Powering External Devices

Firstly, hi everyone, I'm new on here and new to Arduino so Hello World :slight_smile:

I'm after a bit of advice if anyone would be willing to help me out.

I'd like to connect this relay board :

http://www.ebay.co.uk/itm/170745298865?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

to my shiny new mega but I'd like to power the reay board from an external psu rather than drawing too much power from the Arduino pins.

Please could you tell me, if I bought a 5v 200ma psu, would I connect +5 from the psu to vcc on the relay board and psu ground to gnd on the relay board and then psu gnd to gnd on the Arduino ?

Like this :

PSU
(+) ---------->To Relay Board VCC

(-) ----------|--->To Relay Board GND
|
|--->To Arduino GND

If I do this then will it mean that the 15-20ma each relay requires to actuate will be drawn from the external psu?

I know each pin can supply up to 40ma and so I should be fine without an external psu but the problem is that I also need to connect 10-15 leds to the digital out pins and I have a rtc on the pwm and also a lcd, a few temp sensors and an sd card so it may be possible that at any one time the drain could exceed the total rating.

Thankyou in advance, I'm currently rubbish at electronics but I'm trying so your help is hugely appreciated.

It looks like the relay board uses opto-isolators to isolate the input pins from the relay drivers. I suspect that the three pin jumper in the corner lets you isolate the power. Without a schematic or physical sample it's hard to tell.

I think your wiring will work. I can't be sure that you can drive all 8 relays with 200 mA.

rickpbush:
Firstly, hi everyone, I'm new on here and new to Arduino so Hello World :slight_smile:

I'm after a bit of advice if anyone would be willing to help me out.

I'd like to connect this relay board :

http://www.ebay.co.uk/itm/170745298865?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

to my shiny new mega but I'd like to power the reay board from an external psu rather than drawing too much power from the Arduino pins.

Please could you tell me, if I bought a 5v 200ma psu, would I connect +5 from the psu to vcc on the relay board and psu ground to gnd on the relay board and then psu gnd to gnd on the Arduino ?

I don't think 200 ma will be enough to power all the relay coils on at the same time if that is a requirement for your application(s). I would specify a 1 amp 5 volt external voltage source.

Like this :

PSU
(+) ---------->To Relay Board VCC

(-) ----------|--->To Relay Board GND
|
|--->To Arduino GND

That is correct wiring

If I do this then will it mean that the 15-20ma each relay requires to actuate will be drawn from the external psu?

The 15-20 ma current requirements are for driving the led input pins on the opto-isolators for the input control of each relay channel, not to drive the relay coil current which is supplied for the relay board's 5V pin. An arduino digital output pin can handle this current requirement for each channel input.

I know each pin can supply up to 40ma and so I should be fine without an external psu but the problem is that I also need to connect 10-15 leds to the digital out pins and I have a rtc on the pwm and also a lcd, a few temp sensors and an sd card so it may be possible that at any one time the drain could exceed the total rating.
Again the arduino digital output pins supply current for the optoisolator inputs on the relay board, they do not provide current for the relay coils which rather comes from the relay board's 5V pin.

Thankyou in advance, I'm currently rubbish at electronics but I'm trying so your help is hugely appreciated.
You'll get there in time.
Lefty

Thanks so much guys, this is really helpful info. I will get a 1amp 5v supply to power the relay board as per your advice.