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8.2 WATLOW Series 988 User’s Manual
General Software Features, Chapter 8
General Software
Burst Fire
Requirements
• This feature only works with zero-cross, solid-state devices. It will not
function correctly with random-fire devices.
• To enable burst fire the Series 988 must have an open collector or solid-
state relay output: controllers with option “B,” “C” or “K” selected for
output 1 (98_ _-_ _*_-_ _ _ _), output 2 (98_ _-_ _ _*-_ _ _ _), output 3 (98_
_-_ _ _ _-*_ _ _) or output 4 (98_ _-_ _ _ _-_*_ _).
• The time burst is appropriate for fast loads or very tight control. It pro-
vides advantages only for PID control, not for on/off control.
• The short time bases used by burst fire makes it incompatible with the
heater current feature (98_ _-_4_ _-_ _ _ _). The heater current option
requires a minimum of 300 milliseconds on time to get a reading.
• The Series 988 has built in zero-cross detection circuitry, eliminating the
need for external firing circuitry to trigger SCR’s. The controller will not
allow burst fire to be selected if its zero-cross detection circuitry is not
functioning.
• The feature is enabled by selecting burst fire [brst] at the cycle time
prompt for the appropriate output in the PID Menu — [Ct1A], [Ct1b],
[Ct2a] or [Ct2b].
• Only the 988 and 989 can use the burst fire feature. The low-voltage
units (986 and 987) cannot use burst firing.
Overview
Variable, time-base burst firing from the Series 988 provides the most
even distribution of power with the lowest level of noise generation (RFI).
An SSR or SCR firing card translates a command signal into a burst of ac
cycles. The output is zero-cross fired, which always allows at least one full
ac cycle to pass within the variable time base. Burst firing is the preferred
mode to control resistive loads.
The Series 988 detects when the ac sine wave of the load will cross the 0-
volt point. It uses this information to switch the load on or off only at a 0-
volt point, minimizing RFI.
The burst fire time base in the Series 988 varies from a maximum 1.66-
second time base (1-percent output; 1 cycle on, 99 off) down to a 33.3-mil-
lisecond time base (50-percent output; 1 cycle on, 1 off). The graphs on
the next page show how the time base varies with the percent output.
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CAUTION:
The burst fire fea-
ture only func-
tions with zero-
cross, solid-state
devices. It will not
function correctly
with random-fire
devices. Verify
that the output
switching device
is compatible
before selecting
burst fire. Failure
to follow this
guideline could
result in damage
to equipment.
C8.wc 5/14/1999 8:55 AM Page 8.2