Solid State Relay Redesign

A solid state relay was sent in to us for repair recently. From viewing the board it was clear that a couple of tracks had been blown off the board. Running some external wires to replace the blown tracks would have been a quick fix however it is quite unusual for tracks to be damaged like this without a more serious cause being evident. As such we investigated further and traced out the circuit.

Close up of blown control tracks on solid state switch

You can see the missing control track which was blown off when mains shorted to part of the control circuitry

The circuit itself was quite a nice design. Back to back SCRs with appropriate fusing. Opto-isolated inputs and some indicators to pass on the state and fault conditions. However the board layout was poor. Despite the use of opto-isolators the high voltage and low voltage tracks were run in very close proximity to each other. There were numerous places where a breakdown from one track to another would lead to mains voltage being applied to the control circuitry. Even worse, there was nothing stopping the mains voltage from going back out the control terminals and livening up the control circuitry throughout the rest of the machine.

Due to the safety and reliability issues with this device it was decided to redesign the circuit board to make it safer. Whilst carrying out the design we were able to strip out a reasonable amount of unused circuitry that is present in this model but not required by our customer. This freed up enough space to be able to put in clear isolation gaps between the high voltage and low voltages sections. The diagram below shows the redesigned board with the high voltage and low voltage sections highlighted.

Redesigned solid state switch highlighting separation of mains and control circuitry

Redesigned solid state switch highlighting separation of mains and control circuitry

As you can see there is a clear barrier between the high and low voltage sections. No copper tracks break this barrier and the only components that cross it are the two opto-isolators. Designing the board in this way makes the chances of a breakdown from 240V to the control circuitry very small. It is common to find this sort of design detail in switch mode power supplies and the drive circuitry for any form of inverter (VF drives and UPSs). The design could be improved further by using cut outs to separate the closest component pins on either side of the barrier (such as under the opto-isolators). This stops any liquid or dust ingress from sitting on the board and providing an easy path for a breakdown across the barrier.

With the new design completed we had a batch of 15 boards produced by Seeed Studio using their Fusion PCB service. They run a highly automated PCB manufacturing service that is great for low cost prototyping and small batch jobs. I was caught out by not specifying the cutout for the SCR gates properly and as a result we had to mill out the slot ourselves. Other than that the boards arrived quickly and were of a very high standard. We assembled and tested one of the cards and then sent it back to the customer to be fitted in their machine. It functioned correctly so now we are in the process of replacing all of the other solid state relays across their two manufacturing lines.

Photo showing the redesigned and completed solid state relay card

The assembled redesigned solid state relay card

If you have a piece of electronics that isn’t up to modern safety standards give us a call. We would be happy to redesign and manufacture a new component to meet your needs.