RC Snubber for relay modules

[geologeek]

Hi all, hope you and yours are keeping well!

So I need to redo my reef-pi setup as I am transferring it to a new aquarium and not only that I have just recently had a few of my relays in the relay modules fail on me. Initially it was my heater and ATO relays that died from over a year's worth of switching, so I replaced them with SSR modules.

These worked great for 6 months, however last week I had one of my heaters go caput and after a loud bang the thing quickly filled with water and sizzled a bit.

In doing so it took out the relay it was connected to, and then when I turned off the return pump to deal with it, the return pump would not turn back on......so another relay gave up the ghost!

So I was wondering if adding RC Snubber module to the relays would help with their longevity?

They are quite cheap:


AliExpress - RC snubber module

Now the other question is how are they actually wired in? From what I can see you simply wire them across either the COM and NO or NC contacts.......But I have seen them wired such that they are across the live and neutral lines on the 240v AC:

Which is it? Or does it not matter?

My new setup will have a 5vdc 16 channel relay module (won't be using them all) and I will put some decent omron AC/AC SSR between the relay modules and the heaters and motors just for some extra piece of mind. Just wondering if the RC snubbers will be if any practical assistance in reducing failure of the mechanical relays?

Any guidance would be greatly appreciated!

Cheers,

Levi

 

[Peace River]

*bump* #reefpi

 

[DaJMasta]

So you had a heater fail which killed a relay?

Maybe you need an inline fuse - if the heater went drawing too much power it could have welded the relay contact inside of the relay itself - a destructive teardown of the relay may tell you (but also if it's just latched in the wrong position).

An RC snubber could reduce spiking voltage or inrush current, but my guess is that's not the issue - a heater is an almost entirely resistive load, so it shouldn't have so strong of an inductive component as to spike the voltage at power on by much, and I wouldn't expect any snubber or PTC inrush restriction to be necessary.

Do you have an oscilloscope or relatively quick multimeter? It would be worth checking the coil drive side of the relay to see if they're still being properly driven. Even a cheap relay should stand up to more than a year's worth of switching unless you're switching dozens of times an hour, which would be a really rough control loop for a heater, but switching a relay also can create inductive spikes on the drive lines, since the coil is a primarily inductive load, and you need to protect the logic switching them on (usually with a diode in parallel), and if the board wasn't well designed, it could be that the drive lines are being cooked by over/undervoltage.

Also worth checking the current limit on your modules and derating them for being likely pretty cheaply built. If your module is rated for 220V, 2A, and you're running a 400+W heater on it, you could just be overdrawing what it was designed to do and burning them out prematurely, and because of the potential questionable design and origin of a lot of these cheap electronic modules that are available, I would derate what they claim to be able to do by at least 2/3 to make sure you've got some margin for the loads you expect.

 

[geologeek]

So you had a heater fail which killed a relay?

Maybe you need an inline fuse - if the heater went drawing too much power it could have welded the relay contact inside of the relay itself - a destructive teardown of the relay may tell you (but also if it's just latched in the wrong position).

An RC snubber could reduce spiking voltage or inrush current, but my guess is that's not the issue - a heater is an almost entirely resistive load, so it shouldn't have so strong of an inductive component as to spike the voltage at power on by much, and I wouldn't expect any snubber or PTC inrush restriction to be necessary.

Do you have an oscilloscope or relatively quick multimeter? It would be worth checking the coil drive side of the relay to see if they're still being properly driven. Even a cheap relay should stand up to more than a year's worth of switching unless you're switching dozens of times an hour, which would be a really rough control loop for a heater, but switching a relay also can create inductive spikes on the drive lines, since the coil is a primarily inductive load, and you need to protect the logic switching them on (usually with a diode in parallel), and if the board wasn't well designed, it could be that the drive lines are being cooked by over/undervoltage.

Also worth checking the current limit on your modules and derating them for being likely pretty cheaply built. If your module is rated for 220V, 2A, and you're running a 400+W heater on it, you could just be overdrawing what it was designed to do and burning them out prematurely, and because of the potential questionable design and origin of a lot of these cheap electronic modules that are available, I would derate what they claim to be able to do by at least 2/3 to make sure you've got some margin for the loads you expect.

Thanks for the reply,

The heater was a 200w unit but was quite a few years old, I just heard a bang and when I went to check what it was I could see water bubbling into it, the seal either went when it blew or was the cause of the blow out.

I do have inline fuses on each of my outlets but possibly the 5amp glass fuses are too high a rating as it did not blow, just took out the relay.

I've not yet been able to take the unit apart to diagnose as I just had to move a few sockets around to keep it going whilst I plan out the new control unit and buy the new bits and pieces.

I will try and figure out how to scope it out with a cheap finirsi oscilloscope I have so that it won't happen again......and put the heating and pumps on decent SSR's.

And if course get some 3 amp glass fuses for my inline fuses.

Thanks again for the information!

 

[theatrus]

Hi all, hope you and yours are keeping well!

So I need to redo my reef-pi setup as I am transferring it to a new aquarium and not only that I have just recently had a few of my relays in the relay modules fail on me. Initially it was my heater and ATO relays that died from over a year's worth of switching, so I replaced them with SSR modules.

These worked great for 6 months, however last week I had one of my heaters go caput and after a loud bang the thing quickly filled with water and sizzled a bit.

In doing so it took out the relay it was connected to, and then when I turned off the return pump to deal with it, the return pump would not turn back on......so another relay gave up the ghost!

So I was wondering if adding RC Snubber module to the relays would help with their longevity?

They are quite cheap:


AliExpress - RC snubber module

Now the other question is how are they actually wired in? From what I can see you simply wire them across either the COM and NO or NC contacts.......But I have seen them wired such that they are across the live and neutral lines on the 240v AC:

Which is it? Or does it not matter?

My new setup will have a 5vdc 16 channel relay module (won't be using them all) and I will put some decent omron AC/AC SSR between the relay modules and the heaters and motors just for some extra piece of mind. Just wondering if the RC snubbers will be if any practical assistance in reducing failure of the mechanical relays?

Any guidance would be greatly appreciated!

Cheers,

Levi

Snubbers are designed to prevent relay contact wear from arcs - as the relay switch points aren't synchronized with the zero crossings in AC, or are switching inductive loads (hearts are somewhat inductive, though not motor-level inductive), arcs form as the relay opens and closes. Over time this can spot-weld or blow-open the relay so its permanently closed or open.

The extra energy from this switching operation can also cause inductive spikes back into the equipment. Something like a heater should be pretty immune from this, but some more sensitive electronics will suffer (though most power supplies shouldn't... some cheap ones who knows).

The risk with snubbers is the MOVs (the blue cylinder) will also degrade over time and transients, and slowly become more and more conductive (and in turn more heat). There are protection devices better suited for this (like combo EGT+MOVs). Also, with the capacitor and resistor network also present in the MOV, some very low load devices like a power supply will simply always be on from the power leaking across them

Can it hurt? Probably not. Have the mechanical relays failed in your case? Or have the SSRs failed?

 

[geologeek]

Have the mechanical relays failed in your case? Or have the SSRs failed?

Originally it was the mechanical relays that went so I replaced with SSR and it was these that went out this time with the heater and then the return pump.

 

[theatrus]

Originally it was the mechanical relays that went so I replaced with SSR and it was these that went out this time with the heater and then the return pump.

I would say, yes, these would help, especially around SSR modules.

If you do want to measure the actual kickback with a scope, use an isolated probe which can handle high voltage transients, or else you will blow the scope out as well. Not super cheap for something halfway decent, so I'd recommend just the belt and suspenders approach with the snubber.

 

[geologeek]

Thanks theatrus, your help as always is much appreciated!