Watlow CPC400 - Manual (Page 103)

CPC400 Series User’s Guide Chapter 4: Tuning and Control
Doc. 0600-2900-2000 Watlow Anafaze 89
Distributed Zero Crossing (DZC)
With DZC outputs, the PID algorithm calculates an output
between 0 and 100 percent, but the output is distributed on
a variable time base. For each ac line cycle, the controller
decides whether the power should be on or off. There is no
fixed cycle time since the decision is made for each line cy-
cle. When used in conjunction with a zero crossing device,
such as a solid-state relay (SSR), switching is done only at
the zero crossing of the ac line, which helps reduce electri-
cal noise.
Using a DZC output should extend the life of heaters. Since
the time period for 60 Hz power is 16.6 ms, the switching
interval is very short and the power is applied uniformly.
DZC should be used with SSRs. Do not use DZC output for
electromechanical relays.
The combination of DZC output and a solid-state relay can
inexpensively approach the effect of analog, phase-angle
fired control. Note, however, DZC switching does not limit
the current and voltage applied to the heater as phase-an-
gle firing does.
Three-Phase Distributed Zero Crossing (3P DZC)
This output type performs exactly the same as DZC except
that the minimum switching time is three ac line cycles.
This may be advantageous in some applications using
three-phase heaters and three-phase power switching.
Analog Outputs
For analog outputs, the PID algorithm calculates an output
between 0 and 100 percent. This percentage of the analog
output range can be applied to an output device via a Dual
DAC or a Serial DAC.
Output Filter
The output filter digitally smooths PID control output sig-
nals. It has a range of 0 to 255 scans, which gives a time
constant of 0 to 85 seconds for a CPC408 or 0 to 43 seconds
for a CPC404. Use the output filter if you need to filter out
erratic output swings due to extremely sensitive input sig-
nals, like a turbine flow signal or an open air thermocouple
in a dry air gas oven.
The output filter can also enhance PID control. Some pro-
cesses are very sensitive and would otherwise require a
large proportional band, making normal control methods
ineffective. Using the output filter allows a smaller propor-
tional band to be used, achieving better control.