# RC snubber circuit for 220V AC solenoid valve

#### Sergiu23

Joined Dec 7, 2020
3
Hello fine people!

I've got a 220v AC solenoid valve which I want to control through a 5v relay module with rasp pi.
As I've understood, I have to add an RC snubber circuit in series in order to protect the relay.

And this is where I need help. I need to figure out what resistor and what capacitor to use. I have no idea, I'm a total noob. I just don't want to fry the poor relay module.

The solenoid is like I said, 220v AC, and the only other info I could find on the web (oh.. and I've searched hours upon hours) is that it draws 18VA stable with the inrush being ~36VA.

This is all I know, please help me out with what resistor and capacitor I need to get!

Thanks so much!

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#### Ian0

Joined Aug 7, 2020
8,427
1) 100nF + 100 ohms seems to work adequately in most circumstances
2) To calculate it properly, R should be the same as the series resistance of the solenoid, and the two time constants should be the same, so RC (from the snubber) = L/R (from the solenoid) , so C= L/R^2

But there's a snag - many solenoids are designed so that the moving part completes a magnetic circuit when energised, removing the gap from the inductor and vastly increasing the inductance. This has the effect of allowing a large current when the solenoid is first energised which then reduces when the inductance changes, so that a smaller holding current keeps the valve open. Clever, but it messes up the snubber calculations!
You can measure R, and then calculate L from the power rating.

But wait - it says that the power is constant over a large range of voltage. That means that there must be some electronics inside, so perhaps it doesn't need a snubber after all.

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#### Sergiu23

Joined Dec 7, 2020
3
I think the coil is wound up to equivalate for the different voltage models to consume ~18VA for each model. So this single solenoid does not work on a wide range of voltages. Mine is rated for 220v and it only works on that voltage (+- 10%), there is also a model rated for 110v, didn't find anything lower than that for AC current.

So I don't think it has any electronics inside, but will open now to see.

#### Sergiu23

Joined Dec 7, 2020
3
So here is the valve. I've opened it, but sadly I can't open the coil itself as it's injection molded. Can't see any electronic components from the limited view.

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Joined Jul 18, 2013
27,666
This may help.
I doubt any electronics on board!
Max.

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#### benjamin.clark71

Joined Aug 7, 2017
2
Hello, not sure if you actually need to install a snubber circuit in this case. I don't think you would need to be worried about installing a snubber on the solenoid circuit, because I would guess there's not anything sensitive enough to make this a concern on the 220V circuit. Not sure what kind of relay you are using, but I would imagine for a 5 VDC controlling a 220 VAC circuit, should be a traditional electromechanical type. If that is the case (you should double-check), as long as your MCU board output is rated for enough current to drive relay, the 5V switching will all be done on the board, and there will have already been a snubber installed to support that switching transient if it was needed.

#### Ian0

Joined Aug 7, 2020
8,427
Interesting, but mainly applicable to snubbers on DC switching circuits, not mains.
One interesting point about mains snubbers is that the power dissipation in the resistor depends only on the value of the capacitor (and not on the value of the resistor)