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Figure 66 shows the steps taken during the long cycle.
WAIT 2n
MONITORING
CYCLES
CURRENT
TEMPERATURE
MEASUREMENT
T1(n)
IS T1(n) > OP1
OPERATING
POINT
TEMPERATURE
NO
OP1
IS T1(n) < LOW TEMP LIMIT
AND
T
< HIGH TEMP LIMIT
MIN
AND
T
< OP1
MIN
AND
T1(n) > T
MIN
NO
Figure 66. Long Cycle Steps
The following examples illustrate some of the circumstances
that might cause T
to increase, decrease, or stay the same.
MIN
Example: Normal Operation—No T
1.
If measured temperature never exceeds the programmed
operating point minus the hysteresis temperature, then
T
is not adjusted, that is, it remains at its current setting.
MIN
2.
If measured temperature never drops below the low
temperature limit, then T
THERM LIMIT
HIGH TEMP
LIMIT
OPERATING
POINT
HYSTERESIS
ACTUAL
TEMP
LOW TEMP
LIMIT
T
MIN
Figure 67. Temperature Between Operating Point
and Low Temperature Limit
Because neither the operating point minus the hysteresis
temperature nor the low temperature limit has been exceeded,
the T
value is not adjusted, and the fan runs at a speed
MIN
determined by the fixed T
MIN
automatic fan speed control mode.
Example: Operating Point Exceeded—T
When the measured temperature is below the operating point
temperature minus the hysteresis, T
YES
DECREASE T
MIN
BY 1°C
YES
INCREASE
T
BY 1°C
MIN
DO NOT
CHANGE
Adjustment
MIN
is not adjusted.
MIN
and T
values defined in the
RANGE
Reduced
MIN
remains the same.
MIN
Once the temperature exceeds the operating temperature minus
the hysteresis (OP − Hyst), T
during the short cycle (see Figure 65). The rate at which T
decreases depends on the programmed value of n. It also
depends on how much the temperature has increased between
this monitoring cycle and the last monitoring cycle, that is, if
the temperature has increased by 1°C, then T
2°C. Decreasing T
thus providing more cooling to the system.
If the temperature is slowly increasing only in the range
(OP − Hyst), that is, ≤ 0.25°C per short monitoring cycle, then
T
does not decrease. This allows small changes in
MIN
temperature in the desired operating zone without changing
T
. The long cycle makes no change to T
MIN
ture range (OP − Hyst) because the temperature has not
exceeded the operating temperature.
Once the temperature exceeds the operating temperature, the
long cycle causes T
while the temperature remains above the operating tempera-
ture. This takes place in addition to the decrease in T
would occur due to the short cycle. In Figure 68, because the
temperature is increasing at a rate ≤ 0.25°C per short cycle, no
reduction in T
MIN
Once the temperature falls below the operating temperature,
T
stays the same. Even when the temperature starts to
MIN
increase slowly, T
increases at a rate ≤ 0.25°C per cycle.
Example: Increase T
When the temperature drops below the low temperature limit,
T
can increase in the long cycle. Increasing T
MIN
effect of running the fan slower and, therefore, quieter. The long
cycle diagram in Figure 66 shows the conditions that need to be
true for T
to increase. A quick summary of those conditions
MIN
and the reasons they need to be true follows:
T
can increase if
MIN
1.
The measured temperature falls below the low temperature
limit. This means the user must choose the low limit
carefully. It should not be so low that the temperature
never falls below it because T
the fans would run faster than necessary.
2.
T
is below the high temperature limit. T
MIN
allowed to increase above the high temperature limit. As a
result, the high limit should be sensibly chosen because it
determines how high T
3.
T
is below the operating point temperature. T
MIN
never be allowed to increase above the operating point
temperature because the fans would not switch on until the
temperature rose above the operating point.
4.
The temperature is above T
is turned off below T
Rev. 0 | Page 49 of 76
starts to decrease. This occurs
MIN
MIN
has the effect of increasing the fan speed,
MIN
MIN
to be reduced by 1°C every long cycle
MIN
takes place during the short cycle.
stays the same because the temperature
MIN
Cycle
MIN
would never increase, and
MIN
can go.
MIN
. The dynamic T
MIN
.
MIN
ADT7473
MIN
is reduced by
in the tempera-
that
MIN
has the
MIN
is never
MIN
should
MIN
control
MIN
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