Help with relay

Hello everyone. Just bought a relay board. This one: http://www.ebay.com/itm/171279000686?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

But i have been reading about the current limitation of the arduino, and the max current output of any I/O pin is 40 mA. The board that i have bought is drawing 50-60mA pr channel. The ebay item description also says that it can directly connect to an arduino board.

So my question is; is it safe to connect this directly to the arduino? Or do i need to connect it differently? If not directly, how?

If that specification is to be believed then no you can not safely connect it directly to an arduino without damaging the arduino.

Why not ask the seller a question. I bet he changes the description if it means he is going to miss a sale.

Normally people like this have no idea what they are selling.

But it says it's opto-isolated, and the LEDs in the isolators won't want that much current: they would surely be driveable by an i/o pin with a series resistor?

Could it be that the low side of the relay, the 5V which is switched by the isolator, needs to provide that much current? I see there's a Vcc pin: maybe that's the one....

Do you have a link to a datasheet or schematic?

But it says it's opto-isolated, and the LEDs in the isolators won't want that much current:

I would tend to agree but that is not whet the specification says.

There was a poster the other day asking why his USB cable was getting hot and he claimed that the optos on his device needed 60mA.

I do not have a datasheet on this.

I also think that the current drawn from each I/O pin is much less than the stated 50-60mA, and that this current is drawn from the Vcc connection. But the max output current from the Vcc is 200mA, which also is to little. I think that using an external 5V power supply to the board is the answer on how to connect this?

There is a jumper that can be removed that says JD-Vcc if i remember correct.

Yeah use an external supply for Vcc.

Can you see the traces on the PCB?- maybe you can try figure it out by some detective work.

Does it have a maker's model number to Google a datasheet?

Grumpy_Mike:

But it says it's opto-isolated, and the LEDs in the isolators won't want that much current:

I would tend to agree but that is not whet the specification says.

There was a poster the other day asking why his USB cable was getting hot and he claimed that the optos on his device needed 60mA.

Yep in the absence of a datasheet, it's pure (possibly expensive) speculation....

I am at work and do not have the module in front of me, but will see if i can manage to google a datasheet later.

I have seen other modules with the same relay but without the optocoupler rated at 20mA.

There were little text on the relay board to help me find a data sheet. Is it possible to connect the board, and only trying to activate one relay and measuring the current used with a multimeter? Both the current drawn from the I/O port and from the Vcc? I am taking a chance that the 50-60mA are not all drawn from the I/O port.

relay Data sheet shows 71.4mA for 5v unit. coil consumption 0.36 watts

we would not suggest that because the high in-rush could make your duino act funny.

to test ? knowing you are not going to rely on anything ? maybe

but I would suggest you take a wall wart, connect the 5v and grounds and test them all properly. last thing you would want is that the inrush or emf or short burnt up your duino.

besides, you should have extra wall warts from your old cell phones laying around.....

But with the opto isolator, the coil current isn’t through the i/o pin, it’s across the isolator’s transistor c-e, and presumably they sized the isolator to cater for that.

The current on the i/o pin is that through the isolator’s led. With a current limiting resistor, that can be as low as you want it, provided the the led still fires the base of the transistor.

EDIT: Attached pic is my take on how this thing works… R1 limits the Arduino i/o current to the isolator led.

My test would be to lose the Arduino for a minute or two, put 5V on the board input and ground to power the coil, and put 5V through a resistor and an ammeter to the isolator LED and listen for the relay to operate.

isolated relay.png

Thanks for all your help guys.

I have now tried some different things here and found out that the current drawn from the I/O port when active is a little under 2mA, and the current drawn from Vcc is around 68mA when only one relay is active.

So powering this via an external power supply and the JD-Vcc connector is the way to go. This is actually better than just powering from the arduino as you get full isolation from the arduino and the relays.