Electronic power switch: the thyristor, how does it work?

When high electrical power levels must be switched, electronic “switches” are being used: the thyristor. This page gives more information on what a thyristor is and how it works.

what is a thyristor?
how does a thyristor work?
what are the advantages and disadvantages of a thyristor?
what kind of control methods are being used for a thyristor?
how do I couple a process control with a thyristor?

Wat is a thyristor?

In short: a controllable diode:

Current can only flow in 1 direction through a diode, from the + side (anode) towards the – side (cathode). A diode will block the current flow when the polarity of the voltage over the diode is reversed.
There are also so-called thyristors, these a controllable diodes that can be used for switching high currents on.
With an extra control signal (the so-called “gate”), the moment for switching on the current within the positive side of the sinusoidal voltage over the controllable diode (in forward current direction) can be selected. Only after the gate-signal has been triggered, the current starts to flow.
The controllable diode will automatically block the current flow, when the polarity of the sinusoidal voltage over the diode becomes negative.

Such a controllable diode is also called a thyristor.

Another much used name is a “silicon controlled rectifier” or SCR. Since a diode can be used as a rectifier (current can only flow in 1 direction). But because the diode of a thyristor can be controlled, a thyristor is also called a controllable rectifier.

Principle of thyristor control

Here is a short movie that shows the principle of operation of the controllable diode (=thyristor). In both examples so-called phase-angle control is being used.

By modulating the gate control signal of the thyristor with regard to time within the sinusoidal voltage, the amount of current flowing through the lamp can be controlled.

Source: wikipedia – thyristor
Blue: open-signal
Red: controlled voltage

Source: youtube channel: Engineering Technology Simulation Learning Videos

Using the whole sinusoidal voltage (also the negative side of the sine)

A disadvantage of using only 1 thyristor is that only the positive side of the sinusoidal net-voltage can be switched.
In order to be able to also switch the negative part of the sinusoidal net-voltage 2 thyristors will be connected antiparallel-wise, with the conducting directions reversed from each other.
The forward current conducting direction of 1st thyristor (from anode towards cathode) is connected in parallel to the non-conducting direction of the 2nd thyristor. So the anode of the 1st thyristor is coupled to the cathode of the 2nd connected thyristor in parallel: we call this way of connecting anti-parallel.
The moment for switching on the thyristors is the same for both diodes: the trigger signal comes at the same time after the sinusoidal net-voltage passes 0 Vac. This can be either in positive direction or in negative direction.
Because the sinusoidal net-voltage is either positive or negative, there is always only 1 thyristor in current conducting state while the other one blocks the current.
This way there can be always a flow of current in both directions through the connected electrical load, independent of the actual polarity of the net-voltage.

Another name for 2 anti-parallel conntected thyristors is the bidirectional controlled thyristor, or BCT.

Advantages and disadvantages of a thyristor

In practical situations there are always advantages and disadvantages, here is an overview:

Thyristor advantages:

electronic switch without wear: very high reliability (more than 100x the life-expectancy of an electromechanical relay)
current and power for the electrical heating can be modulated in an accurate way in small steps.
high energy density: switching high currents in a relatively small housing

Thyristor disadvantages:

in non-conducting direction there is always a very small leakage current of a few microamperes (uA). This can be measured with a voltmeter connected to the – side (cathode) of the thyristor. The power that flows through the electrical heating is neglectable since this current is very small (P=UxI, I is very small, so P is also very small)
higher initial costs when comparing to electromechanical relays.

Because of the very high reliability and long life-expectancy of the thyristor with regard to an electromechanical relay, the higher initial costs are earned back quickly.

Modern thyristor housing

In the past you always had to tap & install 2 separate thyristors in a heatsink. You also had to connect them anti-parallel.
Since about the turn of the century thyristors are also available in a complete housing with 2 anti-parallel connected thyristors inside.
Installation of these modules on a heatsink is relatively easy and enables compact systems with a high energy-density. This saves space and costs.

In the pictures above you see a integrated 6 channel thyristor module. These modules are often used in the heat-treatment industry.
The unit consists of a heatsink with forced cooling (2 fans)
6x 2 anti-parallel connected thyristors are installed on the heatsink
Every separate thyristor can control 136A/480Vac maximum
This thyristor unit is suited for resistive loads
Every thyristor can be controlled separately with a 12-24Vdc signal coming from a temperature controller or PLC system.

Control methods

By triggering the opening of the thyristor on the right time within the sinusoidal net-voltage, the amount of power through the connected load can be modulated in small steps. This way of control is also called phase-angle control.
A downside of this on-switching in the middle of the sinusoidal voltage also causes harmonic distortion.
Another way of control is switching a certain number of complete sines on within a certain time limit, and after that letting the thyristor off for a certain amount of sines. This way of controlling is called “burst firing“, the main benefit is: almost no harmonic distortion.
There are of course other control methods being used, but the ones above are the most used by us in most modern applications.

Phase-angle control

because the voltage is switched on almost immediately inside the sinusoidal voltage after the trigger pulse, there is almost immediately a current flow through the heating element and this immediate switching causes harmonics of higher-order frequencies than the 50Hz (or 60Hz) on the net-voltage incl. blind-current (powerfactor is not 1.0)
because of this the cabling becomes more hot and in practice more power flows through the cabling and thyristor than through the heating-element.
this distortion can cause malfunctions in other equipment, and can even disrupt the control of the thyristor itself.
the right red graph in the picture shows what this distortion does with the voltage that is connected to the load.
currents are flowing towards and back unnecessary through the thyristor and heating element
this also causes extra heat in the cabling (because of the skin-effect: higher frequencies run on the outside of the cabling) and the thyristor.

Advantages:

very fine modulation of the power trough the electrical load
besides normal resistive loads also suited for non-linear loads:
- inductive (primary side transformer)
- MoSi2/Kanthal-Super
- SiC (siliciumcarbide)
- incandescent lamps (IR/UV)
average current trough heating element/cabling is lower than maximum (when not the whole sine is activated)

Disadvantages:

causes harmonics and blind current (powerfactor not 1.0) on the net-voltage: filtering necessary (either with transformer and/or capacitor battery)

Zero-crossing or burst-fire control (letting through only complete sines)

To have a good alternative for phase-angle control, various other control methods have been developed, such as:

zero-cross switching: the sinoidal voltage is always switched just after passing the 0 Vac in the sine.
burst-firing: is comparable to zero-cross switching (also only switching just after 0Vac in the sine, this method however, lets a certain amount of complete sines through within a time-period. The amount is dependent on the desired power through the load.

Note: phase-angle control is also used with these control methods (a thyristor cannot be controlled in another way)
The moment for switching on is however, just after the passing of the 0 Vac in the sinusoidal voltage, so at the moment of switching the power over the thyristor is very low (incl. the current).
because of this, there is (almost) no distortion
Note: immediately after swithing on the whole sine is passing the thyristor, so the current through the heating-element and the cabling is maximum !!!
Modulation with these methods is only possible by switching whole sines on/off during a certain time period

Advantages:

almost no harmonics on the net-voltage: no filtering necessary

Disadvantages:

the maximum current is always running through the load and cabling: the sizing must be made accordingly
because of the on/off behavior of the control, this method is less suitable for non-linear loads:
- inductive loads: very high switch-on currents (transformer)
- ceramic heating elements: SiC/siliciumcarbide rods break faster

Easy integration of thyristors in process automation with thyristor units

In order to connect your process control with thyristors in an easy way, completely integrated thyristor units have been developed
Thyristor units can be controlled with an analog signal (i.e. 4-20mA/0-10Vdc) that corresponds with the desired power that must flow through the heating element.
There are versions available that can be controlled with fieldbus interfaces (i.e. ProfiNET/DeviceNET/Modbus). This enables easy integration with SCADA/PLC systems.
There are versions available that can limit the current and/or power through the heating element. This way fuses are not blown out unnecessary and the life expectancy of the heating elements can be extended