# Beyond board power, understanding multiple power supplies and loads in a circuit

I have a project I am looking to power where the secondary load is more than my Arduino can provide amp-wise. I need to know the simplest way to power the whole system. I will be powering the Arduino and a 5v 1.2amp load. Diagrams I have seen show a common ground with two power supplies but I don't know the math for anything beyond a simple circuit. Links to other resources are welcome with all answers. I am looking to learn more about power and circuits in general so more info is better.

EDIT

If I use one supply, how should I design the circuit? e.g. Series or Paralell. Also, how would I go about determining the requirements for that power supply?

• What is the secondary load? Commented Jan 16, 2011 at 16:33
• I got a good answer on why you need the grounds connected when using two supplies here: electronics.stackexchange.com/questions/7018/…
– Dean
Commented Jan 16, 2011 at 16:43

Without knowing more specifics about your particular set-up, here are some ideas:

The best and safest option would be to use one strong (1.5A or 2A) regulated power supply that powers both the microcontroller (arduino) and the heavier load.

It would also be possible to use two separate supplies, but then, you would have to make sure that the stuff connected to the microcontroller will not back-supply the I/O pins, possibly causing damage to the microcontroller (see: clamp diodes), i.e. you would have to make sure that both supplies provide almost exactly the same voltage (within +/- 0.3 V).

• Thanks zebonaut. Back supplying was something I was worried about. When looking for the supply should I think of the two devices as in series or parallel? Do you know of any links that help explain this. Commented Jan 16, 2011 at 19:37
• @JustSmith One way of thinking about the whole series-parallel issue, it may be useful to keep this in mind: parts/devices in series share the same current, and parts in parallel share the same voltage. For IC's with the same voltage requirement, you want to share the same power supply VOLTAGE, so they will be in parallel.
– W5VO
Commented Jan 17, 2011 at 3:51
• @W5VO thanks that clears up my understanding of of the answer. Commented Feb 14, 2011 at 15:03

You usually need a common ground but don't want to connect power rails, even if they are the same potential (don't tie two separate 5V rails together, for example). There are some exceptions. You can communicate across isolated grounds using opto isolators and there are some regulators designed to be safely used in parallel, such as Linear Technology's LT3080. Otherwise, there is not much math. You just need to make sure you can provide enough current for each of you rails' load requirements. The only other thing I would mention is that if you do use separate rails of the same potential, and communicate between them, make sure you can't violate any I/O specs of your devices. If you are using two separate 5V rails, for example, each +/-10% , one rail could be 4.5V and the other 5.5V. Outputting a 5.5V signal to an input pin with a 4.5V supply could potentially damage the IC. This is why, when using the same potential across multiple devices that communicate with each other, it's usually preferred to use a single supply rail for all devices. This isn't always easy to do when connecting kits, however, such as the Arduino to some external device.