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Trigger, commutate, protect

Trigger: the firing circuit is the experiment

A thyristor conducts when a gate pulse arrives, and the output of a phase-controlled converter is set by when in the cycle that pulse arrives. So the trigger circuit is not a supporting detail - it is the thing being studied, which is why this listing contains more trigger circuits than converters.

• The UJT relaxation oscillator (PELT-0004) is the classic discrete trigger: a capacitor charges through a resistor until the unijunction device fires, and the firing angle is set by the time constant. It is the clearest possible demonstration of phase control from first principles, and it is worth teaching before the integrated version.
• The TCA-785 trigger circuit (PELT-0009) is how it is done in practice: the integrated circuit synchronises itself to the mains zero crossing, generates a ramp, compares it with a control voltage and produces pulses at a precisely controlled angle - with a linear relationship between control voltage and firing angle that the discrete circuit does not give. Comparing the two on the bench is the single most useful exercise in this part of the syllabus.
• Gate drive isolation is not optional. The gate signal is referenced to the device's cathode, which in a phase-controlled circuit may sit at mains potential and may be different for each device in a bridge. The gate drive therefore comes through a pulse transformer or an optocoupler. A trainer that brings gate drives out to a common ground is teaching a circuit that cannot be built.
• With an inductive load, a single short gate pulse is not enough. The load current rises slowly, and if the pulse ends before the current has reached the device's latching value, the thyristor turns off again. The remedy is a long pulse or a pulse train, and discovering this on the bench - the circuit works on a lamp and fails on a motor - is one of the genuinely memorable lessons in the subject.

Commutate: the thyristor does not turn off when you stop telling it to

This is the central difficulty of the subject and the thing students must understand before anything else makes sense. Removing the gate signal does not turn a thyristor off. It stops conducting only when the current through it falls below its holding value.

• In an alternating-current circuit that happens by itself at every zero crossing of the supply - natural, or line, commutation. This is why phase control on an AC supply is straightforward and why most of this listing is AC phase control.
• In a direct-current circuit nothing turns it off, so the circuit has to force the current to zero - forced commutation, classically with a charged capacitor switched across the conducting device. The forced-commutation circuits are the most intricate in the syllabus and the least intuitive, and they are also the honest historical explanation for why the thyristor gave way to the transistor and the insulated-gate bipolar transistor in DC drives and inverters: a device that turns off when you tell it to removes an entire category of circuitry.
• The inductive-load experiment is the one to insist on. With an inductive load the current lags and continues to flow past the voltage zero, so the device conducts beyond the point where a resistive load would release it, and the relationship between firing angle and output changes shape. Add a freewheel diode and the waveform changes again. A student who has measured all three cases - resistive, inductive, inductive with freewheel - understands power electronics; one who has only driven a lamp bank does not.

Protect: the components that are not in the textbook circuit

A thyristor circuit on paper has a device, a load and a trigger. A thyristor circuit that survives has four more things, and a trainer that includes them is teaching real practice:

• A snubber - a resistor and capacitor across the device - because a thyristor can be turned on by a fast rising voltage alone, with no gate signal at all. The rate of rise of voltage the device will tolerate is a datasheet limit, and the snubber keeps the circuit inside it.
• A series inductor, because the device also has a limit on the rate of rise of current, and exceeding it damages the device even when the final current is well within rating.
• Fast semiconductor fuses. This one deserves emphasis: an ordinary fuse does not protect a semiconductor. A thyristor fired into a short circuit fails in a fraction of a cycle, and only a fuse specifically designed for semiconductor protection clears fast enough to matter. Where a trainer has such fuses, ask for spares with the order - they are the component students will consume.
• Heatsinking. A device dissipates roughly its forward voltage times its current, and the thermal path from junction to air is what decides whether the junction survives. For teaching this is mostly a reason to understand why the heatsink is the largest thing in a power circuit.

Safety: the oscilloscope earth is the hazard

We would rather devote a section to this than add a line of boilerplate. These trainers connect to the mains supply, and the following provisions should be bought with them.

1. An isolation transformer for the circuit under test. This is the single most important item. Without it, the circuit is referenced to the supply and any earthed instrument touched to the wrong point creates a fault path.
2. Differential or isolated oscilloscope probes - and never a scope with its earth defeated. A conventional oscilloscope has its probe earth bonded to the building earth. Clip it to a point in a mains-referenced circuit that is not at earth potential, and a current flows through the probe lead, the instrument and the building earth: the probe lead can burn out, the instrument can be destroyed and the person holding it can be injured. The correct solutions are a differential probe, an isolated-input oscilloscope, or an isolation transformer on the circuit. The incorrect solution, which is still widely improvised, is to cut or lift the oscilloscope's own earth - that makes the whole instrument case live and is not acceptable in any laboratory. Put it in the standing instructions.
3. Capacitors stay charged after switch-off. Commutation and filter capacitors hold their charge; the circuit is discharged through a resistor and then verified with a meter before anything is touched. "Switched off" is not "safe".
4. "Off" is not isolated. A phase-controlled circuit set to zero output still has mains voltage present on the device. The only safe state is isolated at a visible switch, locked if the bench is shared.
5. A residual current device on the bench supply, a clearly marked isolator within reach, and one-hand working as the habit.
6. The loads get hot and the motors rotate. Lamp banks and rheostats reach temperatures that burn; motor loads need a guard and no loose clothing, ties or long hair near them.

Tell us at the enquiry stage whether your bench already has an isolation transformer, differential probes and a residual current device. If it does not, we will quote them with the trainers rather than leave a laboratory to discover the gap in its first session.

What to tell us when you enquire

• The experiments your syllabus names - single and three phase controlled rectifiers, AC voltage controllers, choppers, inverters, forced commutation circuits, trigger circuits, motor speed control.
• Whether inductive-load and freewheel-diode cases must be demonstrable, which decides the load bank.
• Whether you need discrete trigger circuits, integrated trigger circuits, or both for comparison.
• What the laboratory already has: isolation transformer, differential or isolated probes, residual current device, load bank, tachometer.
• Whether motor loads are included and whether they are guarded.
• Supply voltage, frequency and whether single or three phase is available at the bench; destination country.

Related ranges: power electronics trainers for the range under our other parent, electrical engineering trainers and electrical training equipment for machines, drives and control panels, electric motors, analog electronics trainers and digital electronics lab trainers for the underlying circuits. The full training kits and electronics lab trainer kits categories list the rest.

Frequently asked questions

Why will my thyristor circuit not turn off?

Because removing the gate signal does not turn a thyristor off - it conducts until the current through it falls below its holding value. On an alternating supply that happens at the zero crossing; on a direct supply nothing does it for you and the circuit must force the current to zero, which is what forced-commutation circuits are for. If a student expects a gate signal to act like a switch, this is the point the subject becomes intelligible.

The circuit works with a lamp load and fails with a motor. Why?

Almost always the gate pulse. An inductive load's current rises slowly, and if the pulse ends before the current reaches the latching value the device turns off again. Use a longer pulse or a pulse train. While you are there, add the freewheel diode case to the experiment - with an inductive load the current continues past the voltage zero, which changes the relationship between firing angle and output, and that comparison is the most instructive measurement in the range.

Can we use an ordinary oscilloscope on these trainers?

Only with an isolation transformer on the circuit, or with a differential or isolated probe. A conventional probe's earth is bonded to the building earth, and clipping it to a mains-referenced point that is not at earth potential creates a fault current through the probe, the instrument and the earth - which destroys equipment and injures people. Never lift or cut the oscilloscope's own earth to work around this: that makes the instrument case live. Specify the isolation transformer and differential probes with the trainers.

Is an ordinary fuse enough to protect the devices?

No. A thyristor fired into a short circuit fails within a fraction of a cycle, far faster than a general-purpose fuse clears. Semiconductor protection requires fuses designed for it, and they are a consumable in a teaching laboratory - order spares with the trainers. Protection in power electronics is designed in from the start rather than added after the first failure.

Why do the textbook circuits have extra components around the device?

Because a real thyristor has limits on how fast the voltage across it and the current through it may rise. A fast rising voltage can turn the device on with no gate signal at all, which the snubber prevents; a fast rising current damages the device even within its current rating, which a series inductor limits. Those components are not refinements - the circuit without them works on the bench and fails in service.

Why is so much of the syllabus about thyristors if industry uses transistors?

Partly history and partly because the thyristor is where the concepts are clearest: phase control, natural and forced commutation, and the consequences of inductive loads. It also remains in use at high powers. But the honest point is worth making to students: the insulated-gate bipolar transistor and the power MOSFET turn off when told to, which removes the forced-commutation circuitry entirely - and that is why DC drives and inverters moved to them. Understanding the difficulty the thyristor creates is what makes that development meaningful.

Key takeaways

• Trigger, commutate, protect - every experiment in this range is one of the three, and the trigger circuit is itself the subject.
• A thyristor turns off only when its current falls below holding, which is why AC phase control is simple and DC switching needed forced commutation.
• Insist on the inductive-load and freewheel-diode cases. A lamp bank alone hides most of the physics.
• Snubber, series inductor and semiconductor fuses are part of the circuit, not accessories - and the fuses are a consumable.
• The oscilloscope earth is the real hazard. Isolation transformer, differential or isolated probes - and never a scope with its earth defeated.
• Discharge and verify. Capacitors hold charge, and a phase-controlled circuit at zero output still has mains on the device.

About this page

Ray Export is a manufacturer and exporter of laboratory, scientific and educational equipment based at Plot No. 449, HSIIDC Industrial Area, Saha, Ambala, Haryana 133104, India. Enquiries: [email protected], +91-9817319615.

Written by Palak Mall, Senior Content Writer. Technically reviewed by Arvind Kumar, Lab Equipment Specialist, 12+ years in laboratory and testing instrumentation. Last updated: 17 October 2026.

Notes on this listing, published rather than hidden: the product names carry the padding phrase "For Power Electronics ... For Vocational Training And Didactic...", which has truncated several of them, and one record is named only "Equipment for Education, Engineering and Vocational..." - the padding has replaced the product name entirely, so that record cannot be identified from the catalogue and is being rewritten. This category also overlaps our power electronics trainers listing under a different parent, and we have flagged the structure for a decision. The device behaviour, protection requirements and electrical safety practice described here are general engineering fact rather than claims about a particular model; all specifications, included protection components, fuse types and certification claims will be confirmed in writing on the quotation; for certificates, contact us for the latest copies of the relevant certificates. Verify them before tender submission, and confirm electrical safety requirements for mains-connected laboratory equipment with your own institution and national rules.