TCN4 PT100 Decimal Display and Hysteresis Setting Guide

The TCN4 temperature controller supports PT100 (RTD) input and can show the measured temperature with either an integer resolution or a 1-digit decimal resolution. When the process needs a tighter readout — for example in lab heating, food processing or low-temperature HVAC work — enabling the decimal display gives a clearer picture of the actual sensor reading. This guide also covers how to set the hysteresis (HYS) value used in ON/OFF control, so the output does not chatter on and off around the setpoint.

Watch how to set the PT100 decimal display and hysteresis on the TCN4 temperature controller.

Before You Start

• Confirm a PT100 sensor is connected to the correct input terminals and that the sensor type is already set to PT100 in the controller.
• Have the controller powered on and at the main measurement screen before entering parameter mode.
• Keep the TCN4 product page open for reference if you need the full terminal and specification details.

Note: the decimal display setting changes the display resolution (how many digits are shown after the decimal point). It does not change the control accuracy by itself. The hysteresis setting, covered below, is a separate parameter that controls the ON/OFF action gap.

PT100 Decimal Display Setup

When a PT100 sensor is used, the TCN4 can switch between integer display and 1-digit decimal display. To enable the decimal display:

1. Power on the controller and press and hold the SET key for 3–5 seconds.
2. When Par2 appears, release the SET key.
3. Use the ▲ key to scroll through the parameters and locate dPt.L.
4. Press SET again and hold for 3–5 seconds to confirm.
5. The display now switches to PT100 input with 1-digit decimal resolution.

After this change the measured value is shown with one digit after the decimal point (for example 25.0 °C instead of 25 °C), giving a clearer readout in applications where small temperature changes matter.

Hysteresis Setting (ON/OFF Control)

Hysteresis (HYS) is the ON/OFF control action gap. In ON/OFF mode the output turns ON when the temperature is below the setpoint and turns OFF when it rises above it; the hysteresis value defines how far the temperature must drop back below the setpoint before the output switches ON again. A suitable HYS value prevents the relay or output from switching on and off too frequently, which extends component life and stabilises the process.

To set the hysteresis:

1. Enter the basic setting mode: press and hold the SET key for 3–5 seconds.
2. Locate the HYS parameter using the ▲ ▼ keys.
3. Adjust HYS to the value you need. For example, with HYS = 1.0 °C, the output re-engages when the measured temperature drops 1.0 °C below the setpoint.
4. Confirm and exit the parameter menu to return to the main screen.

Hysteresis only applies to ON/OFF control. It is not a PID tuning parameter — do not confuse it with P, I or D values used in PID control mode.

Practical Example

Suppose you are running a process at a 60.0 °C setpoint with a PT100 sensor. After enabling the 1-digit decimal display, the measured value reads to 0.1 °C. If you set HYS = 1.0 °C, the output turns OFF as the temperature passes 60.0 °C and turns back ON only when the temperature falls to 59.0 °C, giving a stable 1 °C switching band around the setpoint.

Common Problems

• The display still shows an integer after the change: re-enter parameter mode and confirm dPt.L was accepted, and that the input type is PT100 rather than a thermocouple.
• The output switches on and off very rapidly near the setpoint: the HYS value is too small for the process; increase it so the ON/OFF band is wider than the normal temperature noise.
• The temperature reading looks wrong: check that the PT100 sensor wiring (2-wire / 3-wire) matches the controller input configuration and that the sensor type is correctly set.

FAQ

What is the difference between decimal display and hysteresis?

Decimal display changes the display resolution (the number of digits shown after the decimal point). Hysteresis (HYS) is the ON/OFF control action gap — the temperature difference between the output switching OFF above the setpoint and switching back ON below it.

Does enabling the decimal display improve control accuracy?

It improves the readout resolution so you can see smaller temperature changes, but the actual control stability is governed by the control mode and parameters (such as HYS in ON/OFF mode, or P/I/D values in PID mode).

Should I set hysteresis in PID mode?

No. HYS applies to ON/OFF control. In PID mode the output is continuously modulated, so the hysteresis value is not the relevant tuning parameter.

Related Product

This guide applies to the TCN4 Series Digital PID Temperature Controller, which supports PT100 and thermocouple inputs with relay or SSR drive output. For full specifications, wiring help, or one-on-one support, please contact our team.