Using triac as simple switch

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Greetings.

I'm a software guy with enough hw knowledge to be dangerous. That said, I'm trying to learn.

I wish to control multiple 24 VAC water valves. Switching rate is low, as this is a hydroponics related project. Would like to use triacs and happen to have some Littlefuse L201E5 available. Also happen to have some xmegas available from a past project, so would like to use them.

My concerns are a) protecting the xmega from noise and triac failure; and b) proper inputs to the L201E5 triac.

Starting with b), the triac docs talk of zero crossings, positive input, and negative inputs. I understand only some of it, but conclude that it's outside the realm of simple digital IO. I'm looking for a simple on/off switch without worry as to when the input goes off relative to switched load state.

Some designs seem to solve all this by using a MOC3041 triac as inputs to the load switching triacs. At this point I get a bit lost.
- I believe the MOC3041 are sufficient to handle the 24VAC valve solenoids, but am unsure how to prove it to myself.
- I also don't see why this part is called a "triac" instead of a "relay". While the innards may be different, is the function the same?

Words of wisdom in an electrically naive format?

BTW, have read much of what exists here re triacs. Most of those discussions are intended for more complicated uses (eg dimmers, phased motor drivers) and I'm sorry to say they lose me too quickly.

BTW2, why not just use relays? Learning something new and cost.

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I'd use solid-state relays.

Leon Heller G1HSM

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A relay has contacts that arc, get dirty, wear out.
You can switch AC or DC.

A Triac can only switch AC.
It has to be turned on every cycle.
You either use a pulse of current when you want to turn on the Triac.
Or you use a steady current.

It turns off when the cycle passes through zero. It you are using a steady current, the Triac will conduct on every cycle.

No, I have not studied your data sheet. In general, the Triac has a different sensitivity on each quadrant. So your steady current must be large enough for each quadrant. You will be using quadrants 1, 2. i.e. +ve gate, +ve,-ve anode.

If you are happy 'wasting' gate current by using a steady drive, then just use the AVR o/p, or a transistor buffer.

If you want to be efficient, you just use a pulse at the start of each cycle. This can also use a buffer, diac, or triac.

Since an xmega is only 3.3V, you will probably need some form of buffer / pulse driver. An RC across the solenoid coil will 'snub' induced voltage spikes.

A 1/4 inch solenoid valve probably uses less grunt than a large valve in a water treatment works.

David.

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I'd go for the solid state relay. They're usually made of triacs and often an opto-isolator.

If you don't know my whole story, keep your mouth shut.

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steverino wrote:

I wish to control multiple 24 VAC water valves.
...
BTW2, why not just use relays? Learning something new and cost.

"Multiple" = 16

At < 50 US cents for a MOC3041 and about 70 cents for a L201E5 (if I needed to buy them), this is an inexpensive option. 16 SSRs will cost significantly more (and will teach me nothing new).

Thanks, though, for the suggestions.

Steve

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And they will sort out the gate turn-on pulse at the beginning of the cycle. i.e. zero-crossing.

You do not have to worry about snubbers, interference, anything.

You just pay your money and plug it in.
After choosing the appropriate size / rating.

Hydroponics for a couple of cannabis plants does not need very big equipment. OTOH, glasshouses covering several hectares of tomatoes get through large quantities of water.

David.

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Driving a triac is quite easy- well within the realm of simple i/o. You just need to apply a dc voltage between MT1 and the gate. From memory you need at least 2 volts and the current depends on the actual device. Check your datasheet for actual specs. For small triacs this is easily accomplished by just a series resistor between the port pin and gate. For a snubber, a 100r resistor and 100nF cap in series across MT1 and MT2 of the triac will suffice. The fun comes powering the AVR. Since the ov of the AVR is one side of the 24VAC supply, you need to make sure the solenoid current doesnt flow through the AVR side.

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steverino,

I've controlled 30 amp triacs with megas before with no issues. The real simple interface solution is a triac output optoisolator such as the MOC3063 (zero-crossing output) to control your power triac. So long as the LED is lit, it will conduct.

Note: None (that I know of) of the MOC series of traic output optoisolators are designed to carry a load (such as your solenoid valves) directly. They are ONLY designed to switch power triacs. If you use them directly (like you where hinting above), they will fail. Sooner or later. There are other optoisolators that use triac outputs that can be used to carry a small load, but I don't know of any off the top of my head.

The circuit is fairly simple and the datasheet for the MOC3063 has much of the information that you need to get running. I would personally say this is decent project to learn on as you are only using 24VAC instead of line voltage.

 

Clint

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MOC30xx devices are not intended to directly drive loads all by themselves. They are intended to drive the gate of a triac. Paring a MOC30xx device with a small triac is indeed a simple and inexpensive way to do what you want. You *can* directly drive a triac without using an opto, but most triacs are only guaranteed to operate properly in three of the four quadrants. In a nutshell, this means that the 24 vac circuit needs to be in common with the +5v rail, not ground. Trigger current needs to flow from the gate, not into it, for reliable operation. Spending the few cents for the opto eliminates this hassle.

Tom Pappano
Tulsa, Oklahoma

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Another hint: Do *not* use zero-crossing style optos if you are driving inductive loads like solenoid valves. The zero cross circuit can be 'fooled' by the phase lag, and go into a half-wave conduction mode, which will make you and your valves confused and unhappy.

Tom Pappano
Tulsa, Oklahoma

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Plenty of good free info at teccor.com, too.

Tom Pappano
Tulsa, Oklahoma

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I still have my 3rd edition.