Noel_g
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Powering RPI 3B+ With 24VAC

Sat Apr 13, 2019 3:20 am

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/re ... RCbHA0RP4B

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/re ... RCbN8CFQ47

My question is as follows. I plan to apply power to the RPI via 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.

W. H. Heydt
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Re: Powering RPI 3B+ With 24VAC

Sat Apr 13, 2019 4:47 pm

Noel_g wrote:
Sat Apr 13, 2019 3:20 am
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.
If you power through GPIO pins, you are bypassing the on board power protection circuits. At the very least, put a fuse (probably best to be a polyfuse) in your power circuit to protect against overcurrent conditions. To properly protect your Pi against power supply faults, you would need to--basically--replicate the power protection circuitry built onto the Pi.

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davidcoton
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Re: Powering RPI 3B+ With 24VAC

Sat Apr 13, 2019 9:17 pm

W. H. Heydt wrote:
Sat Apr 13, 2019 4:47 pm
If you power through GPIO pins, you are bypassing the on board power protection circuits. At the very least, put a fuse (probably best to be a polyfuse) in your power circuit to protect against overcurrent conditions. To properly protect your Pi against power supply faults, you would need to--basically--replicate the power protection circuitry built onto the Pi.
Actually, for a Pi3B+, a polyfuse is all that is required to replicate the on-board protection. The perfect diode used on earlier models has been removed, and the TVS diode is not bypassed. Type MF-MSMF250/X or equivalent.
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Noel_g
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Re: Powering RPI 3B+ With 24VAC

Sun Apr 14, 2019 12:19 am

What if I soldered to a mini usb cable and used the standard port?

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Imperf3kt
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Re: Powering RPI 3B+ With 24VAC

Sun Apr 14, 2019 1:12 am

Noel_g wrote:
Sun Apr 14, 2019 12:19 am
What if I soldered to a mini usb cable and used the standard port?
Typically not recommended as a standard microUSB connector usually introduces a lot of resistance and therefore, voltage drop.
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stuartiannaylor

Re: Powering RPI 3B+ With 24VAC

Sun Apr 14, 2019 1:27 am

Noel_g wrote:
Sat Apr 13, 2019 3:20 am
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/re ... RCbHA0RP4B

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/re ... RCbN8CFQ47

My question is as follows. I plan to apply power to the RPI via 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.
You could easily add a 2.5A solid state fuse, maybe try a google for this or look for alternatives
http://www.ti.com/lit/ds/symlink/tps25200.pdf

If you supply is steady it should be fine and no need its only if you have a doh moment that bypassing the current pi circuitry is a problem.

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davidcoton
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Re: Powering RPI 3B+ With 24VAC

Sun Apr 14, 2019 8:01 am

Noel_g wrote:
Sun Apr 14, 2019 12:19 am
What if I soldered to a mini usb cable and used the standard port?
That solves the fuse problem but you must make sure the wires are thick enough (18 AWG) -- this applies to power connections via the GPIO header too. Most USB cables have much thinner wires, you need one designed for charging at 2-3A.
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Burngate
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Re: Powering RPI 3B+ With 24VAC

Sun Apr 14, 2019 9:44 am

Noel_g wrote:
Sun Apr 14, 2019 12:19 am
What if I soldered to a mini usb cable and used the standard port?
Actually micro usb.
Imperf3kt wrote:
Sun Apr 14, 2019 1:12 am
Typically not recommended as a standard microUSB connector usually introduces a lot of resistance and therefore, voltage drop.
davidcoton wrote:
Sun Apr 14, 2019 8:01 am
... make sure the wires are thick enough (18 AWG). Most USB cables have much thinner wires, you need one designed for charging at 2-3A.
Why not use something like this?
https://uk.rs-online.com/web/p/micro-us ... s/1792871/
Y1792871-01.jpg
Y1792871-01.jpg (15.84 KiB) Viewed 1127 times

drgeoff
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Re: Powering RPI 3B+ With 24VAC

Sun Apr 14, 2019 12:23 pm

Thin wires are only a problem if they are long.

https://en.wikipedia.org/wiki/American_wire_gauge says for copper wire

AWG18 about 20 milliohms per metre
AWG22 about 50 milliohms per metre
AWG28 about 200 milliohms per metre

So 1 metre of AWG18 or 40cm of AWG22 or 10 cms of AWG28 all drop the same voltage when carrying the same current.

For a 3B+ at max 2.5 amps those lengths would drop 0.1 volts maximum. (The current passes through 2 lengths.)

If the buck converter is situated close to the RPi , AWG18 should not be necessary.

stuartiannaylor

Re: Powering RPI 3B+ With 24VAC

Sun Apr 14, 2019 8:10 pm

drgeoff wrote:
Sun Apr 14, 2019 12:23 pm
Thin wires are only a problem if they are long.

https://en.wikipedia.org/wiki/American_wire_gauge says for copper wire

AWG18 about 20 milliohms per metre
AWG22 about 50 milliohms per metre
AWG28 about 200 milliohms per metre

So 1 metre of AWG18 or 40cm of AWG22 or 10 cms of AWG28 all drop the same voltage when carrying the same current.

For a 3B+ at max 2.5 amps those lengths would drop 0.1 volts maximum. (The current passes through 2 lengths.)

If the buck converter is situated close to the RPi , AWG18 should not be necessary.
Yeah "If the buck converter is situated close to the RPi , AWG18 should not be necessary" as for a long time now it has really perplexed me how all pi PSU are extremely bad by just cable design and there seems to a a total lack short run DC figure8 mains power for the Pi.
If I had an audioDac I would definately want a better power supply and one thing it would like to bypass is micro-usb for power as really its an awful connector for that purpose.

Thing is its a sprinkler system that needs very low processing power and a Pi-Zero powered direct from the GPIO via buck to 24v and prob a great project for remote I/O.
Power wise it will prob never be anywhere near 2.5A and don't spend more on power ancillaries than the zero cost.

Noel_g
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Re: Powering RPI 3B+ With 24VAC

Mon Apr 15, 2019 7:15 pm

This is perfect. I can’t find it on amazon. Need to see where I can get it.

download/file.php?id=29298

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Imperf3kt
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Re: Powering RPI 3B+ With 24VAC

Mon Apr 15, 2019 8:43 pm

Noel_g wrote:
Mon Apr 15, 2019 7:15 pm
This is perfect. I can’t find it on amazon. Need to see where I can get it.

download/file.php?id=29298
Try searching for the following:
micro usb to screw terminal

That gave me multiple hits for both male and female kinds.

However, seems kind of bulky compared to something like this :
https://www.amazon.com/dp/B01KV4RANE/
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Burngate
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Re: Powering RPI 3B+ With 24VAC

Tue Apr 16, 2019 8:59 am

Yes it does look bulky, cf Imperf3ct's suggestion.
I only have two excuses: firstly, I was looking for something that would easily accept the wires on the SMAKN PSU, whose size I didn't know; and secondly, I didn't search very far.
I also like RS - they've been good to me ever since they were Radio Spares.

Noel_g
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Re: Powering RPI 3B+ With 24VAC

Tue Apr 16, 2019 11:22 am

Imperf3kt wrote:
Mon Apr 15, 2019 8:43 pm

However, seems kind of bulky compared to something like this :
https://www.amazon.com/dp/B01KV4RANE/
Oh yes I had found a few of those but then I read reviews and people mentioned how hard it was to solder to those little terminals. I am going to try my original idea of cutting a cable and soldering right to those wires.

Noel_g
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Re: Powering RPI 3B+ With 24VAC

Tue Apr 16, 2019 11:31 am

Imperf3kt wrote:
Mon Apr 15, 2019 8:43 pm

Try searching for the following:
micro usb to screw terminal
I take my prior reply back I did a search and found this. Seems to be a good option.

Poyiccot Micro USB Screw Terminal Block Connector, Micro USB Male to 5 Pin/Way Female Bolt Screw Shield terminals Pluggable Type Adapter Connector Cable 30cm(Micro USB Male) https://www.amazon.com/dp/B07PPXMTLL/re ... TCbSYCMHCK

JohnsUPS
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Re: Powering RPI 3B+ With 24VAC

Tue Apr 16, 2019 12:24 pm

You also want to keep in mind that in addition to powering the Pi, this power supply will also be actuating solenoids to control water flow. When a (or multiple) solenoids are actuated, there is the possibility of the supply voltage being dragged down, which may interrupt the Pi. Just be aware that this may happen. I wouldn't worry about this too much unless it actually happens, then you can take whatever corrective measures necessary.

LTolledo
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Re: Powering RPI 3B+ With 24VAC

Tue Apr 16, 2019 10:15 pm

How many solenoids (and their power requirements) are you planning to activate with your setup?

the linked AC power source (as I see it) is rated at max 750mA
that 750mA will have to be shared by the solenoid(s) and the buck converter powering the RPi3B+

activating a single solenoid may introduce sufficient power transients to your system, the more solenoid there is, the worst it might get.

For better system stability, powering the RPi3B+ with separate AC mains PSU (no chargers please) 5v 2.4A is my recommendation.
The solenoid(s), deriving power from the 24V AC source, will be activated by the RPi3B+ via a [relay/SSR board -- transistor/transistor module] combo.
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Noel_g
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Re: Powering RPI 3B+ With 24VAC

Wed Apr 17, 2019 12:20 am

LTolledo wrote:
Tue Apr 16, 2019 10:15 pm
How many solenoids (and their power requirements) are you planning to activate with your setup?

the linked AC power source (as I see it) is rated at max 750mA
that 750mA will have to be shared by the solenoid(s) and the buck converter powering the RPi3B+

activating a single solenoid may introduce sufficient power transients to your system, the more solenoid there is, the worst it might get.

For better system stability, powering the RPi3B+ with separate AC mains PSU (no chargers please) 5v 2.4A is my recommendation.
The solenoid(s), deriving power from the 24V AC source, will be activated by the RPi3B+ via a [relay/SSR board -- transistor/transistor module] combo.

It would be one valve at a time.

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

https://rayshobby.net/wordpress/underst ... er-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/B00KT ... 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/benchmar ... onsumption

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