OpenSprinkler 3.0 DC: confused by station voltage

[BertrandMT]

Hi Ray (and team?),

context: I’m intending to drive a relay with my OpenSprinkler 3.0 DC. It currently drives a valve, on one station, and does just fine! The supply is 12V DC, so I’m thinking of a 12V DC relay such as this: https://www.amazon.com/gp/product/B00LW15D1M.

In order to prep for this, I’m trying to get a sense of what voltage I see where. And this is where confusion begins:

I’m seeing about 7.5V DC from GND to COM.
I’m seeing (as I would expect, phew) 0V from COM to S2 (in this case, I care about S2) when not engaged.
But: I’m seeing -7.5V DC from COM to S2 when “running”. For what it’s worth, I am seeing about the same thing between COM and S1 when S1 is running the watering valve.

Now, this is quite confusing to me, in particular starting with the negative voltage.

So I would like to:

a. check my assumptions: am I reading all of this correctly?

and

b. if so: what gives? Is this a pulldown sort of scheme?

and

c. implications for how to drive the relay: it’s got a ground (DC-) and power supply (DC+) and signal in (IN). Is it okay to drive S2 to DC-, COM to DC+ and short DC+ to IN? Will that achieve the intended effect?

[Ray]

“I’m seeing about 7.5V DC from GND to COM.” — this is normal. COM presents positive voltage.

“I’m seeing -7.5V DC from COM to S2 when “running”.” — you should measure the voltage as multimeter’s red probe on COM (that’s always the highest voltage), and black probe on S2. That should give you a +7.5VDC, and this is normal. Every zone has an internal MOSFET switch which switches the zone port to GND when activated. In other words, the MOSFET switch is turned off (open-circuit) when the zone is not running, and turns on (connects the zone to GND) when it’s running. COM(+) -> Solenoid wire 1 -> Solenoid wire 2 (zone port) -> MOSFET -> GND(-) completes the circuit and solenoid turns on.

In circuit design, this is also known as ‘open-drain’.

To connect relays, it should be relays DC+ goes to COM, DC- goes to GND (the left most pin on the 6-pin terminal block), signal goes to a zone port like S2. The relay you linked to seems to allow settings input trigger level: either high or low. You should select trigger by a low (L) signal as that’s how OpenSprinkler DC works.

If you don’t need voltage boosting, you can go to Settings -> Advanced, and set the ‘Boost Time’ to 0. The voltage booster is needed if you use the controller to directly control solenoid valves, in which case the boosting helps to engage the solenoid in the beginning. For relays which operate at 12VDC, voltage boosting is not necessary, so you can set it to 0.

[BertrandMT]

Hi Ray, this is very helpful on all accounts. Thanks a ton.

Sometimes, with problems like these, rubber-duckying in an effective approach, and, as I was writing my question last night, I got to about the same conclusion as to how to wire the relay.

The explanation as to how the stations are implemented is great though.

[BertrandMT]

One follow up question: you indicate that voltage boosting can be removed (Settings -> Advanced, and set the ‘Boost Time’ to 0) if driving a relay. On the other hand, if I understand correctly, it is probably required to drive a valve. As far as I can tell, the setting is global (applies to all stations). If I am going to drive both valves on some stations and relays on others, I expect I should keep the Boost Time to a non-zero value, under the expectation that it is required to successfully drive valves, and it will not impact relays overly negatively?

[Ray]

Yes, the boost time setting is global and is not customized for each zone. If you use the same controller to drive solenoids as well, you should leave the boost time as is. It won’t cause any harm to your relays because it’s a very short pulse. If this is a concern, you can always have the common wire for relays run separately from the common wire of solenoids, and connect the relay’s common wire directly to the positive wire of the input power supply (i.e. not to the COM terminal). The boosted voltage is presented on the COM terminal, and obviously not on the positive wire of the input power supply.

[Author]

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