User guide

Watlow EZ-ZONE
®
PM PID Controller • 97 • Chapter 9 Features
Dead Band (cont.)
When the dead band value is zero, the heating output activates when the temperature drops below the
set point, and the cooling output switches on when the temperature exceeds the set point.
When the dead band value is a negative value, both heating and cooling outputs are active when the
temperature is near the set point.
Adjust the dead band with Dead Band [``db] (Operations Page, Loop Menu).
Variable Time Base
Variable time base is the preferred method for controlling a resistive load, providing a very short time base
for longer heater life. Unlike phase-angle firing, variable-time-base switching does not limit the current and
voltage applied to the heater.
With variable time base outputs, the PID algorithm calculates an output between 0 and 100%, but the
output is distributed in groupings of three ac line cycles. For each group of three ac line cycles, the controller
decides whether the power should be on or off. There is no fixed cycle time since the decision is made for each
group of cycles. When used in conjunction with a zero cross (burst fire) device, such as a solid-state power
controller, switching is done only at the zero cross of the ac line, which helps reduce electrical noise (RFI).
Variable time base should be used with solid-state power controllers, such as a solid-state relay (SSR) or
silicon controlled rectifier (SCR) power controller. Do not use a variable time base output for controlling elec-
tromechanical relays, mercury displacement relays, inductive loads or heaters with unusual resistance char-
acteristics.
Time
Temperature
Zero Dead Band
Set Point
Heat Output Active
Cool Output Active
Time
Temperature
Negative Dead Band
Set Point
Heat Output Active
Cool Output Active