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Temperature Controller Basics

Part I:

Parts of a Temperature Controller

All controllers have several common parts. For starters, controllers have inputs. The inputs are used to measure a variable in the process being controlled. In the case of a temperature controller, the measured variable is temperature.

Inputs

Temperature controllers can have several types of inputs. The type of input sensor and signal needed may vary depending on the type of controlled process. Typical inputs include thermocouples, resistive thermal devices (RTDís), and linear inputs such as mV and mA. Typical standardized thermocouple types include J, K, T, R, S, B and L types among others.

Controllers can also be set to accept an RTD as a temperature sensing input. A typical RTD would be a 100 Ohm platinum sensor (pt100).

Alternatively, controllers can be set to accept voltage or current signals in the millivolt, volt, or milliamp range from other types of sensors such as pressure, level, or flow sensors. Typical input voltage signals include 0 to 5VDC, 1 to 5VDC, 0 to 10VDC and 2 to 10 VDC. Controllers may also be set up to accept millivolt signals from sensors that include 0 to 50mVDC and 10 to 50mVDC. Controllers can also accept milliamp signals such as 0 to 20mA or 4 to 20mA.

A controller will typically incorporate a feature to detect when an input sensor is faulty or absent. This is known as a sensor break detect. Undetected, this fault condition could cause significant damage to the equipment being controlled. Therefore this feature is implemented as a safety feature so that the controller can stop immediately if a sensor break condition is detected.

Outputs

In addition to inputs, every controller also has an output. Each output can be used to do several things including control a process (such as turning on a heating or cooling source), initiate an alarm, or to retransmit the process value to a PLC or recorder.

Typical outputs provided with temperature controllers include relay outputs, solid state relay (SSR) drivers, TRIAC, and linear analog outputs.

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A relay output is usually a single-pole double-throw (SPDT) relay with a DC voltage coil. The controller energizes the relay coil, providing isolation for the contacts. This lets the contacts control an external voltage source to power the coil of a much larger heating contactor. It's important to note that the current rating of the relay contacts is usually less than 2A. The contacts can control a heating contactor with a rating of 10-20A used by the heater bands or heating elements.

Another type of output is an SSR driver. SSR driver outputs are logic outputs that turn a solid-state relay on or off. Most solid-state relays require 3 to 32 VDC to turn on. A typical SSR driver turn-on signal of 10V can drive three solid-state relays.

A TRIAC provides the relay function without any moving parts. TRIACís use solid-state devices to control currents up to 1 A. TRIAC outputs may allow some small amount of bleed current, usually less than 50mA. This bleed current doesn't affect heating contactor circuits, but it may be a problem if the output is used to connect to another solid-state circuit such as a programmable controller (PLC) input. If this is a concern, a standard relay contact would be a better choice. It provides absolute zero current when the output is de-energized and the contacts are open.

Analog outputs are provided on some controllers that send a 0-10V signal or a 4-20mA signal. These signals are calibrated so that the signal changes as a percentage of the output. For example, if a controller is sending a 0% signal, the analog output will be 0V or 4mA. When the controller is sending a 50% signal, the voltage will be 5V or 12mA. When the controller is sending a 100% signal, the voltage will be 10V or 20mA.

Reproduced from a handbook by Partlow-West

                   

 

                 

     

 

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Last modified: 29-Oct-2008