Programmable temperature controllers

Select the input card according to the sensor type, required channel density and acceptable measurement uncertainty. The PTC320 handles one 2-wire or 4-wire RTD, thermistor or diode and covers resistances from approximately 1 Ω to 2.5 MΩ, making it suitable where one broadly configurable channel is needed. For four Pt100 channels, the PTC321 provides dedicated 4-wire inputs, a –215 °C to 850 °C range, ±30 mK initial accuracy and 2 mK RMS noise at 10 samples per second. The PTC323 provides two flexible resistive or diode inputs, while the PTC330 provides four isolated E, J, K, N or T thermocouple inputs. Dedicated cards generally suit higher channel density, while multi-sensor cards provide greater configuration flexibility.

The output card should be matched to the heater voltage, required power, load polarity and thermal response time. The PTC420 switches 120 or 240 VAC at up to 5 A and uses an adjustable 1–240 second cycle, so it is intended for relatively slow line-powered heaters. The PTC430 supplies up to 50 W with 20 V and 50 V ranges, while the PTC431 supplies up to 100 W and offers 2 A, 0.6 A and 0.2 A current ranges. The PTC440 provides a bipolar –5 A to +5 A output for heating and cooling with thermoelectric devices. For stable control, the selected range should provide adequate headroom without being unnecessarily large for the connected load.

The chassis can monitor up to 16 temperature sensors and operate up to six PID feedback loops, although the actual combination depends on the installed cards. Four wide slots accept sensor-input or heater-output cards, while the standard analogue and digital cards occupy two narrow slots. The four ±10 V analogue channels can act as PID inputs or outputs, but their 10 mA output limit means that an external amplifier is required for heater drive. Up to four PTC431 cards can be installed, but only two can run at full power simultaneously. Channel count, load power and simultaneous operating conditions should therefore be mapped before the card configuration is finalised.

The acquisition rate determines how frequently measurements are taken and PID outputs are updated, while the logging rate determines how often values are stored. Feedback loops can operate at up to 50 or 60 Hz, whereas logging can be set between 10 points per second and one point per hour for each channel. Internal memory retains the latest one million readings per channel, covering about 1.2 days at a 0.1-second interval or approximately two years at a one-minute interval. USB logging can extend the recording period according to the available storage capacity. In practice, select the acquisition rate for the thermal dynamics of the system and the logging interval for the required time resolution and file size.

A custom calibration table should be used when the built-in curve does not match the individual sensor or when a non-standard transducer is connected. Each input channel can have its own table, loaded through a USB storage device or remote command. The file must contain at least two monotonic calibration points and may contain up to 16,384 characters, which is typically around 400–800 points. RTD calibration points should normally be spaced no more than 10 °C apart, while thermistor points should be spaced by 1 °C or less for finer interpolation. The table should extend beyond the intended operating range because an out-of-range reading blanks the channel and freezes any PID loop using it.

Each input can be assigned upper, lower and rate-of-change limits, allowing the controller to respond to excessive temperature, rapid drift or a missing sensor. A triggered alarm can produce an audible indication, activate a selected relay and set a chosen output channel to zero. The digital I/O card provides four SPDT relays rated at up to 5 A and 250 VAC, together with eight optoisolated TTL lines for external status or control signals. Alarm-driven relay operation requires the digital I/O card to be installed in slot 6. These functions can support equipment protection and automated shutdown logic, but hazardous heating systems should still use an independently engineered protective circuit and power-disconnection method.

USB, Ethernet and RS-232 are included, and an IEEE 488.2 GPIB interface can be added when compatibility with existing instrumentation is required. All controller functions can be read or controlled through these interfaces, while the USB connection appears as a serial COM port on the host computer. The macro language supports variables, loops, conditional statements and subroutine calls, with up to ten programmes running concurrently. Three virtual channels can hold calculated values, follow other channels or participate in cascade-control arrangements. Together with the analogue channels, isolated TTL lines and relays, these facilities can support supervisory control without requiring the host computer to handle every local action.

Switching off the power clears the 512 MB SDRAM used as operating memory, but it does not erase all stored information. A 32 MB flash memory retains the firmware and user settings, while the internal clock continues to retain its date and time. Removable USB media may also contain user programmes, sensor calibration curves and log files. Clearing non-volatile contents requires the relevant I/O cards to be installed, a full factory reset to be performed and the firmware to be reflashed; removable media should be taken out and reformatted separately. This distinction is important in secure facilities because removing mains power alone does not clear retained configuration data or files stored on USB media.