Watlow Series 998 - Manual (Page 111)

WATLOW Series 998 User’s Manual 8.1 Applications, Chapter 8
This chapter details many of the 998 features unique to Watlow controls.
Some of this material is advanced and requires previous background
or experience.
What we will cover in this chapter:
Standard Features
• Burst fired, zero cross output • Dead Band
• Retransmit • Transmitter Power Supply
Burst Fired Zero Cross Output
The Series 998 can provide a variable time base, zero cross output with
superior controllability compared to a fixed time base, time proportioning
control. This eliminates the need for additional hardware associated with
power control devices. When either output 1A or 1B is ordered as a
switched dc (option C) or a solid-state relay (option K) the 998 can operate
as a burst fired control via the cycle time prompt(s).
Here’s an example.
The Series 998 is used to control a heated platen in a semiconductor
oven. In the past, a control with a 4-20mA output was required to be
used with a power control having burst fire control circuitry. This result-
ed in additional system cost. Now the power control is replaced with a
Watlow QPAC with a QCD card, or a Watlow dc input SSR. This accepts a
signal directly from an open collector output on the 998, providing
smoother control at a lower overall system cost.
To achieve this, set [Ct1A] (PID A Menu) to [brSt]. This sets the output
1, PID set A cycle time to burst fire.
Figure 8.1-
Burst Fire Example.
TL W WA
Analog
Input 1
Control Output 1A
Option "C"
Temperature
Tramsmitter
Semiconductor Oven
Heaters
DC Input
SSR
TL TL W WA
PROCESS
1A 2A 1B 2B
CH A
CH B
DISPLAY
SERIES 998
MODE
AUTO
MAN
ç
CAUTION:
Burst fired opera-
tion requires ac
power supplied to
the Series 998.
Failure to supply ac
power to the control
results in no zero
cross references,
and causes the
Series 998 to
default to a cycle
time of one second.
TL W WA
PROCESS
1A 2A 1B 2B
CH A
CH B
DISPLAY
SERIES 998
MODE
AUTO
MAN
Chapter 8 Applications