Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 23)

General Software Features
3.3
Optimizing Your Process System with the WATLOW Series 988
Burst Fire
Overview
Variable, time-base burst firing from the 988
provides a command signal to an SSR or SCR
firing card that translates into a burst of AC
cycles. The output is zero-cross fired and
always allows at least one AC cycle to pass
within the variable time base. The fact that we
are zero-cross switching the power device
means we enjoy the benefits of low radio
frequency (RFI) noise. Burst firing is the pre-
ferred mode to control resistive loads.
The burst fire time base in the 988 varies from
a maximum 1.66-second time base (1-percent
output) down to a 33.3-millisecond time base
(50-percent output). The examples below show
how the time base varies depending on the
percent output.
Requirements
The 988 family has built in zero-cross detection
circuitry. Therefore special firing circuitry is
normally not required for triggering SCR’s. To
enable burst fire the 988 must have an open
collector or solid-state relay output. The feature
is enabled by selecting burst fire
at the
cycle time prompt for the appropriate output.
You should note that the short time bases used
by burst fire makes it incompatible with the
heater current feature (see page 3.7). The
heater current option requires a minimum of
300 msec. ON-time to get a reading.
Only the 988 and 989 can use the burst fire
feature. The low-voltage units (986 and 987)
cannot use burst firing.
Sample Application
The 988 is being used to control a heated platen in a semiconductor oven.
Previously it required a power control requiring a 4-20mA signal to implement
burst-fire control. We have replaced the power control with a Watlow Loyola
QPAC with a QCD card that accepts a signal directly from an open-collector
output of the 988. This gives smooth control at a lower overall system cost.
66% Output 10% Output
TL TL W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
input 1 output 1
limit
sensor
Semiconductor Oven
temperature
sensor
heated platen
QPAC
with
QCD
input
limit
1 ON, 9 OFF 2 ON, 1 OFF
50% Output 100% Output
10 ON, 0 OFF 1 ON, 1 0FF