6.1 Control loop and output level

Each channel is a control loop of its own. The device uses the thermocouple to measure the temperature of the heating zone, compares it with the set temperature and calculates from this how much power the heater is to receive. This calculation is repeated in every half-cycle of the mains voltage, that is fifty times per second – faster than a heating zone can change thermally at all.

Figure 17: The control loop of a channel. Feeding the measured temperature back turns open loop control into closed loop control: the device corrects until the deviation has disappeared.

The output level is an on-time

The result of the calculation is called the output level and is displayed as a percentage. It is neither a power nor a voltage but a ratio of times: the Thermonom switches whole half-waves of the mains voltage on or off. An output level of 25 % means that on average every fourth half-wave is applied to the heater.

Three things follow from this that are noticeable on the display:

Current and power are mean values. Within a switched-on half-wave the full current flows; between the half-waves none flows. What is displayed is measured over several periods, not an instantaneous value.
At a low output level the display becomes restless. Few switched-on half-waves per measuring window give a coarser resolution – the displayed current fluctuates even though the heater is in order.
The current limits only take effect while heating. A channel with output level 0 provides no current measurement that could be compared with a limit.

The Thermonom does not set the voltage continuously. There is therefore no phase-angle control and no way of operating a heater at a reduced voltage; limiting is always done via the on-time.

Set temperature and nominal output level

Which of the two values guides the channel is decided by the operating mode, see Section 6.2, “Operating modes”:

In the operating mode Temperature the set temperature specifies the target, and the controller determines the output level itself.
In the operating mode Manual output the nominal output level specifies the on-time directly. The measured temperature continues to be displayed and monitored, but is no longer controlled.

NOTE

A channel in manual output mode does not hold its temperature. If the heat dissipation changes – different material, different cycle time, open tool – the temperature changes with it, because nobody readjusts.