PLC Digital Signal · Free Calculator

Time Proportioning Output Calculator

Turn a 0 to 100 % PID output into on and off times for a digital output driving a heater, solid state relay or contactor.

PID to digital outputCycle timeSSR and relayContact life

Controller output

Cycle time

Heater power (optional)

Contactor electrical life (optional)

operations

Short cycle times wear out mechanical contactors quickly. Use a solid state relay or a longer cycle time.

On time per cycle3.5 s

Off time per cycle6.5 s

Average power2.1 kW

Switching operations per hour360

Contactor life at this rate (continuous)0.317 years

How this result was calculated

Time Proportioning

On time = cycle time × output % ÷ 100
Off time = cycle time − on time

A 35 % output with a 10 s cycle switches the heater on for 3.5 s and off for 6.5 s, averaging 2.1 kW from a 6 kW heater.

Key insight: choose the cycle time well below the process time constant; a fast process with a long cycle shows ripple in the temperature.

What cycle time should I use?

1 to 2 s with solid state relays, 15 to 30 s or more with mechanical contactors, always much shorter than the process time constant.

Why use an SSR instead of a contactor?

An SSR has no moving contacts, so it can switch every second for years.

What is time proportioning control?

Turning a digital output on for a fraction of a fixed cycle equal to the PID output.

Pulse width modulation and duty cycle

A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.