Double triac mains switching

Projects General Guidance
1

Hey guys!
For my next project I need to individually switch on and off multiple (10-ish) switching power supplies, roughly 100 to 300 W each. They are mains powered and since they are weirdly positioned, I would like to add an additional layer of safety and switch both live and neutral.
Current installation uses relays to switch mains, but it's not working very well. The contacts occasionally stick/weld in closed position. Obviously, using more powerful relays would solve the issue, but there is a space restriction.
The idea is to use two triacs, but this is where I come to you. I must admit I'm not familiar with triacs. Anyway, this is what I have in mind: each switching power supply (in my schematic represented by a light bulb) connected between with two triacs.
In my schematic the optoisolators on the left would be MOC3010 or similar (couldn't find the proper symbol)
Do you see anything wrong with this approach?
I'm looking at the MOC30XX datasheet (Fig. 10, 11, 12, 13). They show additional resistors and capacitors in various arrangements. What would be the most suitable arrangement for my application?

Thanks!

triac.png

2

Shpaget:
Hey guys!
For my next project I need to individually switch on and off multiple (10-ish) switching power supplies, roughly 100 to 300 W each. They are mains powered and since they are weirdly positioned, I would like to add an additional layer of safety and switch both live and neutral.
Current installation uses relays to switch mains, but it's not working very well. The contacts occasionally stick/weld in closed position. Obviously, using more powerful relays would solve the issue, but there is a space restriction.
The idea is to use two triacs, but this is where I come to you. I must admit I'm not familiar with triacs. Anyway, this is what I have in mind: each switching power supply (in my schematic represented by a light bulb) connected between with two triacs.
In my schematic the optoisolators on the left would be MOC3010 or similar (couldn't find the proper symbol)
Do you see anything wrong with this approach?
I'm looking at the MOC30XX datasheet (Fig. 10, 11, 12, 13). They show additional resistors and capacitors in various arrangements. What would be the most suitable arrangement for my application?

Thanks!

Your symptoms tell me you do not have snubbers on the relay contacts to control the reactive voltage spikes you are getting when you turn off inductive loads(power supplies). Using solid state devices will not help your problem.

Paul

3

Correct, there is no snubber circuit installed, although my colleague experimented with some caps and resistors, but didn't get a satisfactory result. I'm honestly not sure what combinations he was testing.
Any suggestion where to start (what R and C values)?
And just to make sure, what arrangement we're talking about?

4

Shpaget:
Correct, there is no snubber circuit installed, although my colleague experimented with some caps and resistors, but didn't get a satisfactory result. I'm honestly not sure what combinations he was testing.
Any suggestion where to start (what R and C values)?
And just to make sure, what arrangement we're talking about?

Certainly the first circuit.

I recently deconstructed a reflow oven we had that was beyond repair. The relay circuits in it were all protected with snubbers that were all in a single plastic package with two wire leads. The R and C values are printed on the package.

I am not an EE and don't know what would work for you. Perhaps a look at one of the electronic distributors catalog (digikey, etc.) would give enlightenment.

Or an engineer on the forum might jump in with suggestions!!!

Paul

5

Thanks. I'll investigate it further, and hopefully somebody else drops by. :slight_smile:

6

Your sketch shows the trials in series...?
0e30dfdcd218fe7299f0ee7967f2e0120a8e207b.png

Apart from the snubber issue, triacs can easily handle 300watts or more each.

You’re chasing the wrong problem.

7

lastchancename:
triacs can easily handle 300watts or more each.

You’re chasing the wrong problem.

I was never worried that a single triac couldn't handle the power requirements. The idea behind two of them is to cut off both live and neutral purely for safety reasons.

8

Still chasing the wrong problem.
a) they’re in series, and
b) if they’re in separate (L/N) legs as you want, the second triac will turn off as soon as the first turns off, as they need a ‘load’ to hold the ‘on’ state.
(read up on SCRs - triacs are basically bi-directional SCRs)

9

What is the relay you currently use?
You say " relays" (plural) but you could get away with just 1.

10

I am aware that they are in series. That was the whole idea behind using two of them.
I am fine with both of them turning on and off at the same time.

What do you think is the correct problem I should be chasing and why do you think that in my case shutting both live and neutral is the wrong problem to be solving?

bluejets:
What is the relay you currently use?

This sort of thing:

11

TRIACS in series, indicates you have no idea what you’re trying to do.
The second triac is irrelevant in this circuit.

Please put warning stickers all over your controlled devices.

I'm really not trying to be mean, but if we spec or design it for you - you won't have learned anything. To figure it out, follow the current path around the circuit from the supply. If you can’t see the issue, please stop now.

For simplicity, use a DPxT relay with snubber, or read about triacs, scrs and scrs. There may even be a reference circuit to draw from.

12

I have a pretty good idea as to what I'm trying to do - disconnect both live and neutral from the device.
Using relays is fine, and current system uses them, however I am trying to improve and fix an issue with the current system, so I am investigating alternatives. Solid state came to mind so I came here to ask for guidance.

Why do you say that the second triac is irrelevant? If the wall plug is plugged so that live goes to the device directly (and not through the triac) then the circuit does not meet the requirement of cutting off live wire in any case.
In case of a single triac, I can not guarantee the orientation of the wall plug and that live will always goes through the triac before getting to the device. As I said, the power supplies are in awkward locations so I want an added safety layer. Stickers will do no good, since whoever is poking around there knows perfectly well there are mains devices around, but again, location is awkward.

I'm all for snubbers, I've said that much already, but don't they protect mainly on opening of contacts? What about inrush current when the power supply gets powered?

13

echo...
To figure it out, follow the current path around the circuit from the supply. If you can't see the issue, please stop now.

14

Shpaget:
I have a pretty good idea as to what I'm trying to do - disconnect both live and neutral from the device.

It is not a good idea to disconnect both live and neutral. In the UK it used to be quite normal for there to be a fuse in both live and neutral, The problem with this is that if the neutral fuse blows it leaves the connected device connected to the live . It is then easy to get a serious shock by connecting oneself between the device and Earth. I once received a massive electric shock as a result of this double fuse approach.

15

Double pole switching is perfectly reasonable, and mandatory in many situations, but your schematic is NOT doing that!
End of story.
If you don’t understand, there’s no point asking the forum, as physics and electrical theory won’t change because it doesn’t suit your preferences.

There may be another issue worth exploring.
TRIACs won't provide the electrical isolation to make the circuit 'safe' that you're hoping for.
In this case relays - or contactors - (wired as you have already shown) will be fine once you address the contact welding issue.

16

Hi,
What relays are you using?
What voltage are you switching? 110Vac or 240Vac?

What advantage will switching both active and neutral have.
If you are going to work on the installation you will have , I hope, opened a master circuit breaker and checked for live devices.

Also check the power factor of your SMPS's as they may not be acting as a resistive load and your triacs may not switch OFF.
Also SMPS have surge currents at turn ON, they have surge limiting built in but there is still a high ON current.

If you do want to play with triacs, try SSR, Solid State Relays, they are ready made assemblies that can take 5V logic level input to control AC mains.
The wiring is a lot cleaner and safer.

Tom... :slight_smile:

17

lastchancename:
Double pole switching is perfectly reasonable, and mandatory in many situations, but your schematic is NOT doing that!
End of story.

I'm perfectly able to accept this. That is exactly the reason I came here with the question - to ask if it would work.

lastchancename:
If you don’t understand, there’s no point asking the forum, as physics and electrical theory won’t change because it doesn’t suit your preferences.

I thought that was the point of the forum, specifically this subforum "Project Guidance
Advice on general approaches or feasibility".
It is not my preference to use triacs. I have a problem and thought I had a solution, but figured to check with people who I consider more knowledgeable about the subject, which is, again, the whole purpose of this forum, or at least that's the impression I got.
It just puzzles me why are you acting like a child that tells they have a secret but won't tell what it is. For the amount of effort you put into your "It won't work" posts, you could have easily written three sentences describing why. I appreciate and thank you for your enthusiasm to help me stay off the dead end path, but I would appreciate even more if you told me why.

lastchancename:
There may be another issue worth exploring.
TRIACs won't provide the electrical isolation to make the circuit 'safe' that you're hoping for.
In this case relays - or contactors - (wired as you have already shown) will be fine once you address the contact welding issue.

That is another part of the answer to the question I asked in the OP. Thanks.

TomGeorge:
Hi,
What relays are you using?
What voltage are you switching? 110Vac or 240Vac?

Thanks for stopping by.
They are of the ebay "Arduino Relay Module" type things, with written rating of 10 A at 240 Vac.
I'm switching 240 Vac
I figured if the SMPS used 1 to 2 A, I'd be safe with them.

TomGeorge:
What advantage will switching both active and neutral have.
If you are going to work on the installation you will have , I hope, opened a master circuit breaker and checked for live devices.

Purely as an additional safety layer.
Sure, when there is work to be done on the device the entire thing gets unplugged from the wall, but on occasion there is unrelated work needed to be done on nearby devices.

TomGeorge:
Also check the power factor of your SMPS's as they may not be acting as a resistive load and your triacs may not switch OFF.
Also SMPS have surge currents at turn ON, they have surge limiting built in but there is still a high ON current.

Yes, I believe that is the source of the contact welding.

TomGeorge:
If you do want to play with triacs, try SSR, Solid State Relays, they are ready made assemblies that can take 5V logic level input to control AC mains.
The wiring is a lot cleaner and safer.

Tom... :slight_smile:

SSRs are definitely an option.

18

Hi,

They are of the ebay "Arduino Relay Module" type things, with written rating of 10 A at 240 Vac.
I'm switching 240 Vac
I figured if the SMPS used 1 to 2 A, I'd be safe with them.

Go and consult an electrical supplier and tell them what you are switching,
you need heavier contacts.

Cut open one of those ebay relays and look at the contact area.

Tom... :slight_smile:

19

Several people have expressed several opinions on this topic.
Last time from me - your current schematic won’t work with active switching elements, and won’t meet any safety issues solution criteria.
Relays - ok.

Tom’s comments about the cheapie ‘Arduino’ type relays are valid.
If you do use them, you should route isolation gaps under them to avoid flashover from the mains. The supplied board rarely has them.
I’ve used hundreds of these - on custom PCBs designed for a mains switching application.
In the same theme, I’ve also used hundreds of SSRs in lighting control applications - typically cycling at 2-5Hz. The only failure ever - was when an installer drilled through a conduit when the system was powered up!

20

TomGeorge:
Hi,
Go and consult an electrical supplier and tell them what you are switching,
you need heavier contacts.

Cut open one of those ebay relays and look at the contact area.

Tom... :slight_smile:

Yeah, I now that I've seen that the contacts are not up to the task I realize that I'll need something better, but when building the first iteration, I thought maybe they'll suffice. I underestimated the peak currents.
I might as well open one up. :slight_smile:

lastchancename:
Several people have expressed several opinions on this topic.
Last time from me - your current schematic won’t work with active switching elements, and won’t meet any safety issues solution criteria.
Relays - ok.

Thank you for you time and input. I promise not to push the double triac thing, mkay?
I'll see what suitable SSR I can get my hands on. Any suggestion? I see that my local shop has S216S02 in stock.

lastchancename:
Tom’s comments about the cheapie ‘Arduino’ type relays are valid.
If you do use them, you should route isolation gaps under them to avoid flashover from the mains. The supplied board rarely has them.
I’ve used hundreds of these - on custom PCBs designed for a mains switching application.
In the same theme, I’ve also used hundreds of SSRs in lighting control applications - typically cycling at 2-5Hz. The only failure ever - was when an installer drilled through a conduit when the system was powered up!

The current board has routed isolation gaps.