Your air conditioner (and whole HVAC system) are a strange mix of parts. On the one side, we have low voltage, DC electronics that couldn’t tickle a fly. On the other, we have high voltage, AC equipment that could turn an ill-prepared reptile or rodent into a charred chunk of bones. How exactly do we mate these two, different systems together safely?
The Safety Issue
In all electronics, we strive to separate high voltage and low voltage. We don’t want them to touch each other. In some systems, you have no choice but to put high and low voltage equipment on the same board, in other cases you try to keep them on separate boards altogether.
If there’s any sort of electrical short between high voltage and low voltage, the low side is going to get destroyed. That’s your best case scenario. 240 volts or 440 volts blasting into some tiny microprocessors and capacitors, which then explode in a terrifying, but mostly harmless show of pyrotechnics. If the board is really well built, a diode will take the brunt of the failure before anything scary does.
Sometimes though, these shorts don’t go like that. Some older equipment or equipment which never passed any reviews like Underwriters’ Laboratories can send high voltage straight to frame components, a button, or other areas where a user could come in contact with it. 220 Volts is not a pleasant experience, let me tell you from first hand experience. You lose all control of whatever’s touching the shorted equipment, there’s a numbing tingle, and if you’re unlucky, long-term heart damage.
We need to exercise as much safety as possible when interfacing high and low voltage equipment. We use low voltage around people as much as possible, because you don’t need 440 volts to measure the temperature and run an air conditioner and therefore, the 9 volt, 5 volt, and lower solutions are just the safest solution. Unfortunately, a compressor probably won’t even buzz at 9 volts, so we can’t go fully one way or the other on the voltage and current being used.
This is where your relays come in. A relay is just a gigantic, chunky, electrical switch. There is a whole family of relays and ways to actuate them, but the easiest way to think of them is a light-switch that can thrown by any machine. In most systems today, that is a controller sending an electrical signal to the relay, which actuates and either turns something on or off. Each relay is designed for a specific voltage, so they can be used pretty much anywhere. You can put them in 220, 440 and high voltage, 10 amps, 20 amps, 100 amps, it doesn’t matter. There’s a relay built to handle that kind of voltage and current.
This allows us to send in 5 volts on one side and have control over massive amounts of electricity. There is however, still a problem: what if that relay shorts? What if some freak failure occurs, and that 440 volts blasts through the relays switching side and down to the control side? It’s not hard to imagine, many relays are just electro-magnetic coils inside that push 2 plates together to make contact.
This is where we take things a step further. We don’t actually need the low voltage to touch a relay to make it work.
There’s another kind of switch we can put on the board. We call it an Opto-Isolator. There are chips which are light-sensitive. When we shine a light at them, they output voltage. This is almost like a solar panel, but much, much less power and they’re only a few square millimeters.
What we do is put an LED against one of these photosensors. The low voltage equipment turns on the LED, which triggers the photosensor, which outputs a steady few volts, which actuates the relay. This allows power to flow only one way.
In this set up, if the relay manages to short, the power will flow to the photosensor, which will probably fail with a puff of smoke, and that’s as far as it goes. There’s no conductive path between the LED and the photosensor, and therefore, no path for electrical power to get back onto the low voltage boards and cause any harm or damage.
The Typical Setup in Practice
In practice, this might mean a few relays in use. Your thermostat might run on about 5 volts of power for it’s actual temperature sensing, wifi, and powering the processor and memory inside. These then actuate a relay which closes a circuit to your heater or air conditioner. You can often hear a click when a thermostat uses one of its relays.
The signal from these relays can actually be fed into another relay, which will run something even bigger. Your thermostat can signal the air conditioner’s internal components to start circulating refrigerant, which will mean using a much bigger relay to turn on a compressor and multiple fans. These sorts of relays are the big, chunky ones. They weigh a good couple of pounds and are about the size of a coffee mug. They’re too big to actually mount on a circuit board.