Of Relays, SSRs, Triac, Opto-Couplers ... when to use what?

[falcon74]

Hi,

My limited understanding is that we use those individually or in a combination, to control AC load (inductive / resistive) using DC control signal (say uC). Can someone share a brief summary of when which one (or which combination) is used ?

In my search I've found some material here (http://www.electronicspoint.com/differe ... 56314.html) and here (http://www.zimbio.com/Broadcast+Hardwar ... pler+Triac), but things are not crystal-clear to me.

Given following 230VAC loads can someone suggest preferred (& economical) way to control those --
1. Fluorescent lamps (40W)
2. Incandescent lamps (60W)
3. CFLs (15W)
4. Microwave Oven (1500W)
5. 29" CRT TV
6. 32" LCD TV
7. Electrical Hot-Water Geyser (1200W)
8. Refrigerator 450Ltrs
9. Water Pump (1HP)
10. Power-strip that powers Desktop PCs

Have read about issue of arcing (apparently due to huge inrush current at appliance startup time), and the quick deterioration of relays, eventually leading to shorting. I would like to avoid those issues, but not over-engineer to make the setup too expensive (if avoidable).

cheers,
F

[falcon74]

Okay, replying to my own question. Immediately after posting this question, I did find some answers/explanations on my own, so thought of sharing. However, I would still like to have the views of the experts here on what would be ideal, given my list of appliances, especially -- would be great if someone could point me to a schematic showing usage of EM-Relay or SSR with Snubber.

Here are the highly informative links I found:
1. http://en.wikipedia.org/wiki/Solid_state_relay
2. http://en.wikipedia.org/wiki/Thyristor
3. http://en.wikipedia.org/wiki/Snubber

Also, to the list of gadgets/appliance to control, would like to know what kind of a load are Airconditioners ? And what is a safe way to switch on/off mains power, say right at the mains junction box in a house.

[ericgu]

I've done quite a bit of AC control.

Relays are easy to find in ratings that will handle all of your loads. I would try to overrate the contacts so that you can better handle inductive loads. They are relatively easy to interface *but* you often need a transistor to drive them because microcontrollers can't put out enough current to drive most relays, and certainly not a bunch of them. You also need snubbing diodes across the relay contacts.

Solid-state relays are the easiest to use; you can hook them directly to the a microcontroller and they don't take too much current. Most of them switch at zero-crossing (when the AC voltage is zero), so there is no current when they switch, which limits radio noise. You can also get them without zero-crossing which gives the potential to implement dimming with a fair bit of extra code and some programming skills. They will be pricier than relays but are my solution of preference. They are also generally UL listed, which is important from a safety standpoint.

You can build the equivalent of SSRs with triacs and opto couplers, with some more circuitry to get zero-crossing switching. This looks like it will be cheaper than the SSR route but it takes skill and experience to do well.

Hope that helps.

[ericgu]

Air conditioners are big loads; take a look on the plate. It will either show the wattage (divide by the voltage to get amperage), or the amperage directly.

To switch of mains power, it's not clear if you are asking how to turn it off temporarily or to switch it from a microcontroller.

If it's the former and you don't have a main shutoff in the panel, find an electrician to put on in for you. The power in the panel is unfused in places and can put out 1000s of amps of power. Even the fused stuff can kill you.

If you're asking how to switch it from a microcontroller, I would look at systems that are designed to automatically switch over to generators and get somebody else to install it. That would get you something safe and up to code.