Watlow Series D8 - Manual (Page 137)

Series D8 User’s Guide Chapter 5: Tuning and Control
Doc. 0600-3120-2000 Watlow Anafaze 119
Distributed Zero Crossing (DZC)
With DZC outputs, the PID algorithm calculates an output be-
tween 0 and 100 percent, but the output is distributed on a
variable time base. For each ac line cycle, the controller de-
cides whether the power should be on or off. There is no fixed
cycle time since the decision is made for each line cycle.
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 electrical 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 in-
terval is very short and the power is applied uniformly. DZC
should be used with SSRs. Do not use DZC output for electro-
mechanical relays.
The combination of DZC output and a solid state relay can in-
expensively approach the effect of analog, phase-angle fired
control. Note, however, DZC switching does not limit the cur-
rent and voltage applied to the heater as phase-angle 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 be-
tween 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.