Make a Power Budget for Your Micro Project

A Power Budget is an inventory of project power needs. At some point, you must pick a power supply unit (PSU) to meet these needs. A finished budget helps you shop for the right power supply. Once found, you learn Size, Weight, Capacity and Cost from its sales listing. You know many important details before spending money. You can purchase the PSU or make changes depending on what you learned.

Writing the Power Budget

Lets begin your Project power needs Inventory. First identify Voltage feeds. Then list section-Currents for each feed. This can be written in spreadsheet-form, where;

• Columns denote Voltage(s)
• Rows hold section-names and section-Currents.
• We sub-total the Currents...
• Add a 20% to 50% headroom multiplier underneath.
• At bottom, increased Current sum(s) are derived by multiplying each sub-total by the Headroom factor.

The entire process may take 20 minutes to several hours. If you have the needed information, it can go quickly.

Finding Section Currents

We divide the project into smaller sections that seem logical. Then determine each section's Current info. Its easy to get module and MCU currents from datasheets. But other areas with discrete components may challenge your thinking. Maybe its a transistor or a relay that eats mA when actuated. How can you get the correct current values?

Try this;

• ON-OFF Sections: With items that consume current at varying intervals, assume an always-ON situation. This ensures your PSU can accommodate needed Current when the ON time comes. Use this approach for currents used by LED's, Relays, Solenoids, Motors, Fans, buzzers, etc.
• MODULES: When using purchased modules, get Current data from documentation or datasheet. Typical examples may include; a Real Time Clock, MCU, RF Transceiver, SD, GPS, LCD, etc. If current data isn't available, you can always measure the Module's current with a series multimeter in current-mode. If power info is written in a range, use the high end value.
• DISCRETES: Transistors and MOSFETs are often used as switches. The switched current is already accounted for by the section-current (or load-current) being controlled. A datasheet should reveal any current sensors consume. LED's usually have datasheets (often 10-20 mA per color). Super-brite LEDs require a datasheet to find their specific current demand.
• EXTERNAL FEEDS: If power gets routed out to external devices, note that mA requirement as well. Treat it as another (Always ON) section in your break-down analysis. Even if its an auxiliary feed, assume constant use -- add the current value into your inventory.

Power Budget Headroom

We always need extra capacity. Its essential for long project life and safety. Some folks use 20% as a rule of thumb. Others choose more. Without headroom, your supply runs at full capacity all the time! That's bad for long term reliability (and perhaps safety).

In our budget, we'll have a multiplier row to derive final-totals from sub-totals. This is our headroom allowance. Use at least 20% (multiplier=1.2). More headroom is better but impacts size and cost.

Power Budget and Battery Capacity

You may think battery power is the way to go. But you're not sure how much battery you need. Battery Capacity is rated in Amps-times-hours (Amp-hours). An example is a 10 Ah battery is expected to supply 1-Amp for 10 hours -OR- 10-Amps for 1 hour. This is almost never how things work out. But its a handy way to estimate Project operating-time.

A usual aspect of Capacity calculation is guesstimating a project's Average hourly current. This can be tricky with electronics that varies its current demand (Audio volume, motor ON-OFF times, etc).

• I suggest using your power budget Total-Current for this calculation. Divide it into the battery Amp-hour rating for a decent estimation of operation time.

However, there's a catch! Battery manufacturers often state Ah Capacity on a condition of -typical- current draw! This typical value may be well below what your budget calls out. Be Careful to check on this! If you draw current above the -typical- rating, your battery won't live up to its Amp-hour rating! 

1. I often refer to THIS WEB PAGE for Primary-Battery info.
2. I NEVER USE rechargeable batteries in a DIY (or any home made project) due to Safety, Liability and Insurance issues.

PSU Shopping

So now you know your Power Budget numbers regarding Currents and Voltage. Sometimes you can't find a PSU with multiple Voltages at needed Currents. A work-around is buy discrete power supplies that deliver enough current at a wanted Voltage. Be sure you consider impact on mounting space and wiring.

As you find PSU candidates, take note of Price, Dimensions, Weight and Wiring needs. If you choose a Switch-mode type, pay attention to ripple (AC voltage noise) that can appear on DC outputs. Switchers are smaller but can be noisy. Linear PSU's are larger with generally clean DC.

• NOTE: Wall wart type power supplies may overstate current capacity without warning of possible voltage drop and ripple increase. Be very careful choosing these supplies for your build! You may see a rating of 2 Amps (2000 mA) only to find at 1000 mA the voltage drops 10%.

In Summary

We've seen how to write a Power Budget by breaking a project into Voltage feeds and summing section Currents. We've covered methods for finding each section's Current. Adding headroom ensures the supply doesn't run at full capacity. Knowing budget totals, we shop for PSU candidates with confidence. PSU size, weight, cost and wiring information is then found in sales listings. Using discrete supplies helps fill power budget needs when a do-all PSU isn't available.

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