Powering a 240 V AC Relay

Hi guys,

I'm fairly new to this stuff but basically when I have Logic high coming out of the Arduino I want to close a relay switch and power a 2 KW water heater. I've been told that the best way to do it is to use a transistor to flick the relay on and off (Perhaps with a diode there too to even though the circuit is purely resistive?). Anyhow, Any pointers on HOW to actually do this and the values of the diodes, transistor and roughly the amount of power I'm going to need to power such a relay would be super helpful. I wasn't sure if was even possible to power such a diode with the limited output of the arduino... I pretty much just need to learn more about how this stuff works!

Many Thanks!!

See the first schematic in this link:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html

roughly the amount of power I'm going to need to power such a relay

It depends entirely on what current the relay takes.

If you use a 240VAC solid state relay, the control input to the relay can be driven by a single Arduino output pin. No need for transistors, diodes, etc. Example: 150pcs 0.047uF 47nF 100V 5% Mylar Film Capacitors 2A473J | eBay

First of all, what do you mean by 240V relay?

  • 240VAC coil voltage? in that case you cannot use that directly with an arduino, not even with a transistr driver. The only way is to put another relay (mechnical or ssr) between
  • 240V AC contact rating? In that case make shure the rating is 240V ac 2Kw ac1 at least. Or it wont last very long.

The best option for controlling a 240V ac heater is using a SSR with zero cross switching. it can be controlled from the arduino directly and will not wear due to frequent switching.

Thanks a lot for the insight. I meant 240 V rating for the contacts.. I was hoping that the coil would be more around the 5 V mark so it could be driven by an Arduino.. Is the purpose of the transistor to increasing the current leaving the Arduino for the purpose of being high enough to magnetise the coil?

On another note, what tells you that an SSR is more appropriate for this particular application?

Thanks again.

SSRs are usually preferable to electromechanical relays. The coils of electromechanical relays take a fair amount of power to operate (almost always more than an Arduino can provide) and the contacts burn out, bounce and are generally unreliable.

Have a look here:
http://forum.arduino.cc/index.php?topic=197898.msg1460205#msg1460205

SSR dont exhibit that behavior because there is no mechanical breaking. In addition to that, zero crossing switching eliminates the harmonics and interference caused by breaking or making contact at arbtrary points in sinus wave of the voltage