I think it probably is "about" as easy as you think it is.

Even f the output were DC, a full wave bridge is doubly good - expect
inconsistent polarity from installers, also expect sizable back-EMF pulse
when solenoid is turned off. Full wave bridge is first line of defense for
both of those.
Consider further surge suppression on the solenoids.

Probably not relevant, but consider lightning. Long wire runs are often a
better conductor than the soil around them.

You mention the ESP8266 as load, presumably some type of sensors? Do they
represent enough load to matter?


And you're right, this is an interesting application. I'm curious as to the
"why" of irrigating under a building. No idea what part of the world you
are in, but where I grew up, most homes and buildings had drainage and
ventilation systems under them in an attempt to keep them dry.

On Fri, Jun 16, 2017 at 2:34 PM, James Cameron <quozl@laptop.org> wrote:

> How to steal power from an irrigation controller output?
>
> The controller is connected with two runs of about 15m of four core
> sheathed unshielded cable to three valve solenoids in lawn pits.
>
> 240v -->
>     irrigation controller -->
>         solenoid #1
>         solenoid #2
>         solenoid #3
>
> The controller outputs are specified on faceplate as 24V 600mA, and
> are presumably AC, though I've not yet verified that.
>
> Aim is to power two 3.3V circuits in parallel with two of the existing
> solenoids; one at the controller, one with a solenoid in a lawn pit.
>
> Prototype is a bridge of 1N4004 (400V), 47uF 35V capacitor, and a
> Pololu d24v6f3 step-down module (Vin 4.8V to 42V, Imax 600mA) [1].
>
> At the moment, startup is 0.1ms overall.  Don't need it that quick, as
> the solenoids will be on for minutes.
>
> The step-down module has an enable pin (low 0.3V, high 2.3V, 0.020mA),
> so I can easily defer startup beyond initial solenoid current draw.
>
> Load will probably be an esp8266 esp-01 or esp-12 (3.3v, 300mA peak
> during TX, 65mA during RX).
>
> It seems so easy.  So I'm looking for gotchas.  Capturing the waveform
> with a DSO at the controller is on my to-do list.
>
> For your interest, the irrigation system is for maintaining subground
> moisture under an aging building.  Twice a year, soil samples are
> weighed, microwaved until dry, then weighed again, to measure moisture
> content.  Twice a week, water is released into the ground.  Water
> costs about $1 per kilolitre.
>
> References:
>
> 1.  https://www.pololu.com/product/2106
>
> --
> James Cameron
> http://quozl.netrek.org/
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