Thyristor Firing Types Explained


When deciding which thyristor is best for your process, you will need to Thyristors CD Automation 2consider a number of factors such as the load amperage and type, but a major one is which firing type you need.  This sometimes confuses people as they think that thyristors just switch power on and off which is essentially true but at what phase of the cycle they do this can have a huge impact on their usage. 

There are two main types: zero voltage crossover firing and non-zero voltage crossover firing.  Which one will be most appropriate for your operation will depend largely on what level of control you require, the type of load and heating elements and how much Radio Frequency Interference (RFI) you can tolerate.

Zero Voltage Crossover Firing

This type of thyristor only gates when the voltage across it is zero.  It stays on once triggered until the conducted current drops to zero which occurs naturally every ½ cycle in an AC waveform.  They have two firing types: ON/OFF and Time Proportioned on/off which is sometimes called Burst Firing.


These power controllers are controlled in a simple way.  They’re off if no input signal is present, or fully on if an input signal is present. An operator or some external hardware is required to determine when the controller should turn on and when it should turn off.  These are commonly used to replace mechanical switches to reduce RFI, speed up switching times and last longer without wearing out.

Zero crossing thyristor firing

Time Proportional ON/OFF (Burst Firing)

Similar to the previous type of thyristor firing, these can be fully on or fully off but are controlled by a separate controller that monitors the progress of the process and adjusts the output accordingly.  It does this by using a fixed time base with an output proportioned to that time base.  Here’s an example: if the time base is 1 second and 50% of the output is required, the output will be on for half a second.

Burst Firing Thyristor - CD Automation

Non-Zero Voltage Crossover Firing

There’s only one type of non-zero voltage crossover firing type and they are sometimes called phase angle firing thyristors.  Rather than turning on and off when the voltage drops to zero, these thyristors section the AC cycle into much smaller parts and can fire during any of these sections.  This means a much tighter control of the current which is essential for current limitation and soft starting.  A disadvantage to these controllers is that switching high voltages creates large amounts of RFI which can interfere with radio communications and computer operations.  People wanting to use these controllers can use shielding to minimise the interference.  They are also more complex than zero voltage crossover firing thyristors so can be more difficult to service and less reliable.

Phase Angle - CD Automation Thyristor

Combination Controllers

It is possible to combine the two main types to facilitate soft starting using the phase angle controllers which then switch over to the zero crossover controllers once the unit is at full power.

Knowing which of these controllers is best for your operation can be quite difficult.  They also come in a variety of sizes so we at CD Automation can advise you on the best one for your needs.


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