Controlling a generic AC load from dc

[Noggin182]

Hi All, this is my first post here so be gentle

I've got a circuit I could do with some help on. I have 5vdc (12vdc is also available) that I would like to use to control two independent AC outputs. These outputs are actually just IEC outlets and are designed to power anything. I know the easiest option would be to use a power relay but I want to use another solution, just personal preference and I want to challenge myself.

I've been looking at TRIACs. My current thinking is use the 5v output from my Arduino to control an opto isolator (something like MOC3062) connected to a TRIAC (maybe BTA16-600B). I've found several examples of this but they are all slightly different depending on the load being used.

Considering any load could be connected (if any) I'm not sure what I should do about providing snubbers, varistors or series resistors.

The main things that will be connected is audio equipment. So I guess a lot of the load would be inductive as they use transformers but there is also equipment with switch mode power supplies. I will be adding other loads later and it should also operate with no load.

Is this possible using triacs?

Any help would be appreciate, as I said I know relays might be a better option but I really enjoy learning and trying new things.

 

[kabiru]

Yes,triac will also work but it all depend on the operating voltage and the power of the triac and for relay you will need a suitable swiching circute but i will also work too.

 

[Orson Cart]

A relay may save you a lot of grief, if you use a triac or other device and get it wrong the possibility exists of significant volts and power getting back into your uP control and wrecking a fair bit of it.

Regards, Orson Cart.

 

[Noggin182]

Thanks for the quick replies, I should have mentioned that this will be controlling 240V AC.
Karibu, you say it's possible but what protection should I provide? All of the examples I've seen require that you know the load when calculating vales. As for the TRIAC itself, I haven't got any so I'm happy to buy whatever one can do the job. I assumed buying a 16A TRIAC would be easier as the whole circuit is connected with a 13A fuse so I don't have to worry about overloading it and cooling should be easier.
Orson, I'm aware that there could be large transient voltages but I'm not sure how this could affect the controlling circuit if it is optically isolated?

 

[Orson Cart]

Yes, an opto-triac or similar to trigger the triac should keep you and your uP safe, at 1.2 volts of drop say and 10 amps rms (for e.g.) you will need a heatsink to get rid of 12 watts from the triac, if you have a load like a PC which only draws current near the peak of the mains then you need to ensure trigger current is supplied all the time to the triac, not just at the start of an AC cycle. Regards, Orson.

 

[Noggin182]

Brilliant! And yes, there will be constant current provided to trigger/gate. So even though the load may not be inductive a generic solution like this should work?
GENERIC TRIAC SWITCH INTERFACE FOR INDUCTIVE LOAD - Circuit Diagram - Forum - SeekIC.com

This is the TRIAC I am looking at using (BTA16-600B), my main concern is the values of the resistors and cap. Would I also need a varistor to be safe?

---------- Post added at 10:51 ---------- Previous post was at 09:04 ----------

Sorry, I meant to post the link to the datasheet for that TRIAC but forgot, here it is:
http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXywxqt.pdf

 

[lurchman]

If you have a yen for realy old solutions look up saturable core transformers called "mag amps" Rolls Royce used to use them on jet engines for their very high reliability (back in the days when the other option was a post office type 3000 relay).

More importantly for what you are trying to do try to synchronise your device switching with the zero crossing point of the mains, as this will keep switching noise to a minimum on any audio or other equipment and also helps with reactive loads etc.

Speaking of "reactive" loads I don't know how you plan to power your micro controler, but if issolation from the mains is unimportant and you want realy cheap consider using a capacitor to act as the "dropper" in a potential divider.

 

[Noggin182]

Thanks lurchman, interesting stuff. My circuit is powered by an embedded 12v SMPS, my Arduino has an onboard voltage regulator (I think it is an LM317) to regulate the 5v. In the same unit I have a UHF receiver which is 12v. The SMPS is shielded to limit any EMI produced by the high frequency switching and I've added a filter capacitor close to the receiver as any ripple/noise in its power becomes audible on the output.

The MOC3062 (http://www.datasheetcatalog.org/datasheet/motorola/MOC3061.pdf) that I am looking at to isolate my circuit from the power TRIAC has a built in zero crossing circuit.

Looks like I'm nearly there now with this circuit, just need a little reassurance on the resistors and caps across M1 and M2 of the triac. This is the first time I have built anything to deal with 240VAC and I am a little nervous

 

[Noggin182]

I'm still struggling with this after a lot of research and I mean a LOT! I was thinking to start a new thread to attach more attention but thought I'd continue with this post first.

I've got the circuit working with a bulb but don't want to attach anything else until I get the snubber right. My latest concern is the leakage current through the snubber. The TRIAC will be "open" be most of the time so I'd rather avoid the wasted the energy. Secondly as I don't know the load that will be connected the leakage current could cause problems.

I've got a possible idea, it makes sense to me but I haven't seen it done anywhere on the net so I must be missing something. I'm planning on connecting one leg on the TRIAC to Live and the other end to the load. The other end of the load is then connected to neutral. I would prefer to have it this way as the load is external so switching Liv is safer than switching neutral. What would happen if I connected the RC snubber in parallel with the load instead of the TRIAC? This way when the TRIAC turns off any transient voltages will be conducted to neutral and no leakage current would exist...

What am I missing?

 

[Orson Cart]

Another option is a MOV across the Triac (275 or 375VAC), these have reasonably low leakage. snubbers are gnerally only necessary for inductive loads, with a MOV the turn off energy of an inductive load will hopefully be absorbed by the MOV over 1 or 2 1/2 cycles after which the Triac will turn off (the high dv/dt across the MOV will likely trigger it on until the energy is dissipated - hopefully there will not be continuous retriggering)
Regards, Orson Cart.

 

[DXNewcastle]

I've had very sucessful systems constructed using **broken link removed**. Two 12v DC control wires. Two switched 240v AC load wires. Used for audio and may other high and low power loads.
Only two failures over 20 years, working 24/7.

 

[Noggin182]

Thanks both,
Orson, I was contemplating using a MOV as well for added protection. I was wondering if this could replace the snubber but wasn't sure. Taking a couple of cycles to turn off is OK for me providing it won't cause any harm to the equipment.

DXNewcastle, I was actually looking at something similar, I have two loads to control so I was looking at this (**broken link removed**). The problem I have found with most SSRs though is they tend to have internal snubbers, plus I enjoy the exercise involved in designing and planning circuits so prefer custom make solutions