Using Radiant Turn Coordinators on 28 V Aircraft Electrical Systems

Radiant Technical Note 11.6.2025

Using Radiant Turn Coordinators on 28 V Aircraft Electrical Systems

(Not applicable to SafeTurn portable units)

Applies to: Radiant Turn Coordinator products, when operated from 24–28 V class aircraft electrical systems (“28 V” systems).

Does not apply to: Radiant SafeTurn portable units (self-powered; do not add this resistor).

1. Background

Radiant Turn Coordinators use a robust linear power chain:

External Bus → 9.5 V Pre-Regulator → 5.0 V Regulator → Li-Ion Charger → Li-Ion Pack → Internal Loads

The internal pre-regulator is rated to 30 V (with high transient tolerance), and downstream devices are selected for harsh environments. Electrically, these units can be tied to many aircraft buses.

However, a “28 V” aircraft system normally operates:

• In flight: about 27.5–28.5 V

• With real-world variation and hot regulators near the top of that range

Feeding a 9.5 V linear pre-regulator directly from this range at charging current forces it to burn significant power as heat. That’s survivable but unnecessary—and over time, unwanted.

Radiant’s design supports a simple external addition that:

• Reduces internal thermal stress

• Improves reliability margin

• Requires no changes inside the Radiant Turn Coordinator

  
  

2. Recommended Solution for 28 V Systems

For experimental aircraft with 24–28 V systems, Radiant recommends adding a single external series resistor in the power feed the Radiant Turn Coordinator.

Use a 100 Ω, 5 W (or higher) resistor in series with the +28 V supply line feeding the unit.

Radiant does not supply this resistor; it is a standard, easily sourced component.

How it works (conceptual)

• At a configured internal load/charge of about 160 mA, the resistor drops part of the voltage before the 9.5 V pre-regulator.

• The external resistor now dissipates a significant portion of the waste heat instead of the pre-regulator IC.

• The pre-regulator runs cooler while maintaining proper operation over the typical 28 V range.

Example (worst nominal: 28.5 V bus)

Assumptions:

• Bus: 28.5 V

• R_EXT: 100 Ω

• I: 0.16 A

• Pre-reg target: 9.5 V

Then:

• Drop across R_EXT:VR=0.16×100=16.0 V

• Pre-reg input: Vpre-in=28.5−16.0=12.5 V

• Pre-reg dissipation: Ppre=(12.5−9.5)×0.16=0.48 W

• Resistor dissipation: PR=0.162×100=2.56 W

Conclusion: the resistor (properly rated) runs warm and happy; the pre-regulator is kept in a comfortable power range instead of being pushed hard.

At typical in-flight voltages (27.5–28.0 V), numbers improve slightly further.

3. Behavior at Lower / Abnormal Voltages

This recommendation targets normal 28 V operation and thermal robustness:

• At 27.5–28.5 V (normal regulated):

  • Full functionality

  • Good thermal margin on internal regulators

• At lower bus voltages (e.g. 24–26 V, weak bus / alternator offline):

  • The series resistor reduces headroom; charge current may taper or stop.

  • This is expected and acceptable for abnormal conditions.

• At brief higher inputs within device ratings:

  • The series resistor further limits stress on the pre-regulator.

Installers should treat this as good practice for experimental 28 V aircraft, not as certified data.

4. Installation Guidance (Experimental Use)

Basic wiring:

1. Aircraft 24–28 V bus → appropriate breaker or fuse

2. Breaker/fuse output → 100 Ω, ≥5 W series resistor

3. Resistor output → Turn Coordinator +V input

4. Turn Coordinator ground → aircraft ground

Notes:

• Mount the resistor so it has airflow and is mechanically secure.

• Keep it clear of plastics and harnesses sensitive to elevated temperature.

• Use aviation-appropriate wiring, terminations, and strain relief.

• Verify operation at your aircraft’s actual bus voltages after installation.

  
  

5. Example Orderable Part (Installer Convenience)

Radiant does not supply the resistor. A typical suitable example:

• 100 Ω, 5 W flameproof or wirewound resistor, axial or chassis mount

• Example (one of many acceptable choices): a 5 W, 100 Ω wirewound resistor from a major brand via Digi-Key / Mouser / equivalent.

Any equivalent 100 Ω, ≥5 W resistor from a reputable manufacturer, installed correctly, is acceptable.

6. Scope Clarification

• This recommendation supports and applies to:

  • Radiant SafePanel (experimental installations)

  • Radiant RTCC (experimental installations)

• ️ This recommendation does not apply to:

  • SafeTurn portable units

  • Certified aircraft or TSO/STC applications (this is not approved data)