Ted YapoThe SE1B05AXJ relays I will be using have a rated lifetime of 6x10^5 actuations at the rated load (500 mA), or 10^8 at 10 uA. Since the coils are 5V/500 Ohm, they consume 10 mA. This implies a maximum fan-out of 50 for the logic gates, which is an interesting side note, but doesn't help with the lifetime calculations.
Assuming worst case load, 6x10^5 is the number of seconds in 7 days. That isn't a very long lifetime for a clock. Using the 10^8 figure (which is unrealistic), the figure is 3 years. Still not great, but I could presumably swap in fresh relays.
What if I just built an HH:MM version? Then, the 6x10^5 actuations turns into 416 days, and the 10^8 into 190 years. The truth is somewhere in between, but this sounds much better.
I think for now, I'll build an HH:MM version. I can add a seconds stage later.
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You can reduce the current in the contacts by pre-biasing the coils with a resistor. Add a bit of current, just below the release current (or else the hysteresis will stick the position) and you win a bit of margin.
I have worse fanout problems with my RES15s...
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Yes, I saw the pre-biasing trick in your project. The only relays in this design with serious fan-out are the clock drivers, and even those aren't that bad, with maybe a fanout of around 10.
I'm also wondering if I can parallel them to reduce the loading. One of the pair may be slightly faster, and see most of the wear, but the slower one could take over if the faster one degrades. It's like simulating bifurcated contacts with separate relays.
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paralleling wouldn't be so great, I think...
Another way to extend the life length is to prevent spikes and arcs, for example with capacitors across contacts. Arcs occur when opening the contacts and a diode or capacitor could help.
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