Does a replacement fan's start-up current matter?
Yes — the start-up current of a DC fan can be several times its steady-state rating, and a replacement with a different motor or start profile can trip a supply that the incumbent never bothered. Check the datasheet's start-up (or peak) current figure, measure it on a sample, and size the rail for the worst simultaneous start of every fan on the rail.
Why start-up exceeds steady state
At standstill the motor has no back-EMF, so current is limited only by winding resistance and the drive's current limit; the rotor also needs maximum torque to break static friction. Typical start-up currents on small DC fans run a few times the rated steady current for tens to hundreds of milliseconds. Smart-start or soft-start firmware in the fan's driver can extend the ramp and reduce the peak — but not every fan has it, and the profiles differ between makers.
| Parameter | Typical magnitude | Design impact |
|---|---|---|
| Steady current | Rated, e.g. 0.20 A | Nominal supply sizing |
| Start-up peak | ≈2–5× steady (model-specific) | Supply trip / droop risk |
| Peak duration | tens–hundreds of ms | Bulk capacitance sizing |
| Simultaneous starts | N fans on one rail | Multiply — or stagger starts |
How to measure it on a sample
Scope the rail voltage across a shunt (or use a current probe) while the fan starts at full duty from a cold state. Record peak current and duration, then repeat at minimum ambient — lubricant viscosity raises breakaway torque when cold, and the peak grows. Compare against the incumbent's measured profile, not just its datasheet; then check the supply's over-current trip margin and the rail's droop during the peak.
- Measure at both temperature extremes, not only room temperature.
- Repeat with minimum and maximum PWM duty if the host power-ups at a fixed duty.
- Verify no brownout on other loads sharing the rail.
Supply-side mitigations
If the candidate's peak is higher than the incumbent's: add bulk capacitance near the fan header, stagger fan power-up in firmware, ramp PWM from zero instead of power-on at 100%, or specify a soft-start variant from the fan maker. All four are common in custom fan projects — send your rail budget and start sequence when you request a replacement candidate.
Send MEGA Tech your airflow and static pressure target, voltage, size limit, control interface, quantity and environment. We reply with a selection proposal and a sample plan — selection by operating condition, samples validated on your unit.
WhatsApp +86 135 7056 7086 · Email inquiry@cnmegatech.com · Custom fan development