Snubber for AC solenoid?

Hi Gang-

I have an AC solenoid which is being driven by 14 VAC; the current through the solenoid is 2A.

Although the solenoid is being driven by a relay without a problem, energizing the solenoid seems to make nearby digital circuitry unhappy. (this is a complex model railroad system, where dozens of things are being powered from the same 14V AC bus).

I am wondering if I should put some sort of snubber across the solenoid to suppress noise/spikes. I have run across many references that say "put an RC snubber across the solenoid coil," but none say how R and C should be selected.

Any advice?

Thanks - I had a 0.01 high voltage cap lying around so I used it. Things seem better now, though it's hard to be sure that's the reason.

Two questions though -

1) How did you determine the values for R & C?

2) If the concern is not the relay contacts, but noise etc on nearby electronics, why put the snubber across the relay contacts? (I'd like to hope the relay has some snubbing - it's a box designed to switch AC toy train components, quite a few of which are inductive).

the concern is not the relay contacts, but noise etc on nearby electronics

The relay contacts are the cause of the interference. Rather than trying to mitigate the effects of the interference it is better to cure the problem at source. It's like putting ointment on a poison ivy sting against not touching the poison ivy in the first place.

The interference is caused by a rapid rise in current, sometimes called di/dt, (pronounced dee i by dee tee) this is what you do to transmit an electromagnetic signal, just like a radio signal. Instead of a radio signal having a single frequency, this radio signal covers all the band due to the infinite number of harmonics in a very short spike. Therefore it causes more interference.

What I am unclear on is the placement of the snubber across the solenoid coil vs. across the contacts of the relay that switches it. I see a lot of diagrams where the snubber is across the coil, not the switch/relay contacts.

In my application , the 'delicate' stuff is also connected on the unswitched line, physically close to the solenoid:

and the relay is physically quite far from the solenoid. Will having the snubber across the relay still protect the 'stuff'?

Also, I'm still unclear on how to estimate the capacitor values. Many diagrams show 0.1 uF, but KE7GKP recommends 0.01 uF. Is that a function of the voltage, or is it not very important?


Not being an electronics expert or radio ham … how about a shield around the relay (simple box of conducting metal) or just a large sheet between the relay and the “sensitive” stuff?

If I had any I would try it, but some you hardware guys may have a more informed opioin.

Ahh.. yes, of course... (it's always obvious afterwards ;) ) Thanks. As one that occasionally dissambles stuff, I've wondered about such parts, it seemed like just a wire in a fancy mount. The capacitance is between the wire and the screws/nut shell, right?. Makes more sense now.