Tracing a Eurotherm 425 SS Contactor Circuit

We recently received three Eurotherm 425 Solid State contactor’s in for repair. These units were designed to be a low cost replacement for mechanical contactors. They are used for switching single phase resistive loads up to about 40A.

Searching online we were pleased to find a scanned copy of the Maintenance Manual on the Eurotherm website. Getting documentation for a lot of the old equipment that is still in use can be very difficult. Unfortunately this time the document wasn’t much help as the circuits appear to be from a slightly different model. As a result a circuit trace was required.

First up photographs of the top and bottom side of the main circuit board are taken. Annotations are made to identify each of the components.

Component side of a Eurotherm SSC

Component side of a Eurotherm SSC

Annotated solder side of a Eurotherm SSC

Annotated solder side of a Eurotherm SSC

Then with the help of a multimeter, pencil and paper and a lot of patience the circuit can be reconstructed. We have a lot of general electronics expertise and experience which helps enormously when tracing a circuit. It is important to be able to identify likely common sections of a circuit so that you don’t have to actually test each point to every other point.

Schematic of a Eurotherm SSC

Schematic of a Eurotherm SSC

Schematic of a Eurotherm SSC

Schematic of a Eurotherm SSC

With the completed circuit in hand it was then possible to power up each unit and start probing with an oscilloscope to try and locate the sections of the circuit that weren’t functioning correctly. With these models the main problem is that they are a phase control unit with current limit rather than a linear current device. As such their phase range and current limit have to be set manually.

For these three units the faults were:

  • A faulty transformer
  • An ageing tantalum capacitor restricting the phase range
  • An open circuit resistor preventing phase firing.

Once we had the circuit the repairs were able to be performed quickly allowing the units to be put back into production.