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I'm trying to control 6 normally-closed 230v valves using a raspberry pi. These valves draw little current when energised (1W) but do have a maximum inrush current of 550mA. Normally this would be easy -I'd just buy an opto-isolated relay module and be done with it. The problem is that these circuits are going to spend a lot of their time energised during certain parts of the year, and then nearly all their time off at other times. 6 x 70mA is nearly 0.5 amp of resting current on top of the 6W I'm already burning in the valves. Since I live off-grid, I'd like to save as much power as I can, both for those periods when the valves are mainly open and when their mainly closed.. Ideally, I'd also like to be able to power the control circuit directly from the Rapberry Pi, but I don't know if that's realistic - are there devices that can safely switch 230v with a 550mA inrush current that are low power enough to run directly from the RPI?

If I do have to use a relay, the good thing is that these circuits switch infrequently, and response time is not an issue. I guess I could use a bistable relay with some sort of control circuitry to generate pulses on the two inputs as the logic line transitions from 0V to 5V and vice-versa (do these exist pre-made?). But I'm hoping that maybe there's a low-power solid-state solution that will do this "in one"?

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  • \$\begingroup\$ many relays have a normally closed, and a normaly open contact. you can use them accordingly. using the normally closed all the time, and energize the relay when needed so the contact opens and valve turns off. \$\endgroup\$
    – Tirdad Sadri Nejad
    Dec 20, 2020 at 8:45
  • \$\begingroup\$ This is used to control a heating system. During certain parts of the year it will be on nearly all the time; during others it will be off nearly all the time. I'd like something that has a low power consumption at all times. Will clarify. \$\endgroup\$
    – hollandlef
    Dec 20, 2020 at 8:49
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    \$\begingroup\$ aha. so a latching relay? the bistable, mechanically latching relay has two coils for on, and off which don't need to be energized all the time. just a trigger, then it stays in that position all the time without any power comsunption. \$\endgroup\$
    – Tirdad Sadri Nejad
    Dec 20, 2020 at 8:57
  • \$\begingroup\$ If you live off grid then you probably have some 12/24V available. If you have not purchased the valves yet, you could use low voltage valves, drop the isolation requirement, and drive them with MOSFETs... \$\endgroup\$
    – bobflux
    Dec 20, 2020 at 9:09
  • \$\begingroup\$ @TirdadSadriNejad Yes, this is my fallback option. It will meet the requirement of low continuous power use, but it's not clear to me that the RPI can deliver enough current to activate these devices without an external power supply - particularly if e.g. all 6 need to switch at once. I'm guessing I will also need to create a simple circuit to convert transitions on a logic line into pulses on the relay switching lines \$\endgroup\$
    – hollandlef
    Dec 20, 2020 at 9:12

2 Answers 2

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Some solutions...

Opto-triac with low LED current. This should not require a special snubber as, during turn-off, the triac will stay on until solenoid current reaches zero.

FET Solid state relay with low LED current. Being FET based, this will need a snubber to dissipate the inductive energy in the solenoid. Or you could use this kind of component to drive a snubberless triac.

These have less than 5mA input current so can be driven by a GPIO.

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  • \$\begingroup\$ Awesome, this looks like exactly what I was looking for. The opto-triac can handle up to 3A surge current and 300mA continuous so easily enough to deal with the load placed by the actuators. I think that's the simplest practical solution and I can live with 150mW energised current draw happily (our solar system isn't that small!). Just going to check for EU availability and then will mark as accepted. \$\endgroup\$
    – hollandlef
    Dec 20, 2020 at 10:26
  • \$\begingroup\$ Cheekily, I don't suppose you know off the top of your head of options that would include at least 6 of these in a single DIP package? \$\endgroup\$
    – hollandlef
    Dec 20, 2020 at 10:28
  • \$\begingroup\$ You're welcome! As far as multiples in a package, I don't think it'll be available. It's not that hard to solder 6 DIPs...... \$\endgroup\$
    – bobflux
    Dec 20, 2020 at 10:32
  • \$\begingroup\$ I really hate online european electronics components stores. Most of them want 20€ to deliver 6 DIP6. Sigh. Anyway, I've found these available here so this is definitely my preferred solution. \$\endgroup\$
    – hollandlef
    Dec 20, 2020 at 11:16
  • \$\begingroup\$ Have you tried reichelt? \$\endgroup\$
    – bobflux
    Dec 20, 2020 at 11:38
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Your ultimate solution would be a bistable double solenoid valve which would require a pulse to open and another to close.

Both the solenoids would be deenergised all the time except for the short on and off pulses.

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  • \$\begingroup\$ Yes, this is most likely the best solution. Unfortunately, finding even normally-open valve actuators that will fit onto an underfloor heating collector and have the correct travel distance is surprisingly challenging. The standard is for NC solenoid valves and so that's what's generally available. My system is already "custom" enough without having to start building my own valve actuators. \$\endgroup\$
    – hollandlef
    Dec 20, 2020 at 10:17
  • \$\begingroup\$ I get the picture, thank you. \$\endgroup\$
    – vu2nan
    Dec 20, 2020 at 13:51

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