Make a Power Budget for Your Micro Project

As you develop a project idea, you run into the need for a Power Budget. How much Current (mA) is used? How much space is needed? What cost should I plan on? What power supply should I shop for?

A power budget helps you answer those questions. Its super easy to write up. All it takes is a spreadsheet app (free these days) and entry of a few key numbers. Once done, you know the Current (mA) for each Voltage in your Project. That knowledge helps you shop for candidate supplies. You also get a sense of size and needed space for your Power Supply Unit (PSU). All this before spending a dime!

Now that you see the benefits of having a power budget, here's how to get started.

Break it down

Your project can be broken down into smaller sections to calculate power usage. List each section in a table row. List each voltage in a column; 3.3V, 5V, 12V and so on. The following table illustrates how this might appear on a notebook or spreadsheet.

If a sub-section of your project is comprised of chips, transistors, LEDs etc., then item datasheets should identify maximum current each device could possibly consume. BTW, transistors that switch LED's or relays usually don't count in the budget since its the relay or LED that determines current needed. Discreet component maximums are likely more than you'll need but its better to have extra mA rather than not enough. If you can test a discrete component in a quick hookup for current draw, use that number.

If power gets routed out to external devices, make a note of that mA requirement as well. Treat it as another section in your break-down analyses.

NOTE: Wall wart type power supplies may overstate current capacity rating without warning of consequences such as voltage drop and ripple increase (AC riding on the DC). Be careful about choosing these supplies for your build! You may see a rating of 2 Amps (2000 mA) only to find that at 1000 mA the voltage drops 10% or more which can lead to improper operation or worse effects on your build.

Add Some Headroom to your Power Budget.

Just knowing the sum of Currents for each Voltage isn't enough. You need to give your PSU a bit of extra capacity. This is important so you don't run your supply at its maximum capability! That's why you see the safety multiplier used just after the sub-totals. Be sure you add an extra 25% to 100% capacity. It guarantees you won't kill your supply anytime soon. While this can mean using a larger more expensive PSU, it gives your Project a better chance for long term reliability.

With your budget complete, you can shop with confidence (at least more confidence) you're getting what is required. You won't spend too much on something bigger that may take up valuable layout space.

Once you have your power-budget down on paper, save it! You may need to review your calculations down the road. 

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