24VAC Water Solenoid Valve Causing Massive Voltage Spike - Causing Triac To Turn On - How To Better Prevent This?

Thread Starter

Mahonroy

Joined Oct 21, 2014
396
Hello, I have a 24VAC transformer that I am using to power some rainbird water solenoids. These are activated by mechanical relays. I also have some triac modules on this same 24VAC power. Basically like this:
1654638693035.png

When I activate a relay which turns on a solenoid valve, everything is fine. As soon as I deactivate the relay, the solenoid valve causes a massive voltage spike (over 100 positive volts, and negative 300 volts). I can clearly see this on the oscilliscope - its about 10 microseconds wide.
This causes the triac modules to randomly turn on, and then turn off after about half of a wave.

My initial thought was to use a TVS diode (bidirectional).
This was the one I tried:
1.5KE36CA-TP
https://www.digikey.com/en/products/detail/micro-commercial-co/1-5KE36CA-TP/1959257

Putting the TVS diodes right at the solenoid (at location "A" in the above diagram) - this completely solved the problem. This still worked even putting it closer to the relay output.
Putting the TVS diodes right at the transformer (at location "B" in above diagram) - this did not work. I saw the voltage spike was still supressed, but the triac would still turn on occationally from the voltage spike - much less frequent though when compared to no TVS diode.
Putting the TVS diodes at the triac input (at location "C" in above diagram) - this also did not work. Same as option "B" - the voltage spike was still supressed, but the triac would still occationally activate from the interference.

So while putting the TVS diode across the solenoid valves fixed the issue, I am looking for a more ellegant solution. It is not practical in this scenario to put a TVS diode across each solenoid valve.

I tried using a 2nd transformer only for solenoids, and this also fixed the problem, but again is not very practical.

Is there a better way of handling this issue? Is it possible to isolate the "dirty" side of the power from the "clean" side of power, with a single component, or a couple components? Hopefully something smaller than a 2nd transformer which did successfully isolate this stuff. I am picturing running a wire from the transformer to some kind of a filter, then from the filter to the triac inputs. But like I mentioned earlier, the TVS diode did not seem to work reliably in this configuration.

Any help or advice on this is greatly appreciated, thanks!
 

crutschow

Joined Mar 14, 2008
30,100
So while putting the TVS diode across the solenoid valves fixed the issue, I am looking for a more ellegant solution. It is not practical in this scenario to put a TVS diode across each solenoid valve.
Why is that?
The TVS diode can be placed across the line anywhere between the valve and the activator (including at the activator).
I see no other obvious solution.
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
396
Why is that?
The TVS diode can be placed across the line anywhere between the valve and the activator (including at the activator).
I see no other obvious solution.
In some cases there are over 100 solenoid valves. Putting 100 TVS diodes across each solenoid valve doesn't seem like the proper solution. Plus what happens if a TVS diode goes bad. At that point, I would rather use a 2nd transformer and not have to use any TVS diodes. But if a 2nd transformer works, it seems like there might still be a better solution to isolate the "dirty" power from the "clean" power.
 

prairiemystic

Joined Jun 5, 2018
275
The reason the triac false triggers is because the voltage change is too fast dV/dt across it with the spike. Do you have a 1k gate-MT1 resistor, this lessens the triac's dV/dt sensitivity. But really nobody wants HV spikes damaging things.

The usual solution is an RC snubber across the triac MT1,MT2/relay contacts. Perfect values depend on the solenoid's inductance, cable capacitance etc. Water pressure kick-back on the pintle can cause a larger spike as well, there can be unexpected generator action with larger water valves adding to the coil's back EMF.

I'd start with 0.033uf-0.1uF film cap and 33R 1/4W, nothing too critical as long as you don't make something resonate.
For larger 24VAC zone valves which are quite nasty about 85-200mH inductance 10R/0.22uF is best but the parts are too big so I use 0.1uF 33R. If you are trying to keep things inexpensive and small, experiment with the smallest capacitor value practical. Maybe 10nF would work.
 

crutschow

Joined Mar 14, 2008
30,100
You might be able to isolate the power to the SCR modules with an inductor in series followed by a capacitor (a few hundred nF) across the lines.
The inductor resistance needs to be low enough to not significantly drop the voltage due to the solenoid current, with an inductance of a couple mH or so.
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
396
The reason the triac false triggers is because the voltage change is too fast dV/dt across it with the spike. Do you have a 1k gate-MT1 resistor, this lessens the triac's dV/dt sensitivity. But really nobody wants HV spikes damaging things.

The usual solution is an RC snubber across the triac MT1,MT2/relay contacts. Perfect values depend on the solenoid's inductance, cable capacitance etc. Water pressure kick-back on the pintle can cause a larger spike as well, there can be unexpected generator action with larger water valves adding to the coil's back EMF.

I'd start with 0.033uf-0.1uF film cap and 33R 1/4W, nothing too critical as long as you don't make something resonate.
For larger 24VAC zone valves which are quite nasty about 85-200mH inductance 10R/0.22uF is best but the parts are too big so I use 0.1uF 33R. If you are trying to keep things inexpensive and small, experiment with the smallest capacitor value practical. Maybe 10nF would work.
I picked up some of these to try:
https://www.digikey.com/en/products/detail/red-lion-controls/SNUB0000/3596201

Can I get away with just one of these across the transformer (24VAC and common), or would I need to place one on every single triac input? This is in a cabinet, so I have a decent amount of room. Would a larger capacitor be better? (e.g. remove a wider range of high frequencies from the line?)
 

prairiemystic

Joined Jun 5, 2018
275
Those Red Lion integrated snubbers are fine but I think quite expensive, same as the Paktron Quencharc brand.
The values 0.1uF/47R are good. These would go across each triac and relay contact and I put one across the 24VAC transformer secondary as well. They tame the voltage spikes there enough to not cause arcing or damage to parts. TVS or MOV would work as well, there are many options.

For low cost I'd just buy a capacitor and resistor and wire them up. Example Kemet R75GF3100AA30J 0.1uF 160VDC USD $0.62 and a Yageo CFR-50JR-52-47R 47R 1/2W USD $0.11 total is 1/10 the price of the Red Lion or Paktron snubbers. There are also cheap chinese snubber boards on eBay or Ali as well "RC Absorption/Snubber Circuit" which include a 470V varistor (they are intended for mains use as well) and are under $1 each. 0.1uF/22R.

I noticed those are 30A relays, just how many solenoids are you switching? Just a note your software could stagger switching them on/off in smaller banks with even 1 sec differences will lessen the inrush and surges. How many VA is the 24VAC transformer rated? Just trying to get an idea of the system's size.
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
396
Those Red Lion integrated snubbers are fine but I think quite expensive, same as the Paktron Quencharc brand.
The values 0.1uF/47R are good. These would go across each triac and relay contact and I put one across the 24VAC transformer secondary as well. They tame the voltage spikes there enough to not cause arcing or damage to parts. TVS or MOV would work as well, there are many options.

For low cost I'd just buy a capacitor and resistor and wire them up. Example Kemet R75GF3100AA30J 0.1uF 160VDC USD $0.62 and a Yageo CFR-50JR-52-47R 47R 1/2W USD $0.11 total is 1/10 the price of the Red Lion or Paktron snubbers. There are also cheap chinese snubber boards on eBay or Ali as well "RC Absorption/Snubber Circuit" which include a 470V varistor (they are intended for mains use as well) and are under $1 each. 0.1uF/22R.

I noticed those are 30A relays, just how many solenoids are you switching? Just a note your software could stagger switching them on/off in smaller banks with even 1 sec differences will lessen the inrush and surges. How many VA is the 24VAC transformer rated? Just trying to get an idea of the system's size.
Yeah they are a bit pricey. I wouldn't mind if I just need 1 in the system. But yeah if I need to put one on every single device then I will probably go a different rout.

So to use this at a triac, I would put one leg of the snubber on the triac input, and the other leg of the snubber on common? Or would I put this in series leading up to the triac input?
 
The RC snubber connects across the triac's in/out terminals as you call it, also they are named MT1 and MT2 or A1 and A2 if you check their datasheet. Just don't wire to the gate G terminal.
So one leg to MT1, other leg to MT2. For relays, it would connect across the relay contacts.
 

MisterBill2

Joined Jan 23, 2018
12,399
Youcan also use disc capacitors, which are a bit lossy and thus do a slightly better job of reducing spikes. Also their terminal leads are more conveniently located. And they are a lot thinner. Then you might not need the series resistor.
 

marcf

Joined Dec 29, 2014
267
Is there some reason why you can't activate/deactivate the solenoids with a TRIAC rather than a relay? The large CEMF spikes would be minimized as the TRIAC would deactivate (below holding current) the solenoids when the AC potential across the solenoids are at a minimum. (As long as the gate voltage was not present).

It also seems that the cost of using a TRAC would be a lot less than a relay.
 

MisterBill2

Joined Jan 23, 2018
12,399
Is there some reason why you can't activate/deactivate the solenoids with a TRIAC rather than a relay? The large CEMF spikes would be minimized as the TRIAC would deactivate (below holding current) the solenoids when the AC potential across the solenoids are at a minimum. (As long as the gate voltage was not present).

It also seems that the cost of using a TRAC would be a lot less than a relay.
You will need to calculate the power dissipated in the triac voltage drop tp decide if a heat sink is needed.Triacs are not "zero loss" devices.
 
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