A Guide to Electrical Lab Equipment for Power Electronics Experiments

An electrical lab for power electronics experiments is built around equipment that can source, switch, measure and safely dissipate electrical power at levels far above those used in a basic electronics lab. The electrical experiment equipment in this lab ranges from a Variac and a regulated DC supply to thyristor trainer kits, rheostat loads and isolated measuring instruments. Each item exists because a converter, chopper or inverter experiment makes a specific demand on the bench.

Power electronics is the study of converting electrical power from one form to another using semiconductor switches such as diodes, SCRs, TRIACs, MOSFETs and IGBTs. Rectifiers convert AC to DC, choppers convert one DC level to another, inverters convert DC to AC, and AC voltage controllers vary the RMS value of an AC supply. Every one of these conversions can be demonstrated on a bench, but only if the lab electrical equipment around the circuit is chosen correctly.

This guide covers what each category of equipment does, how the instruments should be selected, a worked numerical example that links meter choice to converter output, which experiments use which equipment, the safety rules that apply, and how the whole setup relates to Indian university labs and competitive exams.

What Makes a Power Electronics Lab Different from a General Electrical Lab?

A general electrical lab, built around circuit theorems, transformers and machines, works with sinusoidal waveforms and steady readings. A basic electronics lab works with small signals at a few volts. A power electronics lab sits between the two and adds a difficulty of its own: the waveforms are chopped, the voltages are mains-level, and the circuits are switched at speeds that make a casual measurement misleading.

Three consequences follow from this. First, the supply must be variable and, where possible, isolated from the mains. Second, the load must be able to absorb real power without overheating. Third, the measuring instruments must be able to read non-sinusoidal waveforms and must not create a short circuit through their own ground connection.

A laboratory text on power electronics experiments states that completely benign power electronics experiments are not possible, and that damaged components or costly repairs follow when basic safety rules are ignored. That is the right mindset for equipment selection: every item on the list either supplies power, controls it, absorbs it, measures it or protects the person at the bench.

Readers who want the foundation first can start with NvisTech's guide to what equipment an electronics engineering lab needs, which covers the small-signal instruments that this lab builds on.

Which Power Sources Does the Lab Need?

The source is where most beginners go wrong, because a mains socket is not an acceptable supply for a converter experiment. Three types of sources cover the usual syllabus.

A Variac, also called an autotransformer or variable transformer, lets the AC input be raised slowly from zero to the required value. Starting an experiment at zero and winding the voltage up is the standard procedure, because a wiring error then shows up as an abnormal current at low voltage rather than as a failed device at full voltage. One caution matters here: a Variac changes the voltage but does not provide isolation, because its output is still electrically connected to the mains.

An isolation transformer, with a 1:1 ratio, provides the isolation that the Variac lacks. It separates the circuit under test from the earthed mains, so that touching one conductor does not complete a path to ground through the body. A common arrangement is to feed the Variac from the mains and pass its output through the isolation transformer before it reaches the circuit. Some commercial power electronics trainers also offer an optional isolation transformer for the oscilloscope, which the instruments section below explains.

A regulated DC power supply, usually 0 to 30 V with an adjustable current limit, covers the DC side of the syllabus: gate and base drive for device characteristic experiments, the input of a chopper, or a battery-like source for a small inverter. The current limit works as a safety feature as much as a specification, since it caps the fault current when a student wires a short.

Three-phase experiments, such as a three-phase fully controlled bridge rectifier, need a three-phase supply with a three-phase autotransformer or a transformer bank. Many Indian lab inventories list this as a separate three-phase module rather than something students build from loose components.

What Trainer Kits and Power Modules Does the Lab Use?

Most Indian engineering college labs run power electronics experiments on modular trainer kits rather than loose components. The equipment inventory published by NIT Hamirpur's power electronics lab is a representative example. It lists modules for SCR, MOSFET, IGBT and TRIAC static characteristics, single-phase and three-phase half and fully controlled bridge converters, forced commutation circuits, a Jones chopper, a UJT firing module, a push-pull inverter, a modified McMurray-Bedford full bridge inverter, AC phase control using SCR, TRIAC and DIAC, and a PWM inverter control module.

Reading that list as a set of categories is more useful than memorizing it. A device characteristic module holds the power device on a board with its gate drive circuit, bias supplies and meter terminals, so that the V-I curve can be plotted safely. A firing circuit module generates the gate pulses that trigger an SCR at a chosen delay angle, using R, RC or UJT triggering. A converter module combines the power devices, the firing circuit and the output terminals for a controlled rectifier, chopper, inverter or AC voltage controller. A commutation module demonstrates how an SCR is turned off in circuits where the supply does not do it naturally.

For the inverter experiments in particular, NvisTech's NV6001 Inverter Trainer is built so that the basics of inverter operation can be understood step by step, and its NV6003 Power Supply Trainer covers the transformer, rectifier, filter and regulator stages that appear in the first few power supply experiments.

More advanced setups move beyond fixed trainers. IIT Madras describes a DC-DC converter analysis setup in its power electronics laboratory that uses a DSP controller for PWM, with adjustable resistors, inductors and capacitors on flexible power circuit boards, so that buck, boost, buck-boost, forward, flyback and interleaved boost topologies can be built on one platform. The same lab also lists a three-phase inverter setup with a rheostat load, a variable inductor load and a Variac.

Which Loads and Passive Components Are Needed?

A converter experiment is not complete until the converter drives a load, and the choice of load changes the result. Resistive loads show the basic waveform. Inductive loads, usually an R-L combination, show how the output current behaves when it cannot change instantly, which is where freewheeling diodes and continuous conduction become visible.

Tubular rheostats are the standard resistive load in Indian labs, and the current rating on the nameplate matters as much as the resistance. A 50 ohm rheostat rated at 5 A can carry a higher current than a 100 ohm unit rated at 2 A, and exceeding the rating overheats the winding. The load must be chosen after calculating the expected current, which the worked example below demonstrates.

An iron-core variable inductor supplies the inductive part of an R-L load. Lamp loads are useful for demonstrating AC voltage control and dimmer circuits, since the brightness gives immediate visual feedback. A small DC motor or a single-phase induction motor appears in drive experiments such as converter-fed DC motor speed control. Electrolytic and film capacitors of suitable voltage rating handle filtering, snubber and commutation duties, and they should be discharged before being touched, because an electrolytic capacitor can retain charge after the supply is switched off.

Which Measuring Instruments Does the Lab Need?

Measurement is where the lab electrical equipment list becomes most specific, because the wrong instrument gives a confidently wrong answer.

Which Ammeters and Voltmeters Suit Converter Outputs?

A converter output is not a clean DC level or a clean sine wave. A controlled rectifier produces a pulsating waveform with a DC component and an AC ripple. Two meter types respond to it very differently. A permanent magnet moving coil (PMMC) meter responds to the average value of the waveform, so it reads the DC component. A moving-iron or dynamometer-type meter responds to the RMS value, so it reads the heating effect of the entire waveform.

The lab therefore needs both types, and the student needs to know which one to read for which purpose. The DC load voltage of a rectifier is read on a PMMC meter, while the RMS value that determines heating in the load or in a transformer is read on a moving-iron meter or a true-RMS digital meter. A digital multimeter that is not labelled true-RMS can give wrong AC readings on a chopped waveform, so the specification is worth checking before use.

Why Does a Power Electronics Lab Need a Digital Storage Oscilloscope?

The waveform shape is the main evidence in a power electronics experiment: the firing delay, the conduction interval, the commutation spike and the PWM pattern all show up on screen and nowhere else. A two-channel digital storage oscilloscope (DSO) is the standard instrument, and the laboratory text mentioned above assumes at least a two-channel scope with 60 MHz bandwidth as the baseline.

A DSO is preferred over an analog CRO because it can capture a single event such as a turn-on transient and hold it on screen. A scope with only a ground-referenced input, however, creates a safety problem that the next section explains.

Why Is a Differential Probe Needed?

A mains-powered oscilloscope has its probe ground clip connected to earth through the instrument's power cord. Clipping that ground lead onto a floating point in a converter, such as one side of a bridge, creates a direct path to earth and can burn out the probe lead, damage the circuit or injure the user. This remains true even when the scope is switched off, because the earth connection stays in place.

There are three standard remedies. A differential voltage probe, which measures between two points without referencing either to ground, is the usual choice, and educational lab versions are typically rated up to about 1000 V. A battery-powered handheld oscilloscope with isolated channels removes the earth path altogether. An isolation transformer on the scope supply is the third option, and some Indian lab manuals explicitly instruct students to use an isolated supply for the CRO in power electronics experiments. Some practitioners prefer to leave the scope earthed and isolate the circuit under test instead, and cutting the earth pin off a scope plug is never an acceptable shortcut, since it removes the safety earth. Whichever route the lab follows, the rule should be written into the manual rather than left to individual judgment.

What About Current Probes and Power Meters?

A current probe clamps around a wire and produces a voltage proportional to the current, so that current waveforms can be viewed on the scope without opening the circuit. Educational current probes are more expensive than voltage probes and typically go up to about 100 A. A digital power meter, which reads real power, power factor and harmonic content directly, is a valuable addition for experiments on power factor and distortion, though a basic lab can survive without one.

How Do the Readings Change with the Meter? A Worked Example

The following problem is in the style of GATE and university practical examinations. It ties the load, the source and the instrument together.

Problem. A single-phase fully controlled bridge converter is supplied from a Variac set to 100 V RMS. It feeds a highly inductive load with a resistance of 50 ohm, so the load current can be treated as constant (ripple-free) and conduction is continuous. The firing angle is α = 60°. Find the average output voltage, the average load current, the load power, and the RMS output voltage. State what a PMMC voltmeter and a moving-iron voltmeter would read across the load.

Step 1: Find the peak supply voltage. Let Vs be the RMS supply voltage and Vm the peak value.

Vm = √2 × Vs = 1.4142 × 100 = 141.42 V

Step 2: Find the average output voltage. For a single-phase fully controlled bridge with continuous conduction, the average output voltage is

Vdc = (2Vm / π) cos α

Vdc = (2 × 141.42 / 3.1416) × cos 60° = 90.03 × 0.5 = 45.02 V

Step 3: Find the average load current.

Idc = Vdc / R = 45.02 / 50 = 0.90 A

Step 4: Find the load power. With ripple-free current, the load power is

P = Idc² × R = 0.90² × 50 = 40.5 W

Step 5: Find the RMS output voltage. In continuous conduction the output voltage is the supply waveform with alternate half-cycles reversed in polarity at the firing instants, and its RMS value equals the RMS value of the supply, regardless of α.

Vo(rms) = Vm / √2 = 100 V

Step 6: Interpret the meter readings. A PMMC voltmeter reads the average value, so it shows about 45.0 V. A moving-iron voltmeter reads the RMS value, so it shows about 100 V. The two meters, connected across the same terminals, differ by a factor of more than two, and neither is faulty.

Step 7: Check the equipment ratings. The load current is 0.90 A, so a 50 ohm rheostat rated at 5 A is more than adequate, and the 40.5 W dissipation is well within its capability. Each SCR in the bridge conducts for half the cycle on average, so its average current is 0.45 A, and it must withstand the peak supply voltage of 141.4 V in the reverse direction. Real device ratings carry a safety margin above that figure, since switching spikes sit on top of the supply waveform.

The example shows why the lab needs more than one meter type and why the rheostat rating, the Variac setting and the device voltage rating must be checked before the supply is switched on.

Which Experiments Use Which Equipment?

Indian university lab lists show a consistent core. Lists from colleges such as IARE Hyderabad, SVNIT Surat, Velammal Engineering College and SRM Valliammai Engineering College all open with SCR, MOSFET and IGBT characteristics, then move through gate firing circuits, controlled rectifiers, choppers, inverters and AC voltage controllers. The table below maps those experiments to the equipment each one depends on.

Experiment

Main equipment

Key instrument

V-I characteristics of SCR, TRIAC, MOSFET, IGBT

Device characteristic module, regulated DC supply

DC voltmeter and ammeter

Latching and holding current of an SCR

SCR module, DC supply, series resistor

Milliammeter

Gate firing using R, RC and UJT circuits

Firing circuit module, Variac and isolation transformer

DSO with differential probe

Half and fully controlled single-phase converter

Converter module, Variac, rheostat, inductor

PMMC and moving-iron meters, DSO

Three-phase controlled converter

Three-phase supply, converter module

DSO with differential probe

Step-down and step-up chopper

MOSFET chopper module, DC supply, rheostat

DSO, DC meters

Single-phase and three-phase inverter

Inverter trainer, DC supply, R and R-L loads

DSO with differential probe

AC voltage controller using SCR and TRIAC

Phase control module, Variac, lamp or rheostat load

True-RMS meter, DSO

Converter-fed DC motor speed control

Converter, DC motor, tachometer or speed meter

PMMC meters, DSO

Simulation of converters

Computer with MATLAB or PSPICE

Software scopes

Several university lists also include a simulation component. The Dronacharya power electronics lab manual (BEE 653), for example, adds software-based experiments in Scilab or MATLAB after the hardware list, and other Indian manuals add PSPICE exercises. Simulation helps with design and prediction, but it assumes ideal devices, so the hardware measurements remain the part that teaches students how real converters behave.

How Should Power Electronics Experiments Be Run Safely?

Safety in this lab is mostly procedure, and most of it costs nothing. The published JoVE guidance for electric machines and power electronics labs gives a useful baseline, and it matches what Indian lab manuals say.

Circuit changes should be made only when the circuit is switched off and all sources read zero voltage and zero current. Every supply should be started from zero output and raised gradually. Exposed metal should be treated as live until it is verified otherwise. Higher-power wiring should be kept apart from low-power control electronics, so that interference and accidental connections do not reach sensitive equipment. Loose wires should be avoided, the bench should be kept uncluttered, and warm equipment such as rheostats should be labelled and cooled. Metal jewellery should be removed, long hair tied back, and safety goggles worn throughout.

Beyond those basics, the equipment itself needs protection. Fuses or MCBs of the correct rating should guard the Variac output and each converter. Residual current protection and a solid earth at the bench protect the user. A common lab rule is that no one wires or modifies a circuit with the supply on, and a second person checks the connections before power is applied. Capacitors should be discharged through a resistor before handling, and a lab notebook with the intended settings should be prepared before the Variac is touched.

How Is This Equipment Used in Indian Engineering Colleges?

In Indian engineering colleges and polytechnics, the power electronics lab is usually a third-year practical for electrical and electronics branches, following university lab manuals aligned with AICTE model curricula. The textbook references named in these manuals are commonly M.H. Rashid's Power Electronics and D.W. Hart's Introduction to Power Electronics, and the minimum experiment count is often stated as eight or ten from a longer list.

Equipment in these labs is largely trainer-based. Published inventories show a typical pattern of device characteristic kits, converter and chopper modules, inverter trainers, regulated supplies, rheostats, a few digital storage oscilloscopes shared across benches, and a small number of differential or current probes. Scopes and probes cost far more than panel meters, so a lab in-charge benefits from planning the scope-to-bench ratio before purchase.

The lab practical exam typically asks a student to wire an assigned circuit, observe a waveform and explain what is seen within a fixed time. That format rewards students who understand which meter reads which quantity and who can explain a firing angle from a scope trace.

Does This Equipment Matter for GATE, SSC JE, and RRB JE Preparation?

The equipment itself is not examinable, but the concepts it demonstrates are. Power electronics is a section of the GATE Electrical Engineering syllabus, and the topics listed for it include static V-I characteristics and firing circuits for thyristors, MOSFETs and IGBTs, DC-DC buck, boost and buck-boost converters, rectifiers, inverters and PWM. Coaching and preparation sites estimate the subject at roughly 6 to 10 marks in GATE EE depending on the year, which is why many aspirants treat it as a rank-deciding subject alongside machines and power systems. Aspirants should confirm the current syllabus and past paper distribution on the official GATE website, since coaching estimates vary from site to site.

Numerical questions of the type in the worked example above, covering average and RMS output of a controlled rectifier, appear regularly in GATE-style papers. Understanding why a PMMC meter and a moving-iron meter read differently also connects to the measurements section of the syllabus.

For SSC JE and RRB JE electrical papers, power electronics is generally a smaller part of the written syllabus than machines, power systems and basic electrical engineering, and aspirants should check the latest official notification for the exact topic list. Hands-on lab familiarity carries more weight in the technical interviews and practical assessments of PSUs such as BHEL, Power Grid and NTPC, where a candidate who has wired a converter and read its waveform can answer applied questions more convincingly than one who has only read the theory.

Which Setup Fits a Basic Lab and Which Fits a Well-Equipped Lab?

The budget decides how much of the list a department can buy at once, and the safe order is protection first, then sources and loads, then instruments. The comparison below shows how the categories scale.

Category

Basic setup

Well-equipped lab

AC source

Variac with fuse protection

Variac with isolation transformer, three-phase autotransformer

DC source

Fixed or single-output regulated supply

Dual-output regulated supply with current limit

Trainer modules

Device characteristic and one converter kit per bench

Separate chopper, inverter, AC controller and three-phase converter kits

Loads

Rheostats and lamp load

Rheostats, variable inductor, DC motor and drive setup

Meters

PMMC and moving-iron panel meters

True-RMS meters and a digital power meter

Oscilloscope

Shared DSO with isolation transformer

DSO per bench with differential and current probes

Software

Free simulator

MATLAB or PSPICE licences for design and pre-lab work

A basic setup can still teach device characteristics and simple converters well. The waveform-heavy experiments, such as inverters and chopped AC, depend on the isolated measurement tools that a well-equipped lab adds.

Where Should a Power Electronics Lab Start?

A power electronics lab should begin from the experiment list that its syllabus requires and work backward to the equipment, not the other way around. For a department building or upgrading this lab, the order of purchase follows the risk: isolation and protection first, then variable sources and rated loads, then trainer modules, and then the measurement tools that make the waveforms visible.

For students, the practical takeaway is that each piece of electrical lab equipment has a reason connected to the converter being tested. Knowing why a differential probe replaces a normal scope lead, or why two meters read two different values across one load, is what turns a routine lab session into usable exam and interview knowledge.

Students and lab in-charges who want to see how the inverter and power supply stages work on real hardware can explore NvisTech's NV6001 Inverter Trainer and NV6003 Power Supply Trainer, or pair this guide with the broader electronics lab equipment list when planning a complete department setup.

FAQs

Q: What equipment is used in a power electronics lab?

 A power electronics lab uses a Variac and isolation transformer, a regulated DC supply, trainer modules for SCR, TRIAC, MOSFET and IGBT circuits, rheostats and inductive loads, PMMC and moving-iron meters, and a digital storage oscilloscope with a differential probe. The lab electrical equipment is chosen to supply, control, absorb and measure power safely.

Q: Why can a normal oscilloscope be dangerous in a power electronics experiment?

 A mains-powered scope connects its probe ground to earth. Clipping that ground onto a floating point in a converter creates a short to earth through the instrument. A differential probe, an isolated handheld scope or an isolation transformer on the circuit under test removes the hazard.

Q: What is the difference between a Variac and an isolation transformer?

 A Variac varies the AC voltage smoothly but remains electrically connected to the mains, so it provides no isolation. An isolation transformer separates the circuit from the earthed mains but does not vary the voltage. Power electronics labs commonly use both together.

Q: Which experiments are part of a typical power electronics lab in India?

 Most university lists include SCR, TRIAC, MOSFET and IGBT characteristics, gate firing circuits, half and fully controlled converters, step-up and step-down choppers, single-phase and three-phase inverters and AC voltage controllers. Many also add MATLAB or PSPICE simulation. The exact list differs by university, so the official syllabus should be checked.

Q: Why do a PMMC meter and a moving-iron meter show different readings on a rectifier output?

 A PMMC meter responds to the average value of the waveform, while a moving-iron meter responds to its RMS value. A pulsating converter output has different average and RMS values, so the two meters disagree even though both are working correctly.

Q: Does electrical experiment equipment knowledge help in GATE EE and PSU interviews? 

Yes, indirectly. The equipment is not examined, but the concepts it demonstrates, including converter output calculations and device characteristics, are part of the GATE EE power electronics syllabus. Practical familiarity also helps in PSU technical interviews, where candidates are asked to explain circuits they have wired.