I want to set up a switch that will turn my geyser / hot water boiler on and off by sms/phone text. (I'm from South Africa, we call things by different names, so I'm trying to give all alternatives!)
So I can get the shield to give me GSM/GPRS connectivity, but I need to drive a relay that can switch a 17A heater element. I'm assuming 17A as the element is a 4000w element: 4000w = 240v x ~17A. To have a margin of error, and to deal with the spikes and drops in our power supply, I assume a 30A or so relay should cover it.
Ideally I'd like to drive a latching relay straight from the Ardiuno (5v DC). Is such a thing possible, or am I going to have to two step it e.g. 5v switching a 24vac relay and that relay switching a 24vac / 240 vac relay? That would involve getting extra power supplies etc and I'd like to avoid it if possible.
Your Arduino can switch a relay directly if the relay coil is rated 5V and less than about 30 mA. For larger relays you can use a transistor to switch a 5V 400 mA coil. For relays with coils >5V or current requirement >400mA you will need a secondary power supply.
DigiKey has a bunch of 5V relays that can switch 240VAC/30A. They all have 185mA coils so you'd just need a small transistor to switch the current. Lots of examples out there.
Best bet is what is called a Solid State Relay (get heat sink for it as well). It has no moving parts or any of the wonky stuff associated with the magnetic coil of a mechanical relay and needs no additional parts to be turned on from a 5v 30ma output from a microcontroller. To hook it up just attach the output from your microcontroller (or whatever) to the positive dc side and ground to the ground of your dc circuit. On the other side (the AC side) of the SSR just cut your hot wire and splice into the relay. I've used them to control a heater as well. It works beautifully. At 20A I would defenitly get a heat sink (make sure you put heat sink compound in between the relay metal back and heat sink). In case you are wondering the metal plate on the back of the heat sink is not electrically active. You can buy both on ebay (usually together) from China for about $10 USD. Great deal! Oh make sure the SSR you buy has the right Amp rating.