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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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Technical Information SheetTIS#: 246 Page 2 of 4



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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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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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

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