How much current on 5V pin?

Hi all,

First post. Great resource, thanks for all inspiration.

I'm using an Arduino to monitor and control my home brewery. I have built my own shield with 4 relays that could all be on at the same time. They are 5V relays and I am using transistors to drive the relays. I'm re-etching and redesigning the control board and right now I have a 5.5V 2A wall wort that supplies power to the relays and Arduino is powered via USB. Well, since I built the relay board as a shield, I'd like the Arduino and relay board to be controlled from the same wall wort and I read that the Arduino may not reliably supply 5V on the pins from a 5.5V supply.

So, how much current can the Arduino supply on the 5V pin if it's supplied with a 9V wall wort? I know the digital pins can only supply 40mA. Do I supply the Arduino 9V and let it power the relays via the 5V (controlled by the transistors via the pins) pin or do I supply my shield with the 9V, power the Arduino via the VIN pin, add a 7805 voltage regulator to my shield and power my relays from that?

If I remember, they only drain about 100mA each so wouldn't take more than about 400mA at once.

Never tried it, but you seem to be just over the limits according to this.

Adding a separate regulator is probably the safer route for prolonged use if the relays actuate at the same time with any frequency.

Note the reminder that driving inductive loads from the regulator requires filtering to protect the processor on Arduino.

You could tie your existing 5V supply to the arduino AFTER the regulator, and not worry about it. You can feed in the 5V through the shield pins (power will flow either way) or via a kludged-together USB cable with power only...

I am curious to hear what all you are controlling and how the brewery will work. What kind of equipment will you be using? I am a homebrewer myself and would like to know more.


Hey thanks all for the quick replies. I'll just put a reg on my shield, I have the room. If all 4 relays were to pull in at once, which they shouldn't but could, it sounds like I'd have a voltage drop that could glitch or crash Arduino. Thanks for the filter advice, I have a few small rad caps hanging around, I'll add one.

Caleb, I'll be outlining more of the project on my home brew forum, under the equipment forum. In a nutshell I want to monitor and log temperatures and be able to control pumps and burners if I want. I have wanted to learn how to program for a long time but i am a very hands-on learner and of all the online tutorials I can't seem to get past "Hello World" before I get bored. By choosing the Arduino platform and picking an application I really know and understand I felt I had an opportunity that would keep my attention and propel me forward. I was right, I've learned a ton already and I'm just scratching the surface!

My final project will have a browser based application that will have a graphic representation of my brewery. I'll be able to click on various objects to turn pumps on or off and set critical temperatures. All the while it will read from Arduino and log important temperatures so I can go back and see what happened. I'll build a second one to monitor and control fermentation temperatures including having a Lager routine to drop my fermenter into lagering temp on a specific schedule.

I have 6 LM34's that will read mash, hot liquor, boil, vorlauf return, chilling H20 in and out. The mash and hot liquor sensors control 2 of the relays and the relays control direct spark ignition controls and valves for those two burners. The other two relays control the 2 pumps. All the relays have override switches so I can control the entire thing manually. Mostly I want this for the logging part but hey, why not control something if you can. I've had a digital HLT for a while now and it's like having a helper just sit and watch that vessel. When I brew, I brew back to back 10G batches so things get a little hectic as I'm finishing sparge and brining batch one to a boil while I'm trying to refill and reheat my strike water for batch 2.

It's been a very challenging project so far and I can't wait to get it incorporated into my actual brewery. I just finished making the temperature probes out of the LM34's and my first shield and sketch is working more or less as anticipated. I've chose some different switches and connectors so I'm etching a version 2 shield.

I think the Arduino is absolutely the most bang for the buck I've ever got out of a toy under $35! Oh and sorry to everyone else for all the beer-geek jargon :-?