Intermatic ST01 Timer Losing Time

[bababrown]

I have experienced the same problem with the ST01 timer referenced in an old post with the same title. I have two ST01 timers. The first switches 120 VAC to 5 LED bulbs and has worked flawlessly for several years. The second switches 120 VAC to a transformer which supplies 12 VAC to a string of LED landscape lights. Very occasionally, like two times in a year, this timer loses its time setting and flashes 12:00. It then blinks and resets to manual mode. The ST01 timer is battery powered. It has no neutral connection and simply switches the line to load wire. It apparently uses an electromechanical switch as I can hear it click. Intermatic suggests adding a line filter at the input to the switch with inductive loads. I tried that and the timer reset to 12:00 every time it switched! I then moved the line filter to the output of the timer just before the transformer. It now seems to work correctly although the problem was so occasional I will not know for some time. My theory is that the inductive kick from the transformer leakage inductance is getting capacitively coupled into the timer circuit.
bababrown

 

[GoldDigger]

The other alternative would be to put a snubber (RC) network across the contacts (or in parallel at transformer.)

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[dfmischler]

I use some of these to turn off walk-in cooler compressor units (using a contactor) for defrost cycles. They definitely need a snubber or filter of some kind for inductive loads. I was involved (some years ago) in testing the noise filtering circuit now sold by Intermatic as the ET-NF. Works well with the filter circuit across the load.