Question Powering RPI 3B+

[Noel_g]

I am new to RPI and have basic understanding of electronics so I want to make sure I am doing things right.

My project is a Sprinkler valve controller. I have learned that those solenoid run at 24VAC so I am getting this power supply.

Orbit Sprinkler System Power Source Transformer 57040 https://www.amazon.com/dp/B000VRYVYS/ref=cm_sw_r_cp_api_i_PCZRCbHA0RP4B

It seems redundant to get a 5V power supply to power the RPI so I am also getting a power converter.

SMAKN® DC 5V/3A(MAX) AC/DC TO DC Buck Power Converter Voltage Step Dowm Power Supply Waterproof Input AC 7-36V/DC 8-50V https://www.amazon.com/dp/B00RE6QN4U/ref=cm_sw_r_cp_api_i_hFZRCbN8CFQ47

My question is as follows. I plan to apply power to the GPIO. Do I need to add anything in between the converter and the GPIO for protection? The converter can output up to 3A so I don’t know if that’s a problem.

 

[jay32267]

I see something that might screw you up.

The output of the transformer is 18 watts,

From this you need to subtract what the valve will use and what's left is available for the SMAKN.

The SMAKN "can"output 15 watts....but it's going to need more than this to output the 15 watts because it's not 100% efficient.

So I don't think you will be able to use the SMAKN to it's full power because I don't think you will have enough from the transformer....especially after you subtract the valve.

If you don't NEED all the 15 watts.....you might be OK.

 

[Noel_g]

Thanks for the info.

From what I have read, I believe that a valve uses < 2 Watts when powered by 24 VAC.

https://rayshobby.net/wordpress/understanding-24vac-sprinkler-valves/

"...under 12VDC, the Orbit valve above will dissipate 12 * 12 / 32.3 = 4.5 Watt; whereas under 24VAC, the same valve only dissipates 0.2 * 0.2 * 32.3 = 1.3 Watt (note that only the resistive portion dissipate power, inductive portion does’t)."

If I am doing this right, the power supply can output .750 * 24 = 18W
The valve will consume < 300mA but let's go higher .400 * 24 = 9.6
leaving us 8.4 W for RPI and the relays

https://www.amazon.com/gp/product/B00KTELP3I/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

The relays use 20mA so that's 0.02 * 5v = .1W (is that right?)
and the RPI is 5.1 W based on this:

https://www.pidramble.com/wiki/benchmarks/power-consumption

So based on this, am I okay? also, going back to my original question, do I need to worry about "frying" the RPI by powering via GPIO off the SMAKN?

 

[Alabalcho]

You do not apply power to GPIO. You provide 3.3 or 5VDC to RPI, and these 3/5VDC drive RPi and relays.

The GPIO pins drive transistors, which in turn drive relays, which in turn drive your valves.

 

[Noel_g]

According to this site I should be able to provide power via GPIO

Power your Raspberry Pi: expert advice for a supply — The MagPi magazine

Don’t let your Raspberry Pi suffer from a shortage of volts. Our expert guide to safe and portable power supply for the Raspberry Pi will help

www.raspberrypi.org

Is that not accurate? This is what I wanted to check as to not damage the RPI.

from the article:
"A more technical (and dangerous) way to power the Raspberry Pi is via the GPIO pins.
The 5V GPIO pins on a Raspberry Pi are connected to the 5V rail. Typically they provide the remaining power from the Raspberry Pi (that isn’t being used to run the board itself). So you can hook up the GPIO 5V pins to a 5V source and feed energy directly to the board.

1. Connect a 5V source to Pin #2 (5V).
2. Connect the ground of that source to Pin #6 (GND).

Please be aware that there is no regulation or fuse protection on the GPIO to protect from over-voltage or current spikes.
If an incorrect voltage is applied, or a current spike occurs on the line, you can permanently damage your Raspberry Pi."

 

[Alabalcho]

Ohh, that's what they call GPIO, just the pins... I'm sure you've read all the warning there, so yes - you can power it thru 5VDC/ GND pins. However, I'd get a HAT power, just to be on the safe side. You can also check various relay hats, there may be a hat which provides power as well.