Can we fix this faulty time switch?

These plug in time switches are so generic that it's actually viable to just buy a new one. But for those who like the challenge of repairing something, it is viable to repair many of the issues that occur with them.

It's also worth mentioning that the same issues are associated with much more expensive equipment, including some that is obsolete, and therefore unreplaceable.

The main faults that occur are:-

The voltage dropper capacitor gradually reducing in value. This is common with the metalised plastic film capacitors. The typical symptoms are the relay buzzing, not pulling in at all, or resetting the controller each time it does activate, resulting in the unit clicking on and off continuously.
The capacitor can easily be measured with a suitable capacitance tester, or even just swapped out to see if it fixes the issue. Don't use a higher value capacitor as it puts more stress on the zener diode.

The memory backup cell failing. Just as they do in solar lights, the capacity of these cells diminishes over time, and their impedance (internal resistance) rises resulting in rogue behaviour.
You can change the cell or battery for a new one, but be aware that settings will probably be lost in the process. This is especially important in some older industrial equipment where the battery may be the only thing maintaining the full software of a machine. The PLC may be obsolete and the company that wrote the software may be out of business. That would leave the options of trying to find an identical machine to copy the software - with the risk of trashing that machine too, rewriting the software from scratch (pretty much a full product development to fix a single machine) or scrapping the machine.
If the device is "mission critical" then source a good quality cell/battery. In this case I used a matching one from a solar garden light, but those are made down to a price, so the longevity of the new cell is an unknown variable.

The zener diode failing short or open circuit, or its solder connections getting baked resulting in a dry joint. In normal use the zener diode in this unit has to dissipate the full relay coil current at 24V, and that can be a lot of power. It's easy enough to replace the zener diode with a suitably rated replacement.

The relay can fail in a few ways. It's rare the windings fail on these lower power relays, but thermal cycling can cause a termination of the fine windings to fatigue and fail. The more common failure is the contacts. They can start arcing and burn the side of the case or they can weld together if they have been switching a high inrush current load like discharge lighting or a lot of power supplies.
If there are signs of burning then it's time to replace the unit. If the contacts have welded and the load is stuck on, then a sharp tap of the unit on a hard surface will often free them. But you have to consider reducing the load or using some sort of inrush current limiting if that is happening. For very high loads it's viable to use the time switch to power a contactor so the relay is not handling the full load.

The electrolytic smoothing capacitor. These can go high impedance over time and result in roque behaviour. It's an easy thing to just swap in a new capacitor, noting that the low ESR type is needed for high frequency switch-mode style power supplies.

And this unit has one extra point of note. It has a metal oxide transient suppressor. Those can wear out over time and start passing current continuously, with a risk of a lot of heat being generated.


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