Faultfinding Temperature Controllers

One of the most important aspects of faultfinding any electrical or electronic system is to employ a logical and safe approach. Isolating and locking off supplies as appropriate. 

Issues with Temperature Controllers can be categorised into hardware or software faults. The first step should be to try to establish the nature of the fault by duplicating the error. Making use of any available indications on the controller display is also important. 

For example, a temperature controller will show the current measured temperature. Is this reading present and stable? 

There are also indications for the output stage of the controller which can be helpful in establishing if the controller logic is working correctly.

The below list is not exhaustive but gives a brief overview of the kinds of problems that can be encountered. We have numbered the lists in likelihood of occurrence.

Hardware Related Faults

1. Incorrect connections - refer to the manual and double check. Power supply polarity should be observed if applicable i.e. 12~24VDC + and -. Sensor and output terminals can be confusing, if in doubt refer to the full manual or contact us. Ensure all connections are sound and terminal blocks pushed firmly into place

2. Open/short circuit wiring - if you are competent with a multimeter test the inputs (sensor, power supplies) test the outputs (pulse, relay). This type of issue can be found in established processes which worked prior to failure

3. Physical damage - check visually for mechanical damage, ensure all voltages are within limits i.e. supply voltage does not exceed rated tolerances. If the controller display will not light up under any form of power i.e. main supply or USB if applicable then this can indicate physical damage to the internal power supply or other internal components

4. Internal component failure - followed the steps above and the controller still behaves abnormally? Internal components can fail but we would advise a full parameter reset before reaching this conclusion

Software Faults

1. Parameter errors - a very common issue, whereby one parameter can completely prevent the process from operating as expected. Carefully re-enter all parameters or perform a Full Reset and start again

2. Internal failure - internal software error is perhaps the least likely of all problems you can encounter but it is indeed possible. Note, many modern controllers will accept Firmware updates

Faultfinding Solid State Relays

Solid state relays comprise input and output circuitry. The input stage is responsible for 'switching' the output. The principle being a small control current can be used to turn on and off a larger current in the output stage. The Output commonly utilises a Triac which allows the current to be switched on and off much faster than a conventional relay or contactor.

Input Stage

The correct operation of the input stage can normally be checked by referring to the Status LEDs fitted to most SSRs. The LED will light to show the input stage is active and the output stage should have switched ON.

Failure of the input stage can be caused by incorrect or out of tolerance voltages. If the LED is not lighting up when the control supply is applied it is possible this stage has failed. 

A suspected failed input stage can be confirmed with the use of a multimeter. If the correct voltage is applied to the input stage and the LED is OFF it is likely the SSR will require replacement.

Output Stage

Of the two stages it is our experience the Output stage is the most likely to fail. Heat being a common issue which can lead to premature failure. The Output stage can fail in the closed or open state. Either failure will require the replacement of the SSR.

An open circuit type fault can be suspected if the input stage is working normally and the voltage supply is present at the output terminals. This can be confirmed with a multimeter and current clamp. The load current being absent in the output stage when the input stage is ON.

A short circuit output fault will lead to the SSR being permanently ON. This is regardless of the status of the Input. It is normally quite straightforward to diagnose this type of error. Again, a multimeter and current clamp can be helpful in confirming. The load current being present in the output stage when the input stage is OFF. 

Failed SSR Symptoms - Open Circuit Output Stage

In a heating system an open circuit Output stage manifests in a lack of heat - too cold!

• The measured temperature will not rise to meet the setpoint
• The supply to the element is present at the SSR Load Input
• The element(s) are serviceable, correct ohms
• There is an input signal to the SSR
• No current is measured in the Output (Load) stage

Failed SSR Symptoms - Short Circuit Output Stage 

In a heating system a short circuit Output stage manifests in an excess of heat - too hot!

• The measured temperature rises well above the setpoint
• The Temperature Controller and Input stage have no control over the output stage
• In many cases the Temperature Controller can be turned off and the system will continue to heat until some high level secondary limit is reached
• In the absence of a high level limit the system can reach dangerous temperatures and should be isolated and repaired immediately