Best way to quickly switch AC power


#1

Hello Friends,

I am building a project that requires switching on and off a solenoid valve in a short time interval, ~.5s. The solenoid is AC which I have no experience with, most of my projects have been low risk dc stuff. From what i have found so far there seems to be a few popular ways to switch power; relays, MOSFETs, or SSR’s.

I have had trouble in the past interfacing with the 5v relay’s i have and don’t think that will be the fastest and easiest way to do it. I have used MOSFETS in the past with a lot of help from this community for switching DC power, but am nervous about using them with AC. I am leaning towards using a solid state relay. The SSR seams to be pretty robust compared to the former two options. Am i correct in leaning this way? What sort of protection should I include? kickback diode’s and such.

Any advice is greatly appreciated, here the post i referenced above;


#2

SSRs are pretty straight forward to use and DC/AC-SSRs do swich at zero-crossing of the switched lines (which you would need to incorporate yourself with the other options).
Since they don’t use coils for switching you won’t need flyback diodes on the SSR, but you’ll still need one on your solenoid.


#3

Thanks for the information.This one seems to be a popular recommendation. Whats different about something like this one? I know it says 5V and probably will have some trouble interfacing with it but they just look so much different. Will i need to sink the heat from these if i am keeping the switch off for long periods of time? am i on the right track with the circuit below?


#4

There are some major differences.

The former one is a single SSR with a much higher switching current (25A vs. 2A) and third higher voltage (380VAC vs. 240VAC) - providing the the given specs are correct.
While the latter one is a quad-SSR but with the lower specs.
With the given threshold of 2.5~5.0V for the HIGH signal you might get away with the 3.3V logic of Particle devices, but AFAICT the board wants to be powered off 5V.

You shouldn’t need a heat sink when you only switch a solenoid - especially not for the off state.


#5

Thanks for all the information. I think i am going to move forward with the former option. I don’t want to have to build driver circuits for the 5V board, especially if i want to keep it simple (stupid) :slight_smile: .

Thanks again.


#6

You should not use a flyback diode (D1) with an AC relay since it will clip half the AC wave. Most AC SSRs have a snubber circuit to reduce the relay’s inductive effects.


#7

I’m sorry i probably should have done more research prior to replying to your previous post. Right after replying to that I realized it was AC and felt dumb haha. Then i started looking in to snubber circuits and RC snubbers seem to be standard. What i didn’t find is a clear rule of thumb for spec’ing those components.

For example, I am using main power, 120V 60HZ (i think) and the current draw of the load when switched shouldn’t be more than 2 amps (essentially a guess), what would the capacitance and resistance one should look for?

With all that said, I don’t need a snubber circuit because this SSR is “zero switching”, is that correct?


#8

@jjlee32, most SSRs have snubber circuits built-in, including the one you want to use. So, no need to build your own. Snubbers are included to reduce the stress on the SSR when switching inductive loads (think resonance). Zero switching doesn’t alleviate that need.


#9

@peekay123 and @ScruffR, you guys are great. The support of the community here is next level.