Zero cross control (also known as "burst firing") is the preferred method for switching nichrome heaters. Zero cross control offers these benefits:

Low electrical noise and minimal harmonics.
Power factor of 1.

The Power Series offers three zero cross modes:

Variable time base
Fixed time base
DC contactor

This method is known as "zero cross" because the Power Series switches the load only when the AC sine wave crosses the 0-volt point.

Variable time base is the best zero cross mode for most applications. It provides:

The best response time.
The best resolution.
The longest heater life.

In this mode, the Power Series switches the load on and off as often as possible to deliver the shortest possible bursts of power to the heater. This reduces temperature variations in the heater element, which results in longer heater life.

For example, at 50 percent power, the output is on for one AC cycle and off for one AC cycle:

At 40 percent power, the output looks like this:

Note: One AC cycle lasts 16.6 milliseconds at 60 Hz, 20 milliseconds at 50 Hz.

Watch a video clip of variable-time-base control on an oscilloscope. In the video, the power starts at 0 percent and gradually increases to 100 percent, then decreases slightly.

Click here if the video will not play or if you cannot hear the sound.

In fixed-time-base mode, the on-off cycle time has a fixed duration of either one second or four seconds (field-selectable). During this time period, the Power Series completes one on-off cycle, proportioning the "on" and "off" time to provide the requested percentage of power.

For example, suppose the time base is one second and the power supply is 60Hz. At 50 percent power, the output is switched on for 30 cycles, off for 30 cycles, on for 30 cycles, off for 30 cycles, and so on:

At 40 percent power, the power is on for 24 cycles (40% of 60 cycles) and off for 36 cycles:

You might use fixed-time-base control when using a generator as your power source or to prevent light flicker on a "soft" power line.

In DC contactor mode, the Power Series functions like a simple on-off switch. The temperature controller must provide time-proportioned on-off signals.

You might choose this control mode if the temperature controller provides a "burst fire" or "variable burst" on-off command signal. This type of command signal is available from Watlow Series 96, Series 988 and Series F4 temperature controllers.