Trying to drive 24vac solenoid with arduino. Research on driving a/c solenoids says need snubber (flyback) diode across solenoid contacts but then show examples with DC solenoids only. Understand use on DC circuits but across 24vac solenoid a diode is going to conduct half of the time causing problems. Using optically isolated solenoid modules so arduino is isolated from the 24vac side but do I still need snubber diode to protect the relay itself, and if so how to apply it?
AC driven coils don't deserve flyback diodes but the relay contacts should be protected by some snubber.
You can try one of these. Put it across the relay contacts
Why do you think you need a snubber?
It's always good practice to add a diode where a DC relay is involved and the drive is a transistor.
Clarify the solenoid/relay relationship. Driving the solenoid from the relay? I would have though a 24-V AC solenoid would need the full power to operate correctly, so don't add the diode there.
Check your module to see if it has diodes already.
Hi, @jsndgrss5
Welcome to the forum.
How are you powering the solenoids?
Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.
Of your basic circuit to drive the solenoid.
Thanks.. Tom.... 
No, see post #3
You drive the relay coil by DC and have to put a flyback diode over the relay coil.
Relay modules will include that diode already.
Research should have also shown that those solenoids can also be used on DC with voltage levels at 0.7 X AC voltage. This is the equivalent DC power of the AC.
If you measure the resistance of the AC coil, you will see why you cannot operate it with DC current @ 24V.
Hint: Inductive reactance.
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You might try a bidirectional TVS (transient voltage suppressor) diode with standoff voltage of 35 ~ 50V across the solenoid coil.
I'm sorry I ask. I got so many irrelevant answers here. Not trying to drive 24vac solenoid with DC. I just stated that the MANY examples I found online turned out to be DC solenoids powered by DC supply. In those cases a reverse diode (ie snubber) makes sense since as the current collapses there will be back current induced. My project is with 24vac solenoid being supplied with 24v doorbell buzzer transformer. Would need something like resistor-cap in series that would serve to "quench" the arc of the solenoid when contacts open. That arc that occurs at the relay will wear out the contacts over time. I guess I was looking for advise on what resistor capaciter combination works well with low (24) voltage and less than maybe 1/2 an amp current.
I know people just trying to help - and I thank them for that, but at least keep your answers relevant to the question.
Don't be sorry. If you feel that the answers were irrelevant, just go with chatGPT and remember to mention that you don't to want know anything beyond your question.
So @jim-p suggestion on post#3 wasn't ok? Not relevant enough?
A TVS diode works like two back-to-back zener diodes, when the collapsing magnetic field tries to build voltage above the zener point (say, 35V), the diode will conduct current back to the opposite terminal of the coil, clamping the "kickback" voltage to ~ 35V and preventing (suppressing) current arc across relay contacts.
https://www.littelfuse.com/assetdocs/tvs-diode-overview-application-note?assetguid=26685cec-30be-4b68-b9f2-6956384c9129
Hi, @jsndgrss5
Thanks, that is all you needed to say, yes a capacitor / resistor snubber would be good in PARALLEL with the relay contacts.
Tom....
See post #3 and #6
You seem to be focusing on the irrelevant answers for some reason
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