RC Snubber Components

I am having EMI issues with a homemade irrigation control box. There are 11 24VAC solenoids that are being controlled from separate relays. I was thinking of adding a snubber to each valve. Capacitor, resistor, varistor.

I have a typical wall transformer for solenoids outputting about 29VAC at no load, with about 650mA.

Solenoid spec:
350 mA inrush, 190 mA holding, 60 Hz

I was possibly thinking this cap?
FG24X7R2E223KNT06 – CAP CER 0.022UF 250V X7R RADIAL

I have these caps on hand but think they might be small?

Thinking a resistor of around 75 ohm?
I’m a little unsure of a minimum for wattage of the resistor though?

I was thinking this varistor?
MOV-14D121K – 75V max AC, Varistor Voltage (Min) 108 V

Any thoughts or advice would be appreciated.

Do have you any idea of what frequency your issues are occuring?


The precise nature of the problem remains somewhat unclear, though I’d presume it has to do with switching an inductor at the end of a long cable using mechanical contacts.

I’d suggest sizing the resistance of an RC snubber placed across the control contacts so as to limit current flow when the switch closes at peak line voltage to the contacts’ rated switching current, or a bit less. Capacitance value will be influenced by the amount of inductance and resistance in the circuit formed by the load & snubber, though I might suggest selecting it such that the quantity 0.5Rsqrt(C/L) ends up around or slightly above as a starting point. If you choose to incorporate some sort of voltage clamping device, I’d suggest sizing it such that it does not conduct at all under normal line conditions, but is close to doing so. Given your 29V open circuit source, a device with approximately a 35V max AC rating may be more suitable.

No, I’m afraid I do not.
The micro controller resets occasionally only when the relay shuts a solenoid down.

Yes this is exactly right. Sorry for my poor explanation.
I must also admit I am not the most knowledgeable when it comes to some of this.
The setup is basically a micro controller (2560), controlling a relay board (16 channel), with a couple esp Wifi modules for control and getting info from the net.
My issues come from the micro restarting itself “occasionally” only when a relay shuts a solenoid down.

The relays are rated for 250/125VAC 10 amp.

I don’t have a means of measuring certain things but this article talks about some that might be relevant.

I’m am using Hunter 24VAC valves.
This person talks about readings of inductance of about 200mH.
And a reactance of about 73ohm.

Hope this gives you a little more info, and thanks for your help.

I’d suggest 4.7ohm/0.1u/56v across the relay contacts as a starting point (Sample through-hole parts here)

Some sort of radiated phenomenon might well explain matters, but it could also be attributable to the manner in which the control relays are driven. Schematics or references to any off-the-shelf building blocks in use could help shed additional light on things.

The best and most desirable solution to the problem, in this case the Electromagnetic interference (EMI) from the collapse of the magnetic fields in the solenoids affecting the more sensitive equipment and/or communications, is to prevent the EMI in the first place. I have found that a lot of the time, in situations such as were described in this case, is that the solenoid coils do not have any devices (a diode across a DC solenoid, or an MOV or Transzorb kind of device across an AC solenoid), and those should be added. Usually the relay coils (when part of a manufactured assembly such as a PLC output module or similar) already have devices on those coils, so taking care of the solenoids should do the trick. The solid state transient protectors tend to work better than RC snubbers across the relay contacts, and one reason for this is that the back EMF from the solenoid is being clamped right there at the source, instead of trying to snub it way down at the other end of the wiring going between relay contacts and the solenoid.

If you find that it is impractical to attach suitable solid state transient protectors directly at each solenoid, you might consider using solid state switches rather than the relays. These days, there are many inexpensive solid state relays available, and since these switch on and off at the points in the AC cycle where the least amount of EMI will be generated, they solve the problem all by themselves.

And sometimes is comes down to simple grounding practice. Is your controller PLC device grounded according to the manufacturer’s recommendations? Is there a communications cable connected to your PLC device that might be picking up the EMI (rather than something inside the PLC picking up EMI) and thus glitching the operation? Proper grounding and/or shielding of the communication cables sometimes works miracles.

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once you nail down the trouble frequencies and propose some R-C values you may want to look at Quencharc from Paktron, it is an integrated R-C snubber network.