Troubleshooting Insufficient BEC Power for Flight Controller Servos

1. Phenomenon

When using the flight controller (FC), if the servo load is too high, a problem of insufficient servo BEC power supply may occur, leading to:

• Servos jam, jitter, or react sluggishly
• Flight controller restarts due to insufficient power
• Other devices such as VTX (video transmitter) and ESC are affected
  This situation usually occurs on large fixed-wing aircraft, such as when using multiple servos, landing gear, flaps, gimbals, and other equipment.

2. Cause Analysis

Servo power is supplied by the BEC (Battery Eliminator Circuit) on the flight controller:

• Servo BEC on F405 WING APP: Continuous 4.5A, Peak 5.5A
• Servo BEC on F405 WING MINI: Continuous 4A, Peak 5A
• Servo BEC on F405 WING APP V2: Continuous 7.5 A, Peak 14A
  If the total current of the servos exceeds the power supply capability of the BEC, it may lead to insufficient power.
  For example:
  
  90" Venom uses 9 x 13g servos in maximum stall state:
  
• Total servo current = 9 × 1A = 9A (Exceeds the power supply capability of some flight controller BECs)
• May cause insufficient servo power and flight controller restart

3. Solutions

Method 1: Partially Use ESC Built-in BEC for Power Supply
1. Power Supply Analysis
Some servos use the flight controller BEC, and some use the ESC built-in BEC. Taking the 90" Venom as an example:

• Flight Controller BEC Power Supply:
• Ailerons (2 units), connected using a Y-cable (1-to-2), occupying one PWM channel.
• Elevator (1 unit), occupying one PWM channel.
• Rudder (1 unit), occupying one PWM channel.
  (Total 4x1A=4A, lower than the servo BEC peak current, safe)
• ESC BEC Power Supply:
  (Assuming the ESC BEC has a 5A power supply capability)
• Flaps (2 units), connected using a Y-cable (1-to-2), occupying one PWM channel.
• Landing Gear (3 units), connected using a Y-cable (1-to-3), occupying one PWM channel.
  (Total 5x1A= 5A, not exceeding the BEC peak 5A, safe)
  This can effectively share the current load and avoid insufficient flight controller BEC power.
  2. Specific Method
  Use the Y-cable modification method for wiring. The schematic is as follows:
  
  3. Specific Wiring
  Steps:
  1. Prepare Y-cable
  
  2. Single Servo Powered by ESC BEC
• One end connects to the flight controller (remove the positive wire)
• One end connects to the ESC (remove the signal wire)
• One end connects to the servo
  
  3. Multiple Servos Powered by ESC BEC
• Add Y-cables
• Positive connects to ESC positive
• Negative connects to ESC negative
  
  4. Precautions
  ⚠ Flight controller, servos, and ESC must share a common ground, otherwise signals may be distorted!
  ⚠ It is recommended to use the flight controller BEC for critical servos (ailerons, elevator, rudder) and the ESC BEC for non-critical servos (flaps, landing gear)!
  ⚠ Ensure servo current is less than the BEC rated power supply capability, otherwise insufficient power may still occur!
  Method 2: Fully Use ESC Built-in BEC for Power Supply
  1. Applicable Scenarios
  If your ESC has a high-current BEC (8A or higher), you can fully use the ESC BEC for power supply, for example:
  
  9 x 13g servos, total current 9A
  ESC BEC power supply capability 10A
  Fully use ESC BEC for power supply
  2. Specific Operations
  For WING APP Flight Controller
• Use Y-cable modification method
  For WING MINI Flight Controller
• No wiring modification needed
• Directly disconnect the flight controller Servo BEC power jumper
• Servos can be directly powered by the ESC
  
  
  3. Precautions
  ⚠ If using multiple ESCs, only one ESC can be used for power supply, otherwise the ESC BEC will be burned out!!!

4. Verification

How to check if power supply is normal?
1. Observe if servo operation is smooth

• No stuttering, no jitter, no abnormal reset
  2. Check flight controller power stability
• Whether the flight controller restarts
  3. Use a multimeter to measure BEC output
• Whether it stays at 5V / 6V
• Whether there is a significant voltage drop at high throttle

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