## Can I control AC Power with Relays ?

Started by 2 months ago12 replieslatest reply 1 month ago73 views

We have an Air Conditioner at the Factory that doesn't have a timer. We need to control it just to turn on at a certain time, and turn off at the rest of the day. As you can see it is only 1 ON and 1 OFF cycle for the whole day.

My worries is that this AC is made for a factory, so it has high current. The ratings are probably 5.7A during operation and about 11 A at startup.

I was wondering if I could develop a simple circuit with a microcontroller and a relay to make this function, but I'm not sure if it is safe to use relays for that or not. These are the relays available in the market for me:
SSR - 5V - 40A https://ram-e-shop.com/product/re47/
Mechanical Relay 5V - 30 A https://ram-e-shop.com/product/relay-module-5vdc-3...

Also I was thinking of a simple Atmega16L for the microcontroller, would that be sufficient?

I would love to hear any recommendations or suggestions regarding this project.

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Reply by September 23, 2019

I'll concur but add that a homebuilt controller gives insurance carriers an obvious scapegoat if something goes wrong. That it's a commercial / industrial property, I'd have certified HVAC folks do it.

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Reply by September 23, 2019

I would agree with CustomSarge. Work with the supplier of the HVAC system if possible. They're the first port of call for a technical solution to your requirement.

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Reply by September 23, 2019
I think I would just go to the store and purchase a programmable thermostat and set the on and off times and call it a day. As interesting of a project as this could be, if this were a factory, I'd be concerned with all the unknowns related to just removing power and then adding power. For example, there could be hysteresis, ramp up and down times, warm-ups, cooldowns, or other things that are done to minimize wear on the system.

You could go the route you are suggesting, but you also have to consider warranties, etc when talking about commercial equipment.

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Reply by September 23, 2019

So I was thinking of an alternative solution, the AC already has a remote control that controls it's on and off with the built in controllers of the AC. Is it possible to make a simple circuit that produces the same result of using the ON/OFF buttons on the remote control?

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Reply by September 23, 2019

Yes. There are several ways to do that Depending on how the remote is setup. For example:

1) If the remote is IR, you can figure out what the protocol is by either research and reverse engineering and then use a microcontroller and an IR LED to duplicate the on and off commands.

2) You could solder wires on the button contacts of the remote and then use a microcontroller to, or even a 555 with the desired period, to close the contact as your timer.

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Reply by September 23, 2019

Aha. I think these could be much easier to start experimenting with.

Thank you so much for the help.

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Reply by September 23, 2019

I've built several IR "substitutes" by finding carrier freq (o'scope), and buying an IR receiver (Sharp / Vishay etc). Program a micro to record the pattern, maybe save to flash.

Then a 555 sets carrier and drives an IR LED while the recording micro issues the pattern.

I use one version countless times a day - its relays mute the external TV sound system (internal turned off), by a harmless TV remote button (menu return).

The ways to control high power always start with "what kind of load?". Voltage (AC/DC), steady state current, inductive, resistive, inrush demand all limit your choices.

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Reply by September 26, 2019

Aha. Firstly, thank you so much for helping.

So I don't currently have an Oscilliscope, can I check the frequency using Arduino or any Microcontroller directly?.

Also which of these IR recievers should I use?:
https://store.fut-electronics.com/products/infrare...

https://store.fut-electronics.com/products/infrare...

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Reply by September 23, 2019
However, I would love to know more about how they would have made a timer and a switching circuit for industrial components with high loads. I would appreciate if you have info or could tell me about that.
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Reply by September 26, 2019

The timer is easy, depending on the range, a 555 feeding a high number of stage divider. The CD4020 & CD4040 are a couple I use. The high power switching is not as easy. Solid state and contact relays, MOSFETs and IGBTs are all candidates. Again, it's the characteristics of the load that narrow your choices.

Not having an o'scope makes this tougher. I'd contact local schools / colleges. One with an electronics department usually is happy to help, but use them to learn, don't have them do Your work. You could also ask them if anyone would be willing to help you build this, at least the IR I/O.

A micro could be written to count the carrier at the transmitter LED, but the pattern will give inconsistent results.

"Most" IR control is 38kHz. Since the Vishay TSOP series is ~$.45 each, you could start there. Write the code and make the 555 carrier adjustable. Again, not having an o'scope Really makes this a lot tougher. If you're at the start of learning electronic design/build, an o'scope and freq generator are important tools. PC/Laptop o'scope interfaces are <$100 and are a fine place to start. A whole bunch of simple generators are at "http://www.discovercircuits.com/F/function.htm". Pawn shops and estate sales are options too. But I'm USA, I have no idea what options you have.

Getting started these days is unbelievably easier than when I did - 51 years ago.

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Reply by October 5, 2019

Have you tried the Hantek O'scope ? Probably that's the most affordable option for me right now, but I have no experience with those so I'm not sure if that will do.

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Reply by October 5, 2019

No, seen but never used a USB / laptop 'scope. But the 6022BE at <$60 on Amazon, if it works as reviewed it's a good starter unit. A colleague recently needed to get one and we spec'd a Rygol. An amazing 4 channel 100MHz color LCD for <$350. I paid \$1500 for my 2 channel 60MHz Tektronix, but that was Several years ago.

Test equipment, being a tool, has the adage "the better you get, the happier you'll be".