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The DC powered OpenSprinkler is certainly a welcome addition to the OS lineup. However, I have a question relating to powering the system. The nominal (recommended) supply voltage of 9VDC makes sense as per the blog post Understanding 24VAC Sprinkler Valves and gives a holding current in the 200-300mA range. The Announcement Post gives a power supply range of 5~24VDC which combined with the simple diode of the block diagram gives a much greater range of holding current that will be applied to the soleniods. Is the voltage range of 5-24VDC really permissible, or perhaps there is a more advanced regulation circuit than shown? Also, a 5V supply is unlikely to keep the 7805 regulator output in regulation, an input of more like 7V would be needed assuming the block diagram is correct.
The 5~24VDC operating range just means the controller can work with voltage within this range. For driving 24V AC solenoids, as I mentioned in the post, 9V DC is the optimal as it provides just the right amount of holding current (typical 24V AC solenoid resistance is about 35 to 45 ohms).
In case you want to run 12VDC solenoids, or 24VDC solenoids, you can do so with OpenSprinkler DC by changing to use a 12VDC (or 24VDC) power supply. So you are basically matching the power supply with your solenoid specification. Still, I suspect that even 12VDC or 24VDC solenoids would work just fine with a 9VDC power adapter, because usually once energized these solenoids can stay on at a much lower voltage, and the boost converter will help energize them initially.
You are right that 5V is below the starting voltage of 7805. However, you can actually plug in a USB cable (5V) to OpenSprinkler DC’s USB port to power it, and this bypasses 7805. The circuit design has a diode that back feed the 5V to the COM wire so you can power solenoids using a USB adapter. I’ve tested sprinkler solenoids and it works just fine. In short, you can power 24V AC solenoids with just a USB cable plugged into the USB port. This sounds a bit wild. I don’t want to advertise this because it’s not fully tested and I don’t know how reliably it is. At 5V the holding current is only about 150mA, which is technically below spec, but it seems to work ok.
Thanks for the clarification Ray. My plan is to use a solar/12V battery setup to power things, so I will likely need to find a 12->9V converter rather than risking overheating solenoids. No biggie, just another thing to obtain and wire up.
Are you planning on putting the hardware design up on GitHub at all?
If you are using 24V AC valves, yes, 9V DC would be ideal. But if you are using 12V DC valves, there is no need to do the 12-9 conversion.
The hardware design will be published as soon as we hear some feedback from the initial batch.
Any update on when the design will be published? I am keen to review the design to check it will fit into my system before purchasing.
The EagleCAD files are now in Github. Here is the link to that folder:
https://github.com/rayshobby/opensprinkler/tree/master/OpenSprinkler%20Controller/hardware/v2.3/DCI have finally made some progress with my setup, now the solar panel and battery are installed. I was about to some pipework and solenoids to get started on the plumbing, but now I have a decision of using either 12VDC or 24VAC solenoids. With the 12VDC solenoids the OpenSprinkler would be running directly off 12V, with the 24VAC solenoids I would use a regulated 9V converter. Either is likely to work, but which would be better? Cable distances are modest (about 30m/90ft max) and I plan on using 1.0mm2 (18AWG) cable so wire losses will be minimal with about 1ohm return resistance.
A couple of points I am debating:
– Power usage. Since it is battery powered I want to minimise the energy used. 24VAC coils @ 9VDC will probably win here.
– Reliability. One of the reasons AC is traditionally used is it performance over long cable runs is much better than DC. 9VDC is a reasonably low voltage so becomes more susceptible to noise etc. However, over 30m this is likely to be a non-issue.
– Complexity. Not having a 12VDC->9VDC converter is one less thing to wire up.
– Longevity. 12VDC coils being energised with a 20+V pulse may shorten their life? Probably insignificant.
– Cost. The solenoids I am looking at are the the same cost with with either 12VDC or 24VAC coils so no real issue.
– Expandability. Running on 12VDC might allow a few other things (like lights) to be controlled. No plans for this, but you never know….Overall I think I am leaning towards the 12VDC option, but open to ideas.
Can anyone point in me it the right direction for the amount of current a 12VDC coil typically uses? The power usage of a 24VAC coil is explained in Rays blog post about powering 24VAC coils with DC, but I can’t find much equivalent info for 12VDC coils. I am interested in this to determine the most efficient setup for me considering I am powering the system on a 12V battery and intend on having the coils energised for reasonably long periods because of the use of drippers in my system.
@pb: the amount of current 12VDC solenoid typically use depends on the coil resistance. I don’t have a 12VDC solenoid at hand so I don’t have enough knowledge to tell but I assume it would be similar to the typical current draw of a 24VAC solenoid, which is somewhere between 180 to 300mA.
Going back to your earlier questions:
1. long distance: the typical belief that higher voltage is better for long distance transfer is because when transmitting the same amount of power, higher voltage means lower current, hence the power loss under the same wire resistance is lower. But OpenSprinkler DC is designed to match the holding current of a typical 24VAC valve under 24VAC power, so this is not an issue.2. +20V pulse: you can set the option ‘Boost Time’ to 0, then the controller will not boost the voltage any more.
So I think the key is to find the datasheet / spec of the 12VDC solenoid, which can help you figure out the current consumption and make the final decision.
I am trying to follow this conversation and I am sure my question is answered in this thread, but I am a bit dense and need to be hit over the head in a very obvious way to understand electronics.
Can I wire up my 12VDC power (solar/battery) directly to the DC version of OpenSprinkler and its expansion boards to directly power the unit and my 24VAC valves? If so, will it cause any long term problems with any of the electrical components to do so? While I understand 9VDC is optimal, is it necessary?
I currently own the standard AC version of OpenSprinkler and am very happy with its performance. I use it to control my aquaponics greenhouse, but am looking to expand the system to water my raised beds and lawn with my high nitrogen fish water. I have an independent off grid system for powering everything and the only thing I use AC power for is the OpenSprinkler. I would like to eliminate the inverter if possible, but it seems like I would just be replacing that with a 12VDC to 9VDC converter instead.
“Can I wire up my 12VDC power (solar/battery) directly to the DC version of OpenSprinkler..”
– Yes, you can. 12VDC is slightly on the high side (which means the valves will be drawing more current than necessary), but most valves are very tolerant to a wide range of operating current, so it should be fine. There is no need to step 12VDC down to 9VDC.I am planning an irrigation system controlled with DC powered OpenSprinkler using 24vac solenoids (bermad s-390-3-d, 24vac, 130mA, 2.2Watt) and powered by 12v battery (maybe later will be backed with a solar panel, but it’s not relevant now).
I have two questions regarding this:
1. What do you think, would be better to use 12v (directly from the battery) or 5v (converted with a cigar plug adapter) to power OpenSprinkler.
I mean better in context with the solenoids life cycle first of all.
2. Are you planning a rpi version powered with DC? 🙂Attachments:
5V is probably too low as the holding current it provides is barely sufficient. Basically you can calculate the holding current by measuring the coil resistance of your solenoid, and divide 5V by that resistance. For example, typical 24VAC valves have a coil resistance of somewhere around 25 to 35 ohm. Using 7.5VDC you get about 200 to 300mA, which is up to spec. If you use 5V, you will only get 140mA to 200mA holding current, which is a bit low. It probably still works — after all, OSDC will generate a high impulse voltage to energize the valve so that reduces the requirement for holding current, but still it depends on the specific valve you have.
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