Thyristor Power Controllers – that is what we do

Our extensive range of Thyristor Power Controllers and monitoring equipment is geared to keep projects on-track and on-budget.

Businesses large and small will benefit from our products’ straightforward installation processes, which help initial manpower investment stay minimal. Following that, our accurate product operation streamlines time-spent monitoring and managing systems.

All our thyristor controls are equipped with intelligent technology – this is geared to allow easy use and remote viewing of system data and excellent information flow via integration with key operating systems. Password systems are also included, with an aim to further decrease the risk of human error.

In turn, project waste is further reduced because our thyristors are designed with economical power and energy use in-mind. Added to that, their spot-on performance greatly reduces ongoing costs for every business.

Where we do it: The industries we supply

CD Automation are based in the UK

Our products and applications can be found in vast and varied industries and businesses, including:

  • automotive, defence, aerospace and transportation such as air, sea and rail;
  • education establishments;
  • food and beverage businesses;
  • manufacturing operations such as glass and ceramics, plastics and rubber, pulp, paper and textiles;
  • chemical and pharmaceutical companies;
  • building and construction projects, including heating, ventilation and air-con suppliers;
  • businesses that use ovens, furnaces and kilns and even power generation, waste and waste-water management providers.

How we do it: Our commitment to you

We’re dedicated to delivering superb quality and reliable thyristor products and an excellent customer service. In fact, we’ve even created a ‘Customer Guarantee Scheme’ – an in-house initiative which focuses our operations on giving all our customers the very best product supply, product support and communication experience.

As part of this service, we offer extended warranty opportunities, fixed pricing promises for up to three years, a product-upgrade program and very competitive pricing for multiple-product orders. In turn, we guarantee same-day or next-day delivery for products in-stock and for those that aren’t – we’ve a ten day maximum.

Our team are ready to take your initial enquiry now and offer indispensable and expert advice to help make your decision on the right product for your project. There’s no obligation, just intelligent recommendations. You can speak to them now by calling +44 1323 811100 or email us.


What is a thyristor?

A thyristor is a semiconductor device which acts as a switch. The semiconductor rectifier material either works as a conductor when current flows in one direction or as an insulator when current attempts to flow in the other direction. If the voltage being applied is the standard sinusoidal AC form, only 1/2 of the waveform will be conducted, therefore current flows during half cycles only. To deliver maximum power to the load, both halves of the AC waveform must be conducted and so, two silicon controlled rectifiers (SCR's) must be used connected in an anti-parallel configuration (back to back).

How do thyristors work?

Back to back SCR's allow the full wave current to be conducted. The forward SCR conducts during the positive half of the cycle and the reverse SCR conducts during the negative half of the cycle. Each SCR is turned on at the appropriate time by a trigger pulse applied to the gate (a third leg) and the device will remain on until the instantaneous load current through it drops to zero. The trigger pulses are generated by a drive circuit, which times the pulse to ensure the thyristor unit output is a function of the input control signal & firing mode.

Where would you use a thyristor?

The most common use of thyristors is in AC circuits and for power control. The application or the load being driven or switched dictates which type of SCR & firing mode can be used. There are two main types of load power; resistive and inductive, each will need a different type of firing of the SCR. Resistive elements are split into two groups, variable or fixed resistance and the choice of element is primarily chosen by the maximum temperature required and environment conditions. The most common inductive load is the transformer. Transformers are normally used to provide galvanic isolation between the primary and secondary winding or to change the main supply voltage to the nominal load supply. Thyristors are also used in motor speed controls, light dimmers, pressure-control systems, and liquid-level regulators.

Thyristor or SCR? Terminology explained

Some people use the term silicon controlled rectifier (SCR) interchangeably with 'thyristors'. In fact, silicon-controlled rectifier is a brand name that General Electric introduced to describe one particular kind of thyristor that it made. There are various other kinds of thyristors too (including ones called diacs and triacs, which are designed to work with alternating current), so the terms aren't completely synonymous. So whether it’s called thyristor or SCR, we're just talking about the same semiconductor device.

How to size a thyristor power controller?

Firstly necessary to recognise whether the application is single or three phase. If single phase (1PH) then you can use the calculation; total load (in watts) over the load voltage (L to N or L1 to L2) to give the current value (I = P/V). So a quick example could be, load is 12kW and the voltage (L to N) is 240V giving a current of 50A. Now we normally add a safety margin of 15 to 20% to this nominal current in selecting a thyristor power controller to ensure we allow for any fluctuations in voltage supply or temperature etc. This just means we are not switching at the units max and results in a long thyristor life.

For larger power consuming loads, three phase (3PH) systems are normally used. The voltage between any two legs of a three system is a sinusoidal AC waveform but the voltage waveform between each successive pair of legs however will be displaced in time by 120 electrical degrees from the other two. This corresponds to 6.67 milliseconds for a 50Hz system.

So assuming the load again is 12kW but this time is connected to 3 phase, we would have 12000 watts over the voltage, times by the square root of 3 or 1.73 for convenience. So the current would be 12000 / 415 X 1.73 = 16.8 amps.

Thyristor firing types and their applications

There are several different ways to control or fire the thyristor (SCR). The firing mode is determined by the electronics mounted on the thyristor (SCR) power controller. The electronics package is called the firing circuit. The two main thyristor categories are: zero voltage crossover firing and non-zero voltage crossover firing. The zero voltage crossover firing category includes a simple on/off and a time proportioned on/off, commonly called burst firing.

Zero Crossover firing mode is used with the logic output typically from a temperature controller and the thyristor operates like a contactor. With zero crossover firing, the power controller is off, if no input signal is present or is fully on, if the input signal is present. There are no other conditions possible. The cycle time is determined by the control device, i.e. a temperature controller. Zero crossover minimises EMC interference due to the fact that the thyristor unit switches ON and OFF at zero volts.

The second type of zero crossover firing is Burst Firing and as its name suggest means that the on or off periods are fixed for a specific cycle time (bursts of on or off). The firing circuit determines how long the power controller should be on for a given control signal. Analogue or proportional input is necessary for burst firing and the number of complete cycles must be specified for 50% power demand. This value can be between 1 and 255 complete cycles, determining the speed of firing. When 1 is specified, the firing mode becomes Single Cycle. The main advantages over on/off solid state control is the unit will accept proportional inputs such as 4-20mA or 0-10V. Proportional inputs allow the process controller to vary the load more accurately as the process changes.

Single Cycle is the fastest zero crossing switching method. At 50% input signal, one cycle is ON and one cycle is OFF. At 75%, 3 cycles are ON and one cycle is OFF. If power demand is 76% the unit performs the same as for 75% but every time the unit switches ON the microprocessor divides 76/75 and memorises the ratio. When the sum is one the unit delivers one cycle more to the load. Single Cycle firing is ideal for simple resistance loads and where thermal mass of the load is small or as a low noise alternative to Phase Angle firing with some applications.

Phase Angle controls the power to the load by allowing the thyristor to conduct for part of the AC supply cycle only. The more power required, the more the conduction angle is advanced until virtually the whole cycle is conducting for 100% power. The load power can be adjusted from 0 to 100% as a function of the analogue input signal, normally determined by a temperature controller or potentiometer, phase angle firing is normally used with inductive loads. Filters may need to be used to remove any resulting EMC interference generated and additional features such as soft start and current limit may be required to control the load.

How to select a thyristor for a specific application or load type

Selecting the correct thyristor power controller for your application doesn’t need to become a headache. The first thing to consider is the load and/or element type used. The most common element type is referred to as a fixed or normal resistance type. This element doesn’t change more than 10% in resistance over time or with temperature variations. In our industrial world this accounts for more than 85% of elements used. Typical thyristor firing will be zero crossover either with a DC logic control signal (for zero crossover firing) or an analogue 4-20mA or 0-10V (for burst firing). No other element consideration or protection is really needed, although having the longest cycle time that also controls the load adequately is recommended to extend element life.

For all other load types including variable resistance elements, short-wave infrared lamps, inductive or transformer loads, contacting our friendly technical guys and girls to help choose the right thyristor power controller for your application. Remember we have the right product for all applications.

Advantages of using solid state relays or thyristors over mechanical relays or contactors

Reduced maintenance and operating costs as there are no moving mechanical parts to fail.

Electrically quieter as the device turns on and off at zero volts so doesn’t create RFI interference.

Line distortion is eliminated compared to a mechanical relay.

Finer control and extended heater element life due to shorter cycle times that increase process stability and decrease heater thermal shock.

Heater also shows less expansion and contraction caused by heating and cooling action.