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Q
What PWM and feedback signals do 28VDC BLDC fans use?
APWM control and FG/RD feedback signals should be reviewed with the correct reference ground. The PWM control reference is normally tied to the fan negative return, and interface errors can cause unstable speed control, false fault feedback, or signal noise. FG feedback is used to monitor fan speed through pulse frequency, while RD feedback is used to monitor run or fault status. FG/RD outputs should normally be wired individually when independent fan monitoring is required.
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Q
How does altitude affect fan cooling performance?
AAt low pressure, volumetric airflow and cooling capacity should not be treated as the same value. Reduced air density lowers heat capacity per unit volume and changes pressure performance, so high-altitude fan selection should be reviewed by operating point, air density, thermal load, enclosure resistance, and model-specific speed behavior. For airborne or high-altitude electronics, low-pressure review may reference MIL-STD-810H Method 500.6 when the qualification plan requires altitude or low-pressure testing.
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Q
What is the correct way to read a fan P-Q curve?
AThe operating point of a cooling fan is the intersection between the fan P-Q curve and the system impedance curve. Free-air CFM describes airflow at zero static pressure, but installed airflow depends on filters, heat sinks, cable bundles, card guides, EMI screens, louvers, and duct geometry. To read a P-Q curve correctly, plot or estimate the system resistance curve on the same axes, find the intersection, and confirm that the resulting airflow meets the thermal budget with adequate margin.
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Q
How is a military cooling fan different from a commercial fan?
AA military cooling fan is selected by operating-point airflow, pressure capability, electrical interface, installation constraints, and environmental requirements, not by free-air CFM alone. In defense electronics, the correct fan is the one that can deliver the required airflow inside the real enclosure while remaining compatible with the applicable thermal, vibration, shock, altitude, and EMI conditions. Commercial fans are typically screened by free-air airflow and price, without the environmental qualification traceability that defense platforms require.
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Q
What test documentation should I request before qualification or RFQ?
AFor a serious RFQ, request the P-Q curve, outline drawing, electrical interface definition, connector or lead specification, inrush-current data, PWM/FG/RD logic, acoustic data, bearing-life basis, and environmental test references. For defense and aerospace programs, also request the applicable MIL-STD-810H method matrix, CE102/RE102 pre-screening data when EMC risk exists, and the power-quality reference such as MIL-STD-704 or MIL-STD-1275. The best supplier response ties each document to your platform test plan rather than sending a generic catalog page.
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Q
Can fans run in reverse for purge or dust-clearing cycles?
ASome fan configurations can support reverse rotation or controlled purge operation, but it must be confirmed by model. Reverse operation changes airflow, pressure, noise, motor loading, and bearing stress. In shelter, ground vehicle, or dusty electronics applications, purge cycles may help clear loose particles, but the host controller should verify direction, startup behavior, and thermal margin. Do not assume a standard cooling fan can run continuously in reverse without a model-specific review.
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Q
Why does airflow drop after long operation in dusty, humid, or high-temperature equipment?
ALong-term airflow loss usually comes from one of four causes: inlet or outlet blockage, voltage drop at the fan terminals, incorrect PWM command, or mechanical degradation. Dust and process residue increase system impedance, humidity and salt can raise connector resistance, and high temperature accelerates lubricant and bearing wear. Troubleshooting should record terminal voltage, current draw, PWM duty cycle, FG speed, inlet condition, outlet condition, and any abnormal bearing noise before replacing the fan.
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Q
How should I size power supply margin for startup current and bus transients?
APower sizing should include nominal voltage, continuous current, startup current, and the platform transient profile. BLDC fans often draw 1.5 to 3 times rated current for a short startup interval. Aircraft and vehicle platforms may also impose surge, brownout, reverse polarity, or load-dump requirements. For 28VDC equipment, review the invoked MIL-STD-704 or MIL-STD-1275 profile. For 400Hz AC equipment, verify line/phase voltage, frequency tolerance, inrush behavior, and dielectric withstand requirements.
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Q
What is the difference between FG and RD fan outputs?
AFG is a tachometer output used to calculate real-time fan speed from pulse frequency. RD is a run/stop or fault-status output used to confirm whether the fan is operating. Both are typically open-collector style signals and require an appropriate external pull-up. Use FG when the controller needs RPM data and trend monitoring. Use RD when the controller only needs a discrete fan-running or fan-fault state.
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Q
How do I calculate RPM from an FG tachometer output?
ARPM equals FG frequency in hertz multiplied by 60 and divided by the number of pulses per revolution. If a fan outputs 2 pulses per revolution and the controller reads 200 Hz, the speed is 6,000 RPM. Always confirm the pulse count in the model datasheet before setting alarm thresholds. A wrong pulse count can make a healthy fan look slow or make a real low-speed condition invisible to the host controller.
APWM control and FG/RD feedback signals should be reviewed with the correct reference ground. The PWM control reference is normally tied to the fan negative return, and interface errors can cause unstable speed control, false fault feedback, or signal noise. FG feedback is used to monitor fan speed through pulse frequency, while RD feedback is used to monitor run or fault status. FG/RD outputs should normally be wired individually when independent fan monitoring is required.
