Computer Controls
Most computers work at low voltages. Most equipment operates at high voltages. There is a range of options to integrated these two sides. For this application, we will use a low-cost well-engineered computer-controlled device designed to control 240V AC 10amp power. This will be used to power a device known as a contractor. When 240V AC power is applied to a contactor it energies a relay that operates two switches one for life and one for neutral
This approach has many benefits
- All components operate within their operating environment
- All components are high volume, low cost, proven design, commodity items with high reliability
- When parts fail they are modules that can be replaced quickly and cheaply
The Sonoff Basic is the entry-level model and works well.
Inside we see the PCB and the ESP8266
This is a waterproof consumer unit with Din Rail
There is room inside the unit..
Testing the size of the unit to see what will fit.
As the Sonoff is not DIN-Rail mounting positioning the unit will need care
Checking the unit will fit on the wall near the heater
Sonoff Basic internals with a serial header added: Four pins on the right of the tall push button.
Computer Controller
For the computer controller, we will use a microcontroller-based Sonoff Basic. This is based on the popular ESP8266 family of parts that cost under a dollar and have local storage and Wifi with a few GPIO pins to control relays and read values from sensors.
Note the Sonoff Basic is well designed with isolation gaps front and rear. This ensures the high voltage parts are isolated from the 3.3V microcontroller. There are Sonoff Models that can switch 16Amps. These work well and have no issues. However from a design and safety viewpoint for less money, we can purchase the basic Sonoff, a waterproof enclosure and a contactor that not only works on higher current and switches BOTH live and neutral. Also when parts fail this is a modular system so cheaper to repair and expand.
The Sonoff Basic comes with software to use the manufactures own cloud service. We will replace the software to allow the device to work on the local network so it will operate even if there is no internet connection.
Sure we could use some ESP8266 units with relays but look at the PCBs there are no isolation cuts in the PCB there are no high current rails on the PCB.
In the Sonoff you can see isolation cutouts.
In the Sonoff you can see isolation cutouts.
Solid State Relays and switching a relay directly are options that were considered. Solid-state relays can switch large current safely. Indeed the 3.3V unit shown he is more than capable of controlling 240V. The concern here is that we are toggling the power for a large inductive load. Inrush currents can be large. Also, Solid State relays can generate heat/use power even when off.
Power Control
While the Sonoff Basic has a mains relay rated to 10Amps 240VAC an immersion heater is a large inductive load. It may be replaced by a more powerful unit. For these reasons, the small relay in the Sonoff Basic will be used to control a BG 20Amp Contactor. This contractor will operate the power for the immersion heater.
Theory of Operation
The microcontroller runs software that exposes a web service used to power the Sonoff Basic relay on or off. This 240V AC output is applied across A1 and A2 of the contactor. This powers a relay in the contactor which in turn switches on the live and neutral supply for hot water.
| Manufacturer | BG |
|---|---|
| Range | CUC20 |
| Part Number | CUC20 |
| Condition | New |
| Breaking Capacity | 6kA |
| Current Rating | 20A |
| Voltage Rating | 230 - 240V AC |
| Manufacturer | BG |
Enclosures
The devices will be switching large power circuits and high current high voltages. In an environment that may contain water. Its a hot water tank in a boiler room.
These Consumer units are small https://uk.rs-online.com/web/p/consumer-units/7982252/ and can be found on eBay for £7 delivered new. Docs https://docs.rs-online.com/ab22/0900766b812bea42.pdf
This will house the Contactor, this can be fitted on the integrated DIN Rail. There is room for the Sonoff Basic and the wiring.
There are mounting kits for the Sonoff Basic https://www.itead.cc/sonoff-dr.html