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Technical Information SheetPage 1 of 4
741-F Mille r Drive , Leesburg, Virginia 20175 T +1 703 443-0000 F +1 703 669-1300 www.eurotherm.com
Action Instruments Barber-Colman Continental Industries Eurotherm Chessell Eurotherm Controls
TIS#: 246, Issue 1
Date: 11/15/03
Issued By: Mark Demick
Subject
This document describes the Minimum Pulse Time setting and its affect on Cycle Time in the
3116 and 3216 Series PID controllers.
Controllers
3116, 3216 Controllers
Introduction
All PID controllers having a logic, triac or relay output to interface to an AC voltage load
through a mercury contactor, solid state relay (SSR), or other On/Off AC actuator modulate
the output using a Time Proportioning Output (TPO) algorithm to control the average
power to the load.
This is done by switching the output ON for a period of time (TON) and OFF for a period of
time (TOFF) and repeating this cycle indefinitely. During one (1) cycle the average power
supplied to the load is:
)( CycleTimeTrHeaterPoweerAveragePow ON= ; where Cycle Time OFFONCYC TT)T +=( .
The Duty Cycle is the factor CycleTimeTON and is normally expressed as a percentage.
Duty cycle represents the proportion of maximum available heater power. The average
power (duty cycle) is varied by changing the ratio of TON to TOFF. Over longer periods of
time the average power may be expressed as:
)( TimeTotalCycleTotalTrHeaterPoweerAveragePow ON= ;
Note that HeaterPowerhere is the actual power transferred by the heater and may not be
the same as the heaters power rating.
The PID controller calculates the Duty Cycle (the PID output control signal from 0 to 100
percent) and provides settings in the controller such as Cycle Time and Minimum On Time.
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Action Instruments Barber-Colman Continental Industries Eurotherm Chessell Eurotherm Controls
The Cycle Time setting typically varies from 100 milliseconds to over 100 seconds. The reasons
for a settable Cycle Time have to do with the type of power control device and the type of load
being driven and the effect on the life and precision of temperature control required.
The minimum possible TON or TOFF is one-half cycle of the supply frequency (8.33ms @ 60Hz)
and often is one whole cycle (16.67ms @ 60Hz) to minimize the effects of DC.
50% 75% 90%25%
Off Off Off Off Off On On On OnTime
Average
Power
Duty Cycle =T
ON/ T
CYC
100%
10%
On
0%
Figure 1
3116, 3216 Controllers
The control outputs in the 3116 and 3216 controllers may be either logic or relay output and
are typically used to drive Solid State Relays (SSR) or other AC voltage actuators. There is a
single setting in these controllers for Minimum Pulse Time 1.PLS, 2.PLS and 4.PLS. No setting
for cycle time is offered and is instead calculated from TON and TOFF. TON and TOFF are calculated
using the following formulas:
)1(. ndOutputDemaFractionalPLSnTON =
ndOutputDemaFractionalPLSnT FFO = .
Where FractionalOutputDemand = Controller Working Output (WRK.OP) in % / 100.
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Action Instruments Barber-Colman Continental Industries Eurotherm Chessell Eurotherm Controls
For example, with 1.PLS set to 250 ms, Table 1 shows TON, TOFF and TCYC. Zero (0) and one-
hundred (100) are not shown as the power would be fully Off and On respectively.
% Output TON (ms) TOFF (ms) Cycle Time (ms)
1 252.5 25000.0 25252.5
5 263.2 5000.0 5263.2
10 277.8 2500.0 2777.8
15 294.1 1666.7 1960.8
20 312.5 1250.0 1562.5
25 333.3 1000.0 1333.3
30 357.1 833.3 1190.5
35 384.6 714.3 1098.9
40 416.7 625.0 1041.7
45 454.5 555.6 1010.1
50 500.0 500.0 1000.055 555.6 454.5 1010.1
60 625.0 416.7 1041.7
65 714.3 384.6 1098.9
70 833.3 357.1 1190.5
75 1000.0 333.3 1333.3
80 1250.0 312.5 1562.5
85 1666.7 294.1 1960.8
90 2500.0 277.8 2777.8
95 5000.0 263.2 5263.2
99 25000.0 252.5 25252.5
Table 1
It can be seen from Table 1 that there is but one output percentage where TON or TOFF are
equal fifty (50) percent, as expected. Further note that at 50 percent output, both TON or TOFF
are twice the Minimum Pulse Time so that at 50 percent output and only at 50 percent
output we can say:
seTimeMimimumPulTCYC = 4 @ Duty Cycle = 50%
For a decreasing controller output towards 0 from 50 percent, TON approaches the MinimumPulse Time asymptotically while TOFF increases to maintain the correct Duty Cycle ratio. The
same holds true for an increasing output towards 100 from 50 percent, TOFF approaches the
Minimum Pulse Time asymptotically while TON increases to maintain the correct Duty Cycle
ratio. This is clearly seen in Figure 1 and Table 1.
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Action Instruments Barber-Colman Continental Industries Eurotherm Chessell Eurotherm Controls
For logic outputs, the Minimum Pulse Time may be set to Auto (default) which is 110 milli-
seconds, or between 100ms and 150 seconds. Relay outputs do not have an Auto setting but
can be adjusted between 1 and 150 seconds, 5 seconds being the default.
The Time Proportioning Output (TPO) algorithm is designed so that it never has to put small
duration pulses onto the switching device. This means that for mercury contactors the wear is
reduced for longer life. For Solid State Relays (SSR) or Silicon Controlled Rectifier (SCR) power
controllers, the minimum period is usually long enough to ensure that the zero voltage
switching restriction does not prevent a good approximation to the power requirement. Figure
2 shows the TPO algorithm in chart form.
3116, 3216 Time Propor tioning Output
0
300
600
900
1200
1500
1800
2100
2400
2700
3000
0 10 20 30 40 50 60 70 80 90 100
Output Power Demand in Percent
Timeinmillis
econds
TON (ms) TOFF (ms) Cycle Time (ms)
Figure 2