminals, and what are they called?

Three -- anode, cathode, and gate
Two -- base and collector
Four -- source, drain, gate, and body
Five terminals

Question 10: An SCR is normally turned ON (begins conducting) by:

Applying a brief pulse of current to its gate terminal
Cooling the device to a very low temperature
Reversing the anode and cathode connections
Removing all voltage from the circuit

Question 11: Once an SCR is triggered on by a gate pulse, it will:

Stay conducting (latched on) even after the gate pulse is removed, until the anode-cathode current drops near zero
Turn off immediately once the gate pulse ends
Never conduct any current at all
Only conduct while the gate pulse is continuously applied

Question 12: The main way to turn OFF a conducting SCR in an AC circuit is:

Let the AC waveform naturally cross zero current, which is why SCRs are so commonly used in AC power control
Apply more current to the gate
Physically disconnect the gate wire only
SCRs can never be turned off once triggered

Question 13: A TRIAC is best described as a device that can be thought of as:

Two SCRs connected back-to-back, allowing current to be switched in both directions
A single diode with no gate control
A type of resistor with adjustable value
A device that only works with DC current

Question 14: A common everyday application of a TRIAC is:

A household lamp dimmer switch
A standalone battery charger with no other components
A pure DC-only voltage regulator
A fixed-value resistor

Question 15: What is the key functional difference between a simple diode and a transistor or thyristor?

A diode only allows current in one direction with no external control, while transistors/thyristors have a control terminal that lets you turn conduction on or off
They are functionally identical devices with different names
A diode has a gate terminal but a transistor does not
Transistors can only be used with AC, diodes only with DC

Question 16: Compared to a traditional electromechanical relay, a solid-state relay (SSR):

Has no moving parts or mechanical contacts, switching the load purely with semiconductor devices
Uses a physical armature and spring exactly like a mechanical relay
Only works with DC coils
Cannot switch any electrical load

Question 17: Solid-state power devices (transistors, SCRs, TRIACs) often need a heat sink because:

They dissipate heat internally while conducting current, and excess heat can damage or destroy the semiconductor
Heat sinks are purely decorative and serve no electrical purpose
Solid-state devices never generate any heat
Heat sinks are required only for devices operating on DC

Question 18: Compared to a BJT (which is controlled by current at its base), a MOSFET is generally described as:

Voltage-controlled, requiring very little continuous current at its control terminal (gate)
Controlled only by mechanical pressure
Identical in every way to a BJT
Unable to switch any current at all

Question 19: The three main terminals of a MOSFET are typically called:

Gate, drain, and source
Base, collector, and emitter
Anode, cathode, and gate
Positive, negative, and neutral

Question 20: Because an SCR only conducts in one direction (like a diode) once triggered, it is well suited for controlling:

DC power or a single half of an AC waveform
Only three-phase power, never single-phase
It cannot control any type of power
Only signals under 1 Hz

Question 21: One major advantage of solid-state switching devices (transistors, SCRs, TRIACs) over mechanical switches/relays for many applications is:

They can switch much faster and have no mechanical contacts to wear out or arc
They never generate any heat under any condition
They are always cheaper in every single application
They require no design considerations at all

Question 22: In transistor amplification, a small change in base current or voltage produces:

A proportionally much larger change in the collector current
No change in the collector current whatsoever
A change only in the device's physical temperature
A permanent, irreversible change in the transistor's structure

Question 23: The term "thyristor" is best understood as:

A general family name for latching, gate-triggered solid-state switching devices, which includes SCRs and TRIACs
Another name for a simple fixed resistor
A type of battery cell
A unit of electrical resistance

Question 24: Thyristors (SCRs/TRIACs) are commonly used inside motor "soft starters" mainly to:

Gradually ramp up the applied voltage/current to reduce the high inrush current of a direct motor start
Permanently disconnect the motor once started
Convert the motor to run on DC only
Increase the motor's horsepower rating

Question 25: Unlike a BJT (which stays on only while base current is present), a triggered SCR is different because it:

Continues conducting on its own after the trigger is removed, until current naturally drops near zero
Turns off the instant the gate pulse ends, just like a BJT loses base current
Cannot conduct any current under any condition
Requires a continuous gate signal at all times, more than a BJT does

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score75%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

Timer starts immediately. Progress is auto-saved.

Source: NEC 2026 (NFPA 70)