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IMPORTANT NOTICE: CONSIDER AT LEAST SAFETLY REGULATIONS IN YOUR COUNTRY AND DETAILED RELAY SPECIFICATIONS BEFORE PROCEEDING!
The most difficult part of your home automation system design is to assure safety and reliability. The first concern is how to design power control circuits for large house appliances. The simplest option is to use low voltage controlled power relays.
Relay specifications
However, there are two major things to take into consideration. The first is negative discharge of a relay coil at switch off that can easily damage microelectronic circuits. This can be prevented by connecting a reverse polarized diode (like 1N4007) in parallel with relay low voltage control input.
The second problem is to select a power relay with suitable electrical characteristics and to integrate it into the power supply circuit for your home. There are basically two was of integrating a control relay into a power circuit. A relay may be added in series with each home appliance, or it may be integrated into your home electrical cabinet in series with a circuit breaker. However, the relay maximum (sustained) current and maximum output voltage are very important.
You should never choose a relay that cannot carry short circuit current and that is not designed for the voltage your home is supplied with (ex. 230 V~). You should also correctly determine the maximum sustained current through the relay. If you are adding a relay into the electrical cabinet in series with a circuit breaker it is important for the relay to have at least the same power rating as the circuit breaker. Therefore the relay maximum sustained current must be at least the same as the maximum sustained current of the circuit breaker, while the short circuit current must be 2 or three times the maximum sustained current. The relay must be able to withstand the short circuit current at least as long as the circuit breaker needs to break the current flow.
Connections routing
You should strictly avoid mixing power lines and low voltage control lines. The relay control lines should be routed to each relay from the opposite side than the power lines, if possible. Most relays or relay adapters have the control cable connections from the opposite side. The only differences are the low profile adapters which may have some power connections on the same side as control lines. If needed, a proper additional insulation should prevent accidental connection between a low voltage control line and a power line. It is usually the best option to route all the control cables through one or more cables and only spread them to the control lines nearby the relays. Keep the control lines away from the power lines inside the electric cabinet as far as possible. If there is no more room for the relays in your existing electric cabinet, you should add a new electric cabinet and route the relay controlled power lines to it.
Assembly and testing
MAKE FIRM CONNECTIONS TO THE POWER LINES. A LOOSELY CONNECTED POWER LINE MAY COUSE FIRE IN YOUR ELECTRIC CABINET! BEFORE STARTING ANY WORK IN YOUR ELECTRIC CABINET, YOU SHOULD SWITCH OFF THE MAIN CIRCUIT BREAKER IN YOUR HOUSE TO PREVENT ACCIDENTAL ELECTROCUTION.
Integration of relays into your house or apartment electric cabinet requires some power lines rerouting. It is important to always place a relay behind a circuit breaker. This means that if something goes badly wrong with the relay, the circuit breaker would still be able to break the current.
As you integrate a relay into the house power circuitry, you should always check the operation with the relays powered off. If the relays are connected as power enablers, all the controlled home appliances and lighting should be now without electric power. If the relays are connected as power disablers then all the controlled home appliances and lighting should have power, as before the modification.
The next step is to check the voltage on the relay control lines relative to the ground (GND). A voltmeter should read 0 V on AC and DC scale (or at least not more than a few millivolts). Only then you are able to connect the relays to the computer control board.
Now, switch all the relay controlled lighting and home appliances off with their internal switches and start switching the relays on. It is important to prevent a high power electric appliance to draw any current though a relay before the relay operating conditions are verified to be in accordance with manufacturer specification.
A 12 V= relay with maximum 16 A output current at 230 V~ needs about 50 mA to firmly switch on a house appliance. The low voltage relay power supply must provide enough power for all the relays to operate within the manufacturer specification. Therefore it is advisable to have all the relays of the same manufacturer and of the same type.
Next, you should start checking the control board and relay operation by applying control power to one relay first. You should hear only one fast relay click as it is powered on. Then you must measure the voltage in the relay control lines. The relay manufacturer usually specifies tolerances regarding to nominal relay operating voltage. Usually the minimum switch on and hold voltage is about 0.8 of nominal voltage and the maximum sustained voltage is 1.1 of nominal voltage. This is 9.6 V and 13.2 V for a 12 V relay. The actual voltage on the relay control contacts should always be within these limits, if the relay is turned on. If is not, you should redesign the control board and/or relay power supply.
Normally, the relay power supply voltage may decrease for a few tens of millivolts as more relays are switched on, but the relay control voltage should still remain within the limits mentioned above.
It is not a bad idea to use one of the PIC18F24J50 A/D inputs to measure relay power supply voltage. However, you should build an adapter circuit to convert the input voltage range (typically from 0 V .. 15 V=) to the range of 0 V .. 3.3 V=.
Relay mounting
Generally there are two ways of integrating a relay to your house electric cabinet. If a relay casing is already prepared for rail mounting, you can simply add it into the electric cabinet. However, relays for PCB (printed circuit board) are usually cheaper, but they need an adapter. If you go for the last option, it is important to make a firm connection between the relay and the adapter. The adapter usually includes a safety clip or a plastic holder to prevent relay accidentally loosing contact or falling off (see the photo below). If no safety clip or plastic holder is provided with the adapter, you may improvise by wrapping electronic insulation tape around the adapter and the relay (see the photo below). But this is certainly NOT the best option! However, the reseller should at least offer the option of buying the safety clips or plastic holders separately. It might be better to halt the home automation project until the safety clips are available at your reseller’s store than to go on with the temporary electric insulation tape solution. There will be quite some work to replace the tape later.
Click on the photo to enlarge it.
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