How to evaluate the power needed by my circuit?

Hi all,

I have designed a system that I would like to build into a standalone device. But first, I would like to know how much power it consumes and choose a battery accordingly.
How can I do that? Right now it's running off the USB port for power. I have a simple multimeter to my disposal.
I need 5V (because of the LCD), but I don't know about the amperage. Can anyone advise?

Sure, put your multimeter in mA mode, and wire it in series with the +V line from 3 AA batteries to the 5V line on your circuit.

If your circuit runs from USB, then it is using <500mA of current as that is all USB supplies.

If your circuit runs from USB, then it is using <500mA of current as that is all USB supplies.

That is something you need to care for too, back sometime i used a cut end USB cable for my small power needs and that exerted a Power Surge on that particular USB port and it shut off the USB port temporarily, I have driven GLCD power (including its LED) with the USB on the computer and in one case a very small Micro servo and it ended in power surge both so i would suggest taking care while powering from the computer ,especially a Notebook/Laptop.

I measured under 50mA.

  1. I only had slots for 2x AA batteries, so I took two slots and soldered them together. I put the black negative wire of one to solder it with the positive red wire of the second cart. I then used the negative of one cart and the positive of the other cart to power the system.
    This worked fine and I could measure consumption. But after about 3 minutes of letting everything sit disconnected from power but with the batteries still in place, I started to smell burnt plastic. It turns out one cart was melting. I appreciate how serious this is and I would like to understand what happened here to avoid this mistake again. I only burnt the tip of my finger while getting the batteries out in a hurry but I got quite scared and I am glad nothing worst happened (like a fire). Maybe the platic the cart is made of is not resistant enough to handle twice the voltage? That's the only thing I can think about. There was no short (I guess?) since the circuit was getting power.

  2. How to go from Arduino to standalone Atmega328p running on batteries? I suppose I would need a voltage regulator or something of the like? Where can I find more info?

  3. How to calculate which batteries I need and how long will the batteries last considering the 5V 50mA figure?

Wire up what you need - power from an external source, or 4 AAs with a diode to drop the voltage from 6v down to 5.3V, or from a AAs at 4.5V.
How long it runs depends on the mAH of the batteries.
AA is something like 2500mAH, so ~50 hours.

I have '1284 based board running from a 1000mAH LiPo battery going thru a 3.3V regulator flashing an LED 100mS on, 900mS off - been flashing since midnight Friday, so going on 3 days now. Didn't measure the current draw, just watching to see when it stops.

See replyu #261 on page 18 here
http://arduino.cc/forum/index.php/topic,80483.255.html
Only have the LiPo and an external LED/resistor on one of the pins.