Understanding Zero-Cross and Phase-Angle Control


This technical article provides a brief summary of different types of industrial heaters, heater power controllers and which control product is the best solution for your system.

Types of Industrial Heaters and Controllers

Shift Controls ISPA series use proportional zero-cross Solid state relay (SSR) controllers and the IFPA series uses proportional zero-cross silicon controlled rectifier (SCR) power controllers for standard, resistive heater applications. Zero-cross power controllers are ideal for the most common type of resistive process heaters.

SSR and SCR controllers are time-proportioned (0-100%) solid state power controllers. Zero-cross indicates that power is turned on and off when the AC sine wave is at zero volts. Zero-cross controllers eliminate arcing and electrical interference created by mechanical contactors opening and closing under load. SSR and SCR controllers are solid state, no moving parts are involved, allowing power to be switched up to 20 times per second. SSR controllers are typically used on smaller, single phase heater loads and SCR controllers loads are typically used on larger electrical loads.


Advantages of SCR and SSR solid state switching controllers are a longer controller life, longer heater life, tighter temperature control, and reduced electrical interference, compared to traditional mechanical contactors and mercury switches. Solid state switching allows for much faster switching, compared to mechanical contactors. Time proportioned control greatly reduces the temperature swings that a heater experiences during control cycles, reducing the thermal cycling heater elements are exposed to, thereby increasing the life of the heater.


Resistive Heaters - Ideal for Zero-Cross SSRs and SCRs

Resistive heater element’s resistance is very stable across the heater’s temperature range. This is known as a low hot to cold ratio. Because the resistance is stable with temperature, the current draw of the heater is constant over large changes in temperature. The constant resistance means the heater will have a linear control response, making temperature control easier across a broad temperature range.


Examples of resistive heater element materials with a low hot to cold ratio are:

  • Iron Chromium
  • Aluminum Alloy
  • 80% - 20% Nickel Chromium (Nichrome Wire)


Most common process heaters, heat trace or heat tape, and many furnaces are resistive heaters. For these applications, zero-cross controllers are typically the best solution. Zero-cross controllers offer similar performance to phase-angle controllers, without the electrical interference issues or additional cost.


Inductive Heaters

A minority of applications will fall into the category of having a high inrush current, or high hot to cold ratio, which may require a custom power controller solution. When cold, these heaters have a lower element resistance, which causes high current during initial heat up. The current decreases as the heater element temperature increases and the resistance increases. This type of heater may require the power controller to limit the current to the elements until the heater is up to operating temperature.


Elements with high hot to cold ratios can exhibit this behavior, some examples are:

  • Molybdenum
  • Tungsten
  • Graphite
  • Quartz (Tungsten)
  • Infrared (Tungsten)
  • Silicon Carbide (higher resistance when cold) - higher current draw when hot


Heater loads coupled to transformers have a similar problem - the high inductance of the transformer causes high inrush currents. This inrush current is higher than the expected current draw from the heater element alone. An over-sized or phase-angle SCR with current limiting protection may be required.


Please e-mail us at sales@shift-controls.com with details of your application, we would be happy to help with a solution.

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  • Brad Kettler