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March 25, 2013 at 4:58 am #22378
Just wondering if anyone else has purchased a transformer for their OpenSprinkler Pi yet? I was looking at this one on Amazon. Will it work?March 25, 2013 at 12:12 pm #23448
That transformer should work. The 24VAC input is not that fussy. The current output is only 750mA, so you cannot have more than two zones on at a time. Most sprinkler controllers limit you to that anyway.
However, rather than purchase from Amazon and wait for delivery, I would suggest going to your local big box home improvement store. My local Lowes and Home Depot have bins full of these transformers. Being that spring is here, they are fully stocked for people repairing sprinkler systems. The two highest failure rates seem to be these transformers and the solenoids on the valves. These are usually in-stock. The good thing about the local home improvement store is that if it does not work for you, it is much easier to return.
P.S. most of these transformers fail because the output gets shorted by a wiring problem in the sprinkler system. This often blows an internal fuse link within the transformer itself. Adding a external 1A fuse can help save transformer failures. The wiring problem blows the external fuse which is cheaper and easier to replace.March 25, 2013 at 5:03 pm #23449
Thanks for the info mrburns42. I was looking on Amazon because none of Menards, Home Depot, or Lowes have the irrigation stuff out on the shelves yet here in Minnesota. I still have about 24 inches of snow covering my garden and grass. I also signed up for Amazon Prime so two-day shipping is free and returns are super simple.
Regarding the current output, aren’t these valves a latching solenoid? With the OpenSprinkler Pi I should be able to turn them on and off and only draw current while the solenoid is in action, so does it matter how many are on at the same time? I was planning on powering my Raspberry Pi which I believe takes the same about of current anyway. So now I am just confused.March 26, 2013 at 3:38 am #23450
AC sprinkler solenoids draw current continuously when they are energized. Typical values for larger solenoids are 350mA inrush current and 175mA hold current. So it take 350mA to pull the solenoid in. After the solenoid moves to the energized position, it take 175mA to hold it there.
There are DC solenoids for some battery operated equipment that are latching. However, all the AC systems are a much simpler non-latching version.March 27, 2013 at 2:05 am #23451
Cool. Thanks for the information. My transformer should be here tomorrow, so I should be able to get started soon.April 14, 2013 at 7:10 pm #23452
The first reply above suggests installing a fuse in series to protect the transformer/controller if there is a short in the wiring to one of the solenoids. I have 5 other irrigation controllers, and 4 out of 5 have a 1 Amp fuse in them, so other controller manufacturers have seen a need for them. In the case of the fifth one, the transformer may have become the fuse device. Over the decades, I’ve seen more fuses fail due to old age than due to a problem in the equipment they were protecting, so I view fuses as both a solution and potential problem.
Is there any fuse-type protection on the OpenSprinkler board? If there isn’t, is there any type of solid-state fuse that would do the job, or is a wire-fuse still the most economical device? Some of the 24V transformers that people might have lying around might be more likely to blow something on the board than self destruct without some form of protection.
Where’s a good place to find an inline fuse holder these days, if that is the best solution available?April 14, 2013 at 7:57 pm #23453
In the past OpenSprinkler has always come with a 750mA PTC resettable fuse. I’ve received a couple of report that in the summer when the temperature is extremely high, the resistance on the PTC fuse would increase to a point that it blocks the current supplied to the controller. So as you said, it poses a potential problem. Another issue is that these PTC fuses have a slow response time (typically a few seconds). So if there is a shorting on the solenoid, typically the transformer voltage would drop instantly before the fuse even catches on. So in practice I don’t know how useful it is. In the most recent kit I’ve removed the PTC fuse. Probably a better solution is to use a fast-action fuse. Unfortunately there is not enough space on the board to fit such a big fuse.April 14, 2013 at 8:46 pm #23454
Ray, Thanks for the info. The transformer I’m using is one from a recent Orbit 4 station controller, and it is likely fused. I’ve never had a wiring problem, so I think I’ll live dangerously and skip the fuse. I’m glad you removed the resettable fuse as it would likely be a headache here. I expect the temp in the box will be about 120 deg F during the summer, with the transformer, controller, and a power line Ethernet adapter adding heat and an outdoor temp of 110+ deg F. And that is without any direct sunlight on the box.
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