6.7 Current monitoring

Whether a heater is in order can hardly be read from the temperature: a heater with half its effect still reaches its set temperature, it just needs longer and a higher output level. The current, by contrast, changes immediately and unambiguously. It is therefore the diagnostic means for everything that happens at the heater and its supply line.

Observation	What lies behind it
Current too low	Partial failure of the heater – with several parallel coils one has burnt through. The channel keeps heating, but more weakly.
no current	Heater open, supply line interrupted, fuse tripped, or tool not plugged in.
Current too high	Insulation damage, damp heater, short circuit – or the wrong heater for this channel.

Nominal current and limits

The monitoring needs a reference: the nominal current of the channel. It is not taken from the data sheet but measured – the heater actually installed at the actual mains voltage. Around the measured value the Thermonom places a lower and an upper limit.

This measurement is carried out by the commissioning wizard, see Section 10.1, “Wizard”. It can be repeated individually via the channel setting “Measurement”. You can choose how far the limits lie around the measured value: a fixed band of half an ampere or a whole one, or the previous percentage tolerance around the new measured value, see Section 9.1.5, “Current”.

NOTE

Determine the limits anew after every tool change. A band that was right for the old tool either reports errors constantly on the new one or never reports one – both are worse than no monitoring, because people rely on it.

What a current fault triggers

Ex works a violated limit is a warning: the channel keeps heating, and the message appears in the event log. That is the right default for running production – a partial failure of the heater is a reason to check the tool at the next stop, not to come to a standstill immediately.

With “Current error: turn off” this becomes a shutdown. Useful where a deviating current points to a hazard and not merely to wear.

Independently of the limits that have been set, two protective functions take effect which cannot be switched off:

The switch-on test. Before a channel is driven regularly, the Thermonom emits a single short test pulse and measures the current. If it is above the test current value, the output is short-circuited and the channel goes into error without ever having seen full power. Only one channel at a time is ever tested – this is one of the reasons why the channels start up one after another when switched on.
The overload shutdown. A current far above the permissible value switches the channel off immediately, a moderately excessive one after a short response delay. These limits belong to the device and not to the tool; they protect the power section.

NOTE

Current is only measured while heating – the Thermonom switches whole half-waves (see Section 6.1, “Control loop and output level”). A channel with output level 0 provides no measured value, and at a very low output level the display becomes restless. When judging a current value, therefore always look at the output level as well.