max current draw with relay board, input buttons and rs485

Hello guys,

I am currently developing a system for controlling lights powered by a 12V photovoltaic system.
For this purpose I have some Arduino UNOs connected by RS485 controlling an 8-channel relay board (LINK) and 8 buttons connected by the usual schematics (5v supply with 10KOhm resistor).

Each external component (RS485 chip, relay board, input buttons) will be powered by a separate 5v source, the 5V output of the UNO is not used.

The documentation of the relay board says that it consumes 15-20mA per channel, so I am not sure if the board will be capable of running all 8 channels with input buttons and RS485 communication (and optionally some sensors) simultaneously without going up in smoke. I read that the maximum current draw is 200mA for the whole Arduino board, so this is quite near the limit.

I would be happy and thankful for any suggestions!

I read that the maximum current draw is 200mA for the whole Arduino board, so this is quite near the limit.

That is the limit of the total current of the processor chip sourcing current, not the board.
Any chance of a link that works?

Sorry, here is the correct link. Also corrected it in the original post.
8-channel relay board

There some limits on groups of pins too:

http://www.gammon.com.au/uno

You might want to invest in some MOSFETs to reduce the strain on the processor pins.

Thanks a lot for that interesting link.

According to a discussion HERE, it seems that the 15-20mA rating is wrong. Instead, the input circuit of the relay board only draws 1.5mA from the I/O-pin, but the relay coil behind the input circuit uses about 60mA when it is in operation. That seems to make sense, but I guess I have to check that for myself :slight_smile: The board should arrive here in some days ...

I just put an amp meter inline with the input pin on two diferent relay boards (not the one you specified),
One had 3.8ma draw, the other had 6.2ma draw.

I also would expect their rating is wrong. If you have the relay, and have a meter, give it a test.
If you have not yet ordered it, you may want to bypass that one.