# Relay inductive protection at lower voltage / amperage

I am currently working on my first large project, which involves switching a small solenoid (sprinkler valve) via a relay for ~1-2 second(s) every 1-2 minute(s) for 6 hours / day (< 1,000 / day).

I have been able to find some information about switching higher voltage + amperage inductive loads, but was looking for a bit of guidance re: lower voltage + amperage.

Solenoid is a 24VAC Orbit 57281
AC power supply reduces main voltage to provide 25.5 VAC / 650mA
Relay module is a multi-channel opto-isolated module; 5VDC power supply (https://www.amazon.com/gp/product/B01HEQF5HU/ref=ppx_yo_dt_b_asin_title_o01_s00)

The relay module is using a pretty standard Songle SRD-05VDC-SL-C, which is rated for 10A @ 250VAC

I have read from various sources (including this forum) that if the relay is sufficiently oversized, potential inductive loads don’t present a significant issue. I come from a SWE background and am still working on my EE (quite a ways to go!), so would really appreciate any thoughts. Thanks!

Those relays are unlikely to survive the number of operations you want to use and their rating “ Optimistic” in my view .

I would use decent quality industrial relay or a semi conductor switch ( solid state relay ) from some where such as RS. Bear in mind specification for relay life , can be 100,000 operations .

I don’t see an issue with this, you may want to check your solenoid for inductance and calculate the surge current and holding current. I didn’t see the datasheet. Calculate the impedance using XL= ωL or 2*pi*f*L then calculate for Z by using sqr rt(R^2 * X^2). R being the measured DC coil resistance. You can now use this to calculate current (I=V/Z). the inductance may change when your solenoid is holding vs open. The surge current is the biggest concern to make sure you have enough current to activate this.

How reliable is that relay and how long will it last? I might feel better with an Omron or equivalent where you know they didn’t cut any corners. Big water bills if and when it fails.

hammy: Those relays are unlikely to survive the number of operations you want to use and their rating “ Optimistic” in my view .

I would use decent quality industrial relay or a semi conductor switch ( solid state relay ) from some where such as RS.

Understood. Re: relay quality, I will definitely be upgrading to an industrial-grade relay prior to going to production with this project (additionally, using an industrial-grade solenoid).

While the number of cycles is significantly higher for a SSR, I was a bit of concerned about the leakage (though small) on the solenoid and heat (since the circuit will be normally open, and only closed for ~1k seconds per day).

wolframore: I don’t see an issue with this, you may want to check your solenoid for inductance and calculate the surge current and holding current. I didn’t see the datasheet. Calculate the impedance using XL= ωL or 2*pi*f*L then calculate for Z by using sqr rt(R^2 * X^2). R being the measured DC coil resistance. You can now use this to calculate current (I=V/Z). the inductance may change when your solenoid is holding vs open. The surge current is the biggest concern to make sure you have enough current to activate this.

How reliable is that relay and how long will it last? I might feel better with an Omron or equivalent where you know they didn’t cut any corners. Big water bills if and when it fails.

I don't have a data sheet for the solenoid, as it is a residential-grade sprinkler valve. When I source an industrial-grade solenoid, will definitely calculate / measure this. In my current layout / components though, it sounds like I need not be too concerned re: protecting the relay, given the relay sizing / rough potential induction?

Also, dealing with a compressed air system in an exposed place (failure would be audible), so no concerns about water bills!

A residential sprinkler valve may be designed to rely on water for lubrication. Is it rated for compressed gas, as well?

Paul

Paul_KD7HB:
A residential sprinkler valve may be designed to rely on water for lubrication. Is it rated for compressed gas, as well?

Paul

I would imagine it is not specifically rated for air / inert gas.

I’m only keeping the high-pressure side of the valve pressurized during testing (< 30 min blocks), and regulated well below its rated value (~60PSI v. 120PSI rated). Simply using this for prototyping.

Before going to production, will switch to an ASCO 210 or similar (https://www.asco.com/en-us/Pages/solenoid-valve-series-210.aspx). Inert gas rated to 150 PSI, with a regulator on the high-pressure side to keep PSI @ ~40 PSI.

walkingcactus:
I would imagine it is not specifically rated for air / inert gas.

I’m only keeping the high-pressure side of the valve pressurized during testing (< 30 min blocks), and regulated well below its rated value (~60PSI v. 120PSI rated). Simply using this for prototyping.

Before going to production, will switch to an ASCO 210 or similar (https://www.asco.com/en-us/Pages/solenoid-valve-series-210.aspx). Inert gas rated to 150 PSI, with a regulator on the high-pressure side to keep PSI @ ~40 PSI.

I still have some of the ASCO valves in my junk box. I think their active parts are either stainless steel or chrome plated for corrosion resistance. Good and reliable devices.

Paul

Paul_KD7HB: I still have some of the ASCO valves in my junk box. I think their active parts are either stainless steel or chrome plated for corrosion resistance. Good and reliable devices.

Paul

Great, thanks for the review. This project will ultimately be in a commercial environment (a burst of air triggered by a switch + software delay), in response to customer action. As a result, definitely want to ensure quality and longevity. Will of course have replacement parts on hand for hot swaps as necessary, but quality over quantity wins out in my book.

For prototyping though, I tend to go cheap so I don't have to worry when I inevitably fry something...

Their component parts are all replaceable and the coil is the same size, no matter the voltage. Very nicely designed.

Paul

I'd be surprised if a sprinkler valve can be used for compressed air.

There are a ton of inexpensive air valves designed for that purpose. Just search for solenoid air valve on Ebay or Amazon.

However, I would switch the power with a normal solid state relay. Most can switch 24-240 or more volts. They last virtually forever.

You could of course use the relay to control the power to the transformer too.

P.S Beware of Solid State relays that are too cheap to be true on Amazon & Ebay. They are fake. While they do (usually) work, the internal components are rated for a fraction of the power they claim to handle.

colorado_kid: I'd be surprised if a sprinkler valve can be used for compressed air.

There are a ton of inexpensive air valves designed for that purpose. Just search for solenoid air valve on Ebay or Amazon.

However, I would switch the power with a normal solid state relay. Most can switch 24-240 or more volts. They last virtually forever.

You could of course use the relay to control the power to the transformer too.

P.S Beware of Solid State relays that are too cheap to be true on Amazon & Ebay. They are fake. While they do (usually) work, the internal components are rated for a fraction of the power they claim to handle.

Thanks for your feedback. Will be switching to an industrial-grade solenoid soon (once I prototype everything out and make sure I'm not going to burn out a \$150 component).

I'm going to avoid using a relay on main level power. Will be using 24VAC for numerous applications in the space, so will probably run everything off a single 24VAC power supply.

For the relays, if I were to go with an industrial-grade solid state relay, eg: AD-SSR810-DC-28Z or AD-SSR810-DC-28R (Datasheet) for ~\$20, would the very minor leakage be an issue for the solenoid?

The solid state relays designed for AC are basically Triacs. The challenge in this situation is the due to the inductive load of the solenoid causing a phase shift and the inrush current can knock it out. All the solid state relays for AC work similarly and with different heat sinks. The Chinese ones are no different. The inductance can cause the inrush current to spike up multiples of the steady state. A snubber may be required and a way to switch them at zero cross. If you are burning them out it may not be the device but how it’s being used. Triacs also run hot and as the current goes up you need better heat sinks.

So in short inrush current even with zero cross (soft start) can be 8x steady state, without out this it’s close to 20x.

The leakage current at 25ma seems excessive.

I pulled a Potter-Brumfield OACM-5H from my junk box and looked at the data sheet. Shows 5ma leakage. The SSR is good for 5A at 240 volts. Ran for almost 25 years with no heatsink! In FAct, none of the SSRs in the old convection oven had heat sinks and the SSRs have no provision for heat sinking. They were all stacked together on a circuit board.

Paul

This is a solenoid for a sprinkler valve. You are over analyzing this.

Grab a big cheap SSR and go with it. It’s not like this is the Electromagnet of a wrecking yard crane.

I may have misread that datasheet :) not sure why I thought it said 20A lol