Power Supply Sizing Calculator

Power Supply Sizing Calculator

📹 Video Walkthrough — How to Use This Calculator

Mathematical Formulas

Complete Guide to Power Supply Sizing

Getting the PSU size right is basic engineering. If you’ve got multiple actuators or automation hardware, you don’t want to rely on luck. A calculator moves this from guesswork to solid numbers, taking both steady-state and worst-case (like startup) loads into account.

Understanding Power Supply Fundamentals

A standard power supply takes AC from the wall and outputs the DC voltage your devices want. The core rule is P = V × I, but in real designs, you also need to consider where things go off the textbook path.

For instance, switching supplies usually convert at 80-90% efficiency, so not all the AC in makes it to the output—some turns into heat. Most devices don’t draw steady current, especially anything with a motor: startup phase is where you see current spikes several times higher than the rated value.

Critical Factors in PSU Sizing

Load Characteristics: The requirements aren’t always the same. Heaters and lamps draw steady current and are easy to size. Motors and linear actuators are less forgiving—they pull large startup surges and their back-EMF can bounce current back into the supply if not considered during sizing.

Duty Cycle: If your actuators are intermittent, the average current consumed is much lower than the peaks. Calculating on-duty and off-duty time lets you avoid paying for unneeded capacity, but you still need to cover the highest load events.

Environmental Conditions: Supplies lose output as they get warm. A 100W supply rated at 25°C could drop to 80W at 50°C. If your enclosure or environment runs hotter than room temp, always check the derating curves and factor them in up front.

Practical Design Considerations

Don’t just add device ratings and call it a day—work out what happens at start-up and stalls. For example, an actuator rated 5A continuous at 12V can easily draw triple that just for a split second when it starts moving, or if it hits the stops. That instant can trip your supply if you sized only for the running current.

If your devices can all run at once, add up their peak requirements. But if your control logic only allows one actuator to move at a time, you can size for just the peak of the largest single device or a reduced total—not all at once. Sometimes adding delays or soft start lets you use a smaller, less expensive supply.

Worked Example: Multi-Actuator System

Take three linear actuators at 12V each:

• Actuator 1: 3.0A continuous (36W), 7.5A startup (90W)
• Actuator 2: 2.5A continuous (30W), 6.25A startup (75W)
• Actuator 3: 1.5A continuous (18W), 3.75A startup (45W)

Total continuous power is 84W. If they all start together: 90 + 75 + 45 = 210W. Add a 25% margin for real-world slack: you’re at 262.5W minimum. Round up to 300W supply.

If your control software staggers each start by even 100ms, you never draw all three peaks at once, and you might get away with a 150W supply. Sequencing and startup control save money here—but only if you know your system’s real behavior.

Advanced Sizing Considerations

Ripple and Regulation: For some analog and signal applications, low noise and voltage stability matter as much as raw wattage. Linear supplies give you both, but run hotter and bigger. Most automation setups won’t need this, but it’s worth checking if noise or voltage drop will cause trouble.

Redundancy and Reliability: Mission-critical panels may use several smaller supplies in parallel (N+1 redundancy), so you don’t shut down on a single failure. Each supply then must be able to carry the full load if others drop out—keep that in mind when splitting loads.

Future Expansion: If you know you’ll be adding devices over time, sizing up by 30-50% now typically costs less than replacing the supply later. Pay for what you’ll use soon, not for upgrades you may never make.

Power Supply Types and Applications

Linear Power Supplies: Good for sensitive analog or measurement circuits, where low ripple is critical. Efficiency is poor, and heat output is high, so use only if your application justifies it.

Switching Power Supplies: These are the everyman’s choice for modern controls. High efficiency, compact, and with protective features built in. Unless you have special analog needs, switching supplies almost always make sense for actuators and automation.

Unregulated vs. Regulated: Use regulated supplies if your load varies or precise voltage is needed. Unregulated is only suited for stable, predictable loads.

Protection and Safety Features

Things like over-current and over-temperature protection are standard on good supplies now. Fold-back current limiting (where output voltage drops to limit current) can be a gotcha: your devices won’t run correctly if your sizing doesn’t plan for how these protections interact in worst-case conditions.

Thermal shutdown or throttling is another reason to account for venting and derating. If your supply runs hot, expect output cuts well below the nameplate. Don’t ignore physical placement and cooling in your calculations.

Integration with Control Systems

Many industrial supplies now give you live feedback: voltage, current, even temperature. Watching these over time lets you validate your sizing and catch overloads or derating before something fails.

Remote voltage adjustment is useful if you need to optimize actuator speed or match changing load conditions—just confirm your supply is still within capacity after any adjustment.

If you’re also dealing with force calculations or mechanical loadings, check the dedicated calculators in the full engineering calculators catalogue. There are quick tools for almost any actuator or motion-system math you’ll run into.

Frequently Asked Questions

About the Author

Robbie Dickson

Chief Engineer & Founder, FIRGELLI Automations

Robbie Dickson brings over two decades of engineering expertise to FIRGELLI Automations. With a distinguished career at Rolls-Royce, BMW, and Ford, he has deep expertise in mechanical systems, actuator technology, and precision engineering.