Providing 24VAC and 3.3VDC - BitKnitting/should_I_water GitHub Wiki

The valves are powered by 24VAC. We will use this power supply to run our 3.3VDC Feather.

We go from 24VAC to 3.3VDC in two steps:

  • Smooth AC to DC using a Full-Wave Bridge Rectifier.
  • Step voltage down from 24V to 3.3V using a DC-DC Buck Converter.

Rectifier

Parts Used:

  • 1 50V 330µF cap
  • 1 prototype board
  • 4 1N4007 diodes
    To calculate the size of the capacitor, we use the formula:
  • I = .035A. From previous tests, I'm putting the amount of current used by the Feather as 35mA.
  • t = 1/120 Hz (full-wave) = .0083 s
  • I'll use 1V for ripple. I recall I saw this used in other similar DIY projects. I'm not an expert and it has been awhile...but for now...

Resources

Rectifiers

Overview

The diagram below shows the ins-and-outs of the components that go into:

  • a 24VAC providing power to 3.3VDC components.
  • a relay switch to control turning the water valve on/off through software:

Hardware Components

The hardware components include:

Plumbing

  • a Valve hose connector - One of the connectors attaches to our gardening hose. The other attaches to the Orbit Valve(s).
  • a 24V Orbit Valve - You can get either male or female threaded.
  • a 24V adapter.
  • Some type of enclosure. I don't have any one set thing. I keep going back and forth between some enclosure I 3D Print and some box I found around the house that (seems?) is empty.

Power To The Feather (3.3VDC)

The full-wave rectifier we'll be building is the one in the DIY Full Wave (Bridge) Rectifier Project Youtube video. Here's what it looks like:

Rectifier

A rectifier converts AC to DC Voltage.

The 24VAC adapter comes into a device with those awesome AC peaks and valleys. DC current comes in flat. The first step is to transform these +/- peaks into a flat line.

Step 1 - Flat Line

We start by setting up a full-wave bridge rectifier with four diodes.

The Instructable about building a Full Wave Rectifier by Bits4Bots makes putting together a full-wave rectifier easy peasy.

We add in a capacitor to keep the voltage at a constant value when a load is added to the circuit. But what size should it be? Using the formula:

  • I = .035A. From previous tests, I'm putting the amount of current used by the Feather as 35mA.
  • t = 1/120 Hz (full-wave) = .0083 s
  • I'll use 1V for ripple. I recall I saw this used in other similar DIY projects. I'm not an expert and it has been awhile...but for now...

C = .035*.0083/1 = .0002905 = 291µF.

So we need a capacitory that is >= 291 µF. We'll also need it to spec'd to handle >= 40V.

Step 2: 3.3VDC

The Feather is powered by a 3.3V power source. Now that we have a constant voltage, we need to turn it down to 3.3V. We'll do this with a DC-DC Buck Regulator. I am using A Drok DC-DC Buck Converter.

Switch

At the request of a level change of a GPIO pin on the Feather, the relay turns on/off the 24VAC. When the 24VAC power is on, the valve is opened and irrigation begins.