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I have finally made some progress with my setup, now the solar panel and battery are installed. I was about to some pipework and solenoids to get started on the plumbing, but now I have a decision of using either 12VDC or 24VAC solenoids. With the 12VDC solenoids the OpenSprinkler would be running directly off 12V, with the 24VAC solenoids I would use a regulated 9V converter. Either is likely to work, but which would be better? Cable distances are modest (about 30m/90ft max) and I plan on using 1.0mm2 (18AWG) cable so wire losses will be minimal with about 1ohm return resistance.
A couple of points I am debating:
– Power usage. Since it is battery powered I want to minimise the energy used. 24VAC coils @ 9VDC will probably win here.
– Reliability. One of the reasons AC is traditionally used is it performance over long cable runs is much better than DC. 9VDC is a reasonably low voltage so becomes more susceptible to noise etc. However, over 30m this is likely to be a non-issue.
– Complexity. Not having a 12VDC->9VDC converter is one less thing to wire up.
– Longevity. 12VDC coils being energised with a 20+V pulse may shorten their life? Probably insignificant.
– Cost. The solenoids I am looking at are the the same cost with with either 12VDC or 24VAC coils so no real issue.
– Expandability. Running on 12VDC might allow a few other things (like lights) to be controlled. No plans for this, but you never know….
Overall I think I am leaning towards the 12VDC option, but open to ideas.