Hi there!
there are loads of relay boards on ebay for close to nothing (like this: 1pc Optocoupler Relay Module 5v 1-channel H/l Level Triger E for sale online | eBay)
they are rated for 250V/10A, but if I look at the bottom of this thing and see those small traces, I can't imagine it could withstand 2.5kW. What do you think, are they safe to use? (I would use it at 240V/7A)
Thanks!
You could always add some heavy wire in parallel with the traces if you were worried about them.
Have a good look at the pictures.
The brand "SONGLE" had been made unreadable in all pictures. Why ?
This seems to be the real one: The MicroController and Embedded System Store
With a load of 7A, I suggest to buy from a good shop.
Look again, the heavy traces on the left 3 pins are the high voltage contacts.
The thin ones on the right are the control signal traces.
GoingForGold:
if I look at the bottom of this thing and see those small traces, I can't imagine it could withstand 2.5kW. What do you think, are they safe to use? (I would use it at 240V/7A)
There's not 2500W here. You're figuring, "Oh, 250V * 10A", but the rating should be interpreted as the contacts can switch, at max., 250V and make/break 10A [assuming those ratings are genuine].
Either way, don't push the envelope.
look at the picture of the bottom of the board.
exposed traces so you can lay on some #14 wire and solder.
since you are asking the question, I figure that you have not had a lot of experience with Chinese stuff. limit your use to 50% of the listed rating. and buy spares.
not sure why Songle is fuzzy, but it is easily seen on almost every other relay in that price range.
unlike transistors, you cannot parallel relays, they do not open and close in a reliable speed so invariably, one will open first leaving the entire load on the others.
I would advice against that exactly board that is linked to. Look at the underside. There is under 2mm spacing between the high voltage and low voltage tracks. It doesn't take much dirt or moisture to break that path, and then your Arduino goes "live" and can kill you if you touch it.
Unfortunately, a lot of these "Arduino-compatible" products on eBay are made by people that have absolutely no understanding of good measures and safe practices. I hope no one is harmed by these badly made boards.
Another example of a board that is very dangerous is this: http://www.ebay.com/itm/261536188402 - The copper pour all over the board offers no isolation from the high voltage to low voltage side... crazy!
A much better approach is seen here - Here the designer actually knows about isolation, and had a piece of the PCB milled out, to isolate better: http://www.ebay.com/itm/161371495044
// Per.
Noted that the power traces are in fact, quite robust, reinforced (slightly) by the solder mask leaving a bead of solder among them.
Also the grossly insufficient clearance of the low voltage tracks.
But look at the crazy "isolated" circuit:
Why do you need the optocoupler if your input signal feeds directly the transistor? They mus be very cheap, or maybe an offer.
I suspect it may be an option, R4 or optocoupler.
Diagram is very unhelpful, to say the least.
luisilva:
Why do you need the optocoupler if your input signal feeds directly the transistor? They mus be very cheap, or maybe an offer.
sort of like a seat belt on a parachute. no use, but some people get a happy feeling that it does something.
the real question is why they connect the ground on the opto to the coil voltage ground !
check out the under side of this board
or this one
dave-in-nj:
Check out the under side of this boardor this one
(Links pared.)
Yes, decent clearance on the mains side.
The second one is however, yet another example of using an opto-isolator to not isolate the relay coil circuit. 
luisilva:
The real question is why they connect the ground on the opto to the coil voltage ground !
Note that they are using different "ground" symbols. I think they correspond to the terminals (confusingly) labelled "AGND" and "DGND" and if R4 is not present (either because it was not included in the opto-coupled version, or because you remove it) then you can actually isolate the two. Mind you, said diagram is not helpful at all on that point.
