What Are The Most Common Thyristor Firing Methods?

Thyristors are controlled by sending the correct Signal to the gate connection of the device. It will then continue to let current flow until the gate Signal is removed and the voltage through it reaches zero. There are two main methods of firing the thyristors:

• Zero Voltage Crossover Firing (burst pulse)

• Phase angle firing

What Is Zero Voltage Crossover Firing (burst pulse)?

Zero Crossover Firing sends the gate Signals to the thyristors only when the voltage through it is zero. Therefore, the thyristor will turn on and off only at the zero voltage crossover point of the sine wave which occurs every half cycle. The fully on and off periods are based on burst pulses of time which are set by the firing circuit and can be adjusted to suit the user’s load. For example if the total cycle time is set to 2 seconds and a 50% power output from the thyristor controller is required then the output will be fully on for 1 second and fully off for 1 second, this is shown in the diagram below:

This percentage output works linearly with reference to the amount of time on and off. For example if the total cycle time is set to 2 seconds and a 75% power output from the thyristor controller is required then the output will be fully on for 1.5 seconds and fully off for 0.5 seconds. To achieve this range of outputs a Signal from the user is required which is normally 0-10Vdc or 4-20mA. This Signal is proportional to the percentage power output required, for example when using a 0-10Vdc Signal:

Signal (Vdc) Percentage Output (%)
0 --- 0
2.5 --- 25
5 --- 50
7.5 --- 75
10 ---100

Why Use Zero Crossover Firing (burst pulse)?

Zero Crossover Firing (burst pulse) is the preferred method for standard resistive loads, in particular in the HVAC industry. By ensuring that the thyristors are always switched on at the zero voltage point of the mains half cycle, fast rising high voltage wave fronts are avoided which significantly reduces the level of generated electrical noise. This technique is cost effective, as the requirement for substantial filtering is now eliminated. Another benefit of this method is that it can be used on a three phase system with only two of the phases controlled by thyristors; this again saves cost and reduces heat losses produced by the thyristors.

What Is Phase Angle Firing?

Phase angle firing turns the thyristors on at a specific point in each half cycle of the mains frequency. Varying this switch-on point between the initial and final zero voltage points of the sine wave provides a variation from 100% down to 0% of the load voltage (and hence the output power). For example if a 50% power output from the thyristor controller is required then the waveform would be:

To achieve the range of output percentage a Signal from the user is required which is commonly 0-10Vdc or 4-20mA. Much like burst pulse firing this Signal is proportional to the percentage power output required; however instead of varying the amount of time the thyristors are on and off the Signal is varying the amount of delay in the conduction angle of the waveform.

Why Use Phase Angle Firing?

Phase angle control allows an extremely tight, accurate and smooth control of the load but will generate a significant amount of electrical noise unless substantial filter networks are employed. The level of electrical interference increases as the switch on point approaches the maximum peak voltage of the sine wave. Power factor is a calculation of available power vs. consumed power, at full output the phase angle method provides a power factor of 1 but this decreases as the output of the thyristors is reduced, i.e. the power factor is 0.5 at 50% output. The phase angle control method is used mainly for inductive loads, such as transformer coupled loads, and also some specific resistive loads which benefit from features that phase angle control can offer such as soft start to limit high inrush current.

Are There Any Other Options?

There are a number of other variants of the firing methods detailed above; probably the most interesting one of these is the combination of zero crossover burst pulse and phase angle firing. If the users load will operate on zero crossover burst pulse the majority of the time but would benefit from a delayed or soft start function then this firing method is perfect. This then has the advantages from both firing methods. For example as phase angle control results in a poor power factor at power levels less than 100% the combination method can be used to avoid this and also reduce harmonics and conducted electromagnetic interference. The burst pulse cycle time would be set long enough to allow the phase angle soft start to complete, once this ramps the voltage up over a set time the burst pulse would take over the control until the cycle time has finished.

原文链接：https://www.ppi-uk.com/news/thyristor-controller-firing-methods/

这里面提到了：晶闸管过零触发。这是一个非常特殊的例子，一个周期的正弦波，在过零点的时候，会经过周期的0，50，100位置。并且幅值的0、25、50、75、100刚好对导通0周期，1/4周期，1/2周期，3/4周期，1周期。所以要输出这些比例的电压，是不需要计算的。

    相角触发，可以输出任意幅值比例的电压，但是正弦波输出电压需要积分来计算一个周期内的平均电压。需要通过计算解算出需要延时的时间。这里假设正弦波经过了全波整流桥。

根据这个等式，可以设定电压有效值，解出来需要延时的时间。

    这里有一篇博客讲述STM32单片机控制可控硅调压设计，其思路也是，检测电压为0的时刻，然后通过计算得到延时的时间，从而控制输出电压的有效值。但是这个博客无法观看所有的内容。

博客地址：https://www.amobbs.com/thread-5745511-1-1.html

当然这还有一篇博客：使用 555 定时器和 PWM 信号进行调光器的交流相角控制和电机速度控制

    这个大佬使用硬件的过零检测，NE555（777）定时器，使用arduino主控实现了使用电位器旋钮调节灯泡亮度。这个博客对相角控制解释的非常的详细

博客地址：https://circuitdigest.com/microcontroller-projects/ac-phase-angle-control-for-light-dimmers-and-motor-speed-control-using-555-timer-and-pwm-control

YouTube视频地址：https://youtu.be/xKuK4MrK5TQ

当然，我也搬运了这个视频，在哔哩哔哩就可以看到。

视频地址：https://www.bilibili.com/video/bv1e3411M7jD