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

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James Cameron
http://quozl.netrek.org/
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