Switching AC Loads with TRIAC?

jnnewton · May 22, 2009

i normally deal with all dc circuits. want to build a circuit to turn on/off a couple of big AC solenoid valves and a couple of 1ph ac motors. my line v is 132VAC and the solenoids measure 1A and the motors measure 13A and 27A once running. does anyone know how to do this with triacs? they are much cheaper than relays.

I have found much info on "snubberless", "alternistors", etc. I just want to make this as easy and simple as possible and this seems to eliminate the sizing and selection of "snubber circuit" and other protection components like MOV. I ultimately need to drive these loads from 5V logic from a pic microcontroller.

Any help to speed up my process or simplify the problem would be appreciated, especially a "how to size a TRIAC" type of document.

Hero999 · May 23, 2009

What voltage is your control circuit?

Does it need to be isolated (I assume the answer is yes).

You need an opto-isolator to drive the TRIAC.

https://www.electro-tech-online.com/custompdfs/2009/05/moc3041.pdf

**broken link removed**

tcmtech · May 23, 2009

Just go to an on line electronics supply company and buy SSR's ( solid state relay's)
They are premade and many models are specifically built for switching high inductance loads. The typical input is 3 -32 VDC and no additional current limiting resistor is needed.
Just get one with a higher voltage and current rating than your load takes.

bigcanuknaz · May 23, 2009

There are also some electrical code issues to consider when working with line voltage and motors, if you are connected to the grid. Also, in case of a problem, you might be interested in motor overload protection. I am not sure if it would be code required at 1hp single phase in your jurisdiction.

I would suggest going to:

The best way to buy industrial controls--low prices, fast shipping and superior service.

and buying a contactor and motor overload. You will be able to get away with spending only a few tens of dollars, and will have a durable solution. The contactors there cost not much more than a triac.

naz

tcmtech · May 23, 2009

Personal projects and home owner wiring has very little Code regulations.
Use the correct size of wire for the load and distance it is from the power source and use fuses or breakers that are rated for the size of load you are running.
A 27 amp motor would be past the 80% rating of a 30 amp breaker so the next common size up would be a 40 amp.
Overload devices are great but rarely used on small home owner operated devices. a full thermal type motor overload protection controller costs about the same as a standard 1 hp motor.

Hero999 · May 23, 2009

Use a circuit breaker or fuse with the correct tripping curve for the motor you're using or it will trip every time you start it.

bigcanuknaz · May 24, 2009

Hero999 said:
Use a circuit breaker or fuse with the correct tripping curve for the motor you're using or it will trip every time you start it.

Exactly! The automation direct site also has some great information on proper overload sizing. Lots of great tutorials and guides.

tcmtech said:
... a full thermal type motor overload protection controller costs about the same as a standard 1 hp motor.

they used too. but now you can get one for $35 with a "means to disconnect." Depending on your application, that might be nice insurance to avoid burning out a $110 motor.

I understand that code issues may not apply but it is possible they do:

jnnewton said:
... couple of big AC solenoid valves and a couple of 1ph ac motors.

Just a heads up, just in case. Code is not just for it's own sake. It is logical rules for safety. Those safety rules are still in play, even in a homeowner project. I always follow the code, even if I don't need to get inspected.

naz

jnnewton · May 25, 2009

thanks everyone for replying, and i am familiar with automation direct. that is what i am using right now. the problem is the size. I have the pcb to replace a plc, and it is a fraction of the cost and size for my project. now that that is done, the contactor is the largest component left and I want it to be smaller, thereby leading me to "triacs" which seem very small in comparison and I could put it right on to the pcb. maybe a pcb mount relay is the way to go here to save cost and size?

haxan · Apr 22, 2010

Deleted

worked for pure resistive.

mneary · Apr 22, 2010

A triac or solid state relay with enough surge capacity for your 27A motors is also going to be large, and the heat sink will be even larger.

I would stay with the contactors.

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Switching AC Loads with TRIAC?

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