Application Note: Leister SYSTEM Category of Air Heaters

Leister's product line of air heaters and heater/blower combination tools are available in a range of kilowatts, voltages, sizes, etc. They also offer many of their heaters with a selection of control options: basic heaters with no control electronics (on/off) 'CLASSIC', heaters with power electronics that allow open-loop control of percentage power output via an onboard potentiometer 'PREMIUM', and heaters with full onboard temperature controllers combined with a thermocouple feedback loop (closed-loop control) 'SYSTEM'. This application note is focused on the functionality of the Leister SYSTEM tools, which are the more sophisticated, closed-loop control tools.

Leister SYSTEM Heaters
Leister SYSTEM heaters come with a built in thermocouple, an on-board temperature controller, and a digital display. This gives the user flexibility in how the heater is controlled. Before describing the various modes in which the SYSTEM heater can be used we should first understand some of the terminology:

• Open-Loop Control: A control system in which an input alters the output, but the output has no feedback loop and therefore no effect on the input.
• Closed-Loop Control: A control system in which an input alters the output, but there is a feedback loop which adjusts the input to maintain a desired output value.
  For more in depth information on temperature control in air heaters, please refer to our article.
• Potentiometer: A resistor with two fixed ends and a third variable contact that acts as voltage divider. Often potentiometers are connected through a dial to allow users to control output such as volume on a stereo or in this case percentage power output of a heater.
• Thermocouple: A thermocouple is used for measuring temperature. Two dissimilar metals are joined together, when the joint is heated or cooled a voltage is produced which is correlated to temperature. The thermocouple can feed this information to a display or controller.
• External Controls vs. On-Board Controls: Whether the control electronics are provided by external components, or the on-board electronics incorporated into the tool.

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The Leister SYSTEM heaters can be configured for use in five different control modes. Depending on the tool, the desired mode is selected through a digital settings menu or by setting internal dipswitches. The dipswitch settings for a SYSTEM heater are given in Table 1. The dipswitches (or menu selections) determine two important things – whether the tool's internal closed-loop is activated or deactivated and whether the tool is using the on-unit potentiometer or an external control interface to provide the set point. The available modes of operation are outlined below:

1. Open-Loop using Potentiometer to set percentage power output
   Dipswitch/Menu Settings: Open-Loop and Potentiometer
   The user can set the heater to deactivate the internal closed-loop; the onboard thermocouple still measures temperature but that feedback loop is not used and the internal temperature controller is not active. Using open-loop control means that the user can adjust the percentage power output of the heater but that the temperature may fluctuate with changes in environment without feedback based correction. In this case the dial on the heater is used to set the potentiometer to the desired percentage output power. The digital display on the tool will show the actual temperature measurement from the thermocouple as well as the set percentage of power.
   
   
2. Closed-Loop using On-board Controller and Thermocouple to control temperature
   Dipswitch/Menu Settings: Closed-Loop and Potentiometer
   The potentiometer is used to set the desired output temperature. The internal thermocouple will feed back the temperature measurement to the on-board temperature controller, which will adjust the power output to maintain the set temperature. The digital display will show the set point temperature and output temperature.
   
   
3. Open-Loop using External Controller to set percentage power output
   Dipswitch/Menu Settings: Open-Loop and Interface
   Rather than use the dial on the tool to set the output power level, the heater can be connected to an external potentiometer/manual controller. The control signal sent from the external control device to the heater will be interpreted as percentage power output. The digital display will show the actual temperature reading from the internal thermocouple and the percentage power set point.
   
   
4. Closed-Loop using External Controller and Thermocouple to control temperature
   Dipswitch/Menu Settings: Open-Loop and Interface
   Setting the tool to Open-Loop may seem counterintuitive. This tool setting indicates that the internal closed-loop is deactivated so that it does not compete with the external closed-loop. When using an external controller, the loop must be closed by an external thermocouple; the internal thermocouple cannot be used in this configuration.
   The heater is set to neglect the feedback from the internal thermocouple and on-board temperature controller. Instead, an external temperature controller receives the feedback measurement from an external thermocouple, determines the power output adjustment required, and sends an appropriate control signal to the heater. The digital display on the heater shows measured temperature from the internal thermocouple and percentage power output. For the temperature measurement of the external thermocouple, the display on the external temperature controller should be consulted.
   
   
5. Closed-Loop using External Controller for temperature set point but On-board Temperature Controller/Thermocouple to control temperature
   Dipswitch/Menu Settings: Closed-Loop and Interface
   In some cases a user may want to be able to set the desired temperature remotely but still use the internal control components. In this case, an external potentiometer (or controller set to manual operation) is used and provides a steady control signal to the heater. In this mode, the heater will interpret the control signal as a temperature set point and will use the internal control components to maintain this control loop. The digital display will read the temperature set point and the measured temperature from the internal thermocouple.
   
   

Table 1: Dipswitch settings for a Leister SYSTEM heater.

If after reading this Application Note on the Leister SYSTEM heaters you still have questions regarding their use and set up please contact your technical representative.