Hello everybody,
Maybe it is simple but I'm quiet new and would'nt burn my arduino card even the sensor or my computor ...
I have an Arduino card connected to a sensor (ultrasound) which gives a signal to a Relay (12VDC to 230 VAC - 40A) to switch on and off a toaster.
Someone could help me by seing if my construction is ok ?
Thanks a lot
If you are talking about the relay connections.. then yes that looks fine. Peace rawbush
You can drop the diode to the resistor on the base of the transistor - it isn't needed.
You also need to wire the 12 volt supply ground to the Arduino ground, otherwise the transistor will never turn on.
Lefty
Super !
Yes, it is about relay connections
The diode was to be sure not to burn something if the transistor would die.
Thanks a lot to both of you 
nat
"You also need to wire the 12 volt supply ground to the Arduino ground, otherwise the transistor will never turn on."
You are right ! I had forgotten. Thanks
The diode was to be sure not to burn something if the transistor would die.
I've never heard of something like this occuring; if it did, you have bigger problems. Plus, the diode will have a voltage drop, which could affect the turn-on of the transistor.
If you really want to protect the Arduino, use an optocoupler (but for something this simple, it isn't needed, really).
BTW - ground isn't -12 volts (I hope you aren't using a dual-ended supply - if you are, that will likely burn out the transistor).
It would appear (from the relay shape and pin numbers) that you are using an automotive relay as an interface between the arduino and 230volts AC. Such relays are entirely unsuitable for use on mains voltage since the required separation distances between LV and HV contacts/components does not exist.
Your circuit might well work but it's not high on the safety aspect.
Better to buy a properly rated relay with 12 volt DC coil and contacts suitable for switching 230volts AC. These are readily available and, in many cases, cheaper than automotive relays.
jack
ground isn't -12 volts (I hope you aren't using a dual-ended supply - if you are, that will likely burn out the transistor).
Heu!!! I don't understand.
Is it means that from my transistor, do I have to connect only the GDN of arduino card and not to -12V ?
Your circuit might well work but it's not high on the safety aspect.
safety aspect is the most important thing
Yes, It is a automotive relay, the only one I found with a 40A.
By chance, Would you have some reference of rated relay ?
nat
Your local Maplin or Radio Shack have suitable relays and bases perfectly adequate for the task.
jack
Heu!!! I don't understand.
Is it means that from my transistor, do I have to connect only the GDN of arduino card and not to -12V ?
The term "ground" is a reference term; voltages are referenced from a "ground" level; if that ground level is 0 volts, you can have voltages above or below this level. If you had two 12 volt batteries connected in series, and called the center tap "ground", you would have one end at +12 volts, and the other end would be -12 volts.
"Ground" is a reference level, and considered "0 volts" for reference, with other voltages in the circuit referenced to this.
Since you don't know this, I have to wonder if what you have marked on your schematic as -12 volts is actually "ground" - and you are calling it -12 volts because you are hooking up to the negative (-) terminal of the battery or power supply?
If that's the case, don't call it -12 volts (that means something different) - it is the "ground" (0 volts) reference. And yes, you have to connect the grounds together (unless you really are using a dual-ended supply - but I doubt that because you didn't know what I was talking about when I mentioned it).
Yes, It is a automotive relay, the only one I found with a 40A.
That's 40 amps at 12VDC; which is a world of difference than household current. You need to find a relay with contact ratings of your amperage for the voltage it will be used at (230 VAC); actually, if your amperage is 40 amps, you want a higher rating (never use parts at their ratings; those are max values, and you never want to do this - instead, spec for parts for 20 percent or so over what you need; so for this, if you were expecting a 40 amp load, select a size of 50-55 amps or greater).
You're probably not going to need a 40 amp contact relay for 230 VAC; I doubt a toaster will pull that (the rating of the plug/outlet is probably less). You need to find out what the toaster's maximum amperage rating is, then select a relay with contacts rated for that value at 230 VAC.
It is exact that I used in my sketch "-12V" to say ground (0 volts)
thanks for your informations concerning relay.
nat
That's 40 amps at 12VDC
40 amps at 230 VAC is still 40 amp's. Relays sold through auto outlets are quite commonly rated for max load current at 250 VAC using a 12V DC coil (suggesting they will be suitiable for either AC or DC loads) . Retail channel does not dictate fitness (good or bad) for a specific purpose, you need the relay datasheet for this. The current rating will typically be limited by the relay contacts current capability irrespective of load voltage or AC/DC. Voltage range will typically be limited by physical phase separation within the relay.
Using an "auto-style" relay may not be such a bad idea in terms of switching capability, but other issues such as isolation may still apply. Irrespective of this however, mains precautions always apply. That is anything involving mains is better left to professionals.
I'd disagree with the above. An automotive relay is NOT suitable. It's rated to switch 12v. You are exceeding it's intended use by nearly 20 times. A quick scan of automotive relays gives a manufacturer maximum voltage of between 24v and 50v.
(I understand what you are saying BenF - but I think it's wise to err on the side of caution)
Just looked at the bottom of my 4 slice toaster. It's 1700W at 230V. So 7.39A I'd strongly suggest actually measuring (if you are comfortable with doing this safely).
Based on the raing on the bottom of my toaster - something like this:
http://www.maplin.co.uk/Module.aspx?ModuleNo=218688
or for crazy overkill
http://www.maplin.co.uk/Module.aspx?ModuleNo=37495#spec
In the end only play with this kind of stuff if you're very comfortable with it. It might be cool to have some sort of robot toaster but it's not worth burning your house down.
I personally prefer a Solid State Relay for work like this. You don't need a transistor because you are driving an LED in the SSR. No external Relay voltage is required.
I drive a 2000 Degree 20 Amp Kiln (AKA Really Big Toaster).
Yes, that's a HUGE heat sink, bit it's the only one I had lying around.
Looks like a Tie Fighter...
;D
Well, at 20 Amps, the SSR gets really warm after about 10 minutes holding the kiln (below) at 1265 degrees.
But, the SSR is a quite work horse. I've been abusing this one in various projects since 1980 or so... still ticking. It's rated at 40 amps at 240V.
You can get them pretty inexpensively from places like MPJA (marlin p jones) www.mpja.com
I'm using the PID Library with the extended PWM control available from the Timer library. Best of both methods, I guess.
This is a deal as it has the heat sink.
You can drive this directly from an ardino pin as it only takes a few milliamps and a voltage above 3v but less than 32v to turn it on.