I got a replacement open sprinkler pi, and replacement solenoids. I also bought some self resetting fuses rated for 500 mA (supposed to trigger at 1A, and handle 60V. I modified the open sprinkler pi to have a fuse in series with each valve connection.
I replaced the solenoids, and measured the resistance at the end of the cable, I got reasonable numbers.
I measured the solenoids I removed from the sprinkler valves, and they all measured 25.4 ohms.
Anyway, at this point I reconnected the sprinkler controller and tried to test it. The sprinklers would turn on then less than a second later turn off.
I removed the resetable fuses an tried again, this time with a current probe on the output of the power supply. I was seeing 0.9A when a sprinkler was running with occasional spikes to 2A. I was running each sprinkler in sequence, one minute each, when it got to the sixth and last sprinkler, I saw a spike of more than 4A (I was on the 4A range on the meter), and a triac and its associated gate resistor smoked.
I replaced the triac with a new one, and I checked the cabling. There were no cuts in the cable, on only the very end near the sprinkler valves has ever
gotten wet. I did notice that there was a bunch of extra cable coiled up near the sprinkler valves in the crawl space under my house.
I tried connecting up the sprinkler controller again. This time I only used the AC to power it, I did not connect the USB C power.
When I tried to test it, it turned on the 1st sprinkler, the current spiked, the pi reset, and the sprinkler turned off.
I removed the sprinkler controller, and connected the transformer directly to one of the sprinkler valves. The current was hovering around 0.5A with occasional spikes to 2A.
I tested each of the solenoids I had previously removed from the valves (not the new ones) and found that they only drew between 300 and 330 mA.
At this point I suspect that the power supply is behaving badly with the long cable. The power supply I was using has an LED, which makes me believe that it internally has something more than just a transformer. If there is some form of protection circuit that is partly triggering such that it only shuts off the output on the positive, or only the negative half of the waveform, it could be driving a large DC component into the solenoid. The DC signal would not see the inductance of the solenoid, and therefore could drive much more current. This is a rather sketchy theory, but it is the best I can come up with.
I removed the extra cable from the sprinkler valve end of the cable, this measured 42′, so originally I had 100′ of cable, and now I was down to 58′.
I have another power supply that I am pretty sure is just a transformer. It is only rated for 0.8A.
I tested the solenoids installed in the sprinkler valves by directly connecting them to the transformer. I found that they draw between 250 and 300mA.
I re-install the resettable fuses. These fuses have about 1 ohm of resistance. I re-connect the controller and test. I find that while the solenoids buzz, the sprinklers either don’t turn on, or barely turn on.
Tomorrow, I will bypass the fuses and try again.