Power Supply Design

[davethomaspilot]

I’m thinking about hacking an OpenSprinkler Pi to provide power for a POE powered Wifi station (Ubiquiti NanoStation).

I need 15V, so I’d either add a full wave bridge on the 24VAC and add a 15V buck switching regulator. It’s possible, I don’t need the regulator, if the PicoStation has an integrated switching regulator. I wouldn’t be surprised if it does, since it’s designed to operate on the end of a potentially long ethernet cable….

I could alse just use the rectified voltage from the OpenSprinkler Pi board, with or without another switching regulator.

But, I was surprised to see only 1/2 wave rectification on the OpenSprinker pi board. Seems like the extra diode for full wave rectification would reduce the required filter cap size significantly.

Also, the rectifying diode is 3000 volts. Is this for surge protection? Won’t the TVS protect adequately?

I don’t see the parts likst for V1.42+. The latest I see in the downloads section is for V1.2.

https://docs.google.com/spreadsheets/d/1m3vf6_f4W0eYYH0tNN4oUCskaHpI2ESa2HRswBn0obw/pub?output=html#

Are the rectification parts the same in V1.42+?

[Ray]

The choice depends on how much current is needed on the 15V line. If it’s a small amount of current (say less than 100mA), a 7815 linear regulator would do the job. Beyond that the heat dissipation would become significant, and you should use a switching regulator instead.

OpenSprinkler uses half-wave rectifier because of the way it switches solenoids. Specifically, it adopts a simple way to use triacs to switch solenoids, and this requires the triac and power supply to share a common ground. A full-wave rectifier would not work as it does not allow ground sharing.

The R3000 diode was picked partly because it has a higher forward drop voltage (3-4V instead of the typical 0.7V). It’s not really necessary, so all circuits now use the common S1B SMD diode instead.

There is no part list for V1.42+ — you can open the circuit diagram in EagleCAD and export a BOM yourself using EagleCAD’s default bom script.

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