ESC Voltage Spikes Tested: Capacitors, TVS Diodes, and the Fires They Stop

Alex -

Voltage spikes in ESCs (Electronic Speed Controllers) can spontaneously ignite your drone, especially during disarming or crashes. Chris Rosser’s rigorous tests reveal the true scale of these spikes and how capacitors and TVS diodes handle them.

TL;DR: ESC voltage spikes often double the battery voltage, threatening ESC FETs and causing fires. Adding capacitors across battery leads slashes spike size dramatically. TVS diodes help but lag behind capacitors. Keep capacitor legs short for best results, and consider TVS diodes on motor phases for extra insurance.

The Setup: Standard FPV Motor, ESC, and Oscilloscope

Chris tested a 2207 1850KV 6S FPV motor paired with a Skystars 55A ESC. A 5x4.3x3 prop represented typical FPV gear. An RPM sensor ensured consistent motor speeds for each run.

The ESC’s battery leads connected to copper wires for oscilloscope probes—one on positive, one on ground. Another probe monitored a motor phase wire. The HanTek USB oscilloscope logged data on a PC.

Power came from both 4S and 6S 5000mAh batteries, topped by a power supply for steady voltage.

Capacitors and TVS Diodes: The Spike Mitigation Arsenal

The test components included two capacitors: 470µF and 1000µF, each tested with short and long legs to gauge wiring impact. Also included was a Fedtech spike absorber—three TVS (Transient Voltage Suppression) diodes designed to clamp voltage spikes.

Capacitors smooth voltage by absorbing spikes, while TVS diodes clamp voltage peaks by rapidly conducting when voltage surpasses their breakdown threshold.

Live Data Collection: ESC Beeps and Motor Spin Voltage Spikes

Initial tests with a 4S battery and no extra capacitor showed ESC beeps alone generated spikes up to 18.5V on a 16V pack. Spinning the motor at 5,000 RPM caused oscillating spikes around 18V, increasing with RPM.

Active braking—disarming the ESC while the motor spun—triggered even bigger spikes, peaking at 21V on battery leads and 25V on motor phases. This exceeds the battery voltage by a wide margin, stressing ESC components.

Voltage Spikes Scale with RPM and Battery Voltage

At 10,000 RPM on 4S voltage, spikes hit 20V on battery leads and 22.5V on motor phases. Active braking pushed spikes to 25V on battery leads.

15,000 RPM increased spikes to 21V on battery leads and 26V on motor phases. Braking caused peaks of 27.5V on battery leads and 21V on motor phases.

At 25,000 RPM, spikes reached 30V on battery leads and 35V on motor phases during braking, more than twice the battery voltage. Voltage sag occurred due to current draw, but spikes remained high.

6S Battery Voltage: Even Harsher Spikes

At 30,000 RPM wide-open throttle with no capacitor or TVS diode, spikes soared to 32V on battery leads and 34V on motor phases. Active braking caused battery lead spikes to jump to 44.5V—almost double the average 23.8V battery voltage.

Interestingly, at 6S, voltage dipped at braking start, unlike 4S where it rose. This nuance likely ties to motor commutation timing.

470µF Capacitor with Short Legs: The Spike Slayer

Installing a 470µF low ESR capacitor with legs trimmed short across battery leads radically reduced spikes. At 20,000 RPM, battery lead spikes dropped to 16.5V from 20V, with motor phases spiking only to 19.5V.

During braking, battery lead spikes shrank to about 18V, motor phases to 22.5V. Even at 25,000 RPM braking, battery spikes capped at 17.5V, motor phases at 24V.

This capacitor mounting is optimal, delivering a significant spike dampening effect that protects ESC components.

Capacitor Leg Length: Does It Matter?

Leaving capacitor legs long (20mm) degraded spike suppression slightly—about 0.5 to 1V increase in spike amplitude. At 20,000 RPM, spikes rose to 17V on battery leads and 22V on motor phases.

Braking spikes also increased marginally. Despite this, the difference isn’t dramatic, suggesting that while short legs are preferable, slightly longer legs don’t cripple capacitor effectiveness.

For longer capacitor leads, using thicker wire (like leftover motor wire) can reduce resistance and maintain performance.

1000µF Capacitor: Marginal Gains Beyond 470µF

Increasing capacitance to 1000µF didn’t noticeably improve spike suppression compared to 470µF. At 25,000 RPM, running spikes reached 24.5V battery lead, 32V motor phase; braking spikes peaked at 26V battery lead, 33V motor phase.

This suggests diminishing returns beyond 470µF in typical single-motor setups. Multi-motor or heavily loaded ESCs might benefit, but for most, 470µF suffices.

TVS Diodes Explained: The Spike Choppers

TVS diodes (zener diodes designed for transient voltage suppression) block voltage spikes by staying off until voltage exceeds their breakdown level. Then they rapidly conduct, clamping voltage to a safe ceiling.

This conduction happens in nanoseconds or picoseconds, effectively chopping the spike tops and protecting sensitive electronics.

TVS Diode Performance: Good but Not Great

At 6S voltage, TVS diodes alone allowed spikes up to 27V at 10,000 RPM, similar to no protection at 4S. Braking spikes reached 30.5V with a chopped peak shape indicating TVS conduction.

At 15,000 to 25,000 RPM, TVS diodes capped spikes around 30-32V on battery leads, but motor phases spiked higher—up to 38.5V. The diode clamped spikes but didn't reduce them as effectively as capacitors.

TVS diodes act as a fallback rather than a primary solution.

Conclusions and Recommendations

  • Never run an ESC without a capacitor on the battery leads—the voltage spikes can double battery voltage and degrade FETs, causing catastrophic failure and fires.
  • Use a 470µF low ESR capacitor with short legs soldered directly across battery leads for best spike suppression.
  • Longer capacitor legs reduce effectiveness slightly but are not catastrophic; use thick wire if longer leads are necessary.
  • Increasing capacitance beyond 470µF offers minimal benefit for single-motor setups.
  • TVS diodes provide a useful backup if capacitors fail or are absent but do not replace capacitors.
  • Consider TVS diodes on individual motor phases to clamp spikes that exceed battery lead voltage, protecting ESC FETs further.
  • High-voltage and high-RPM setups—like world record drones—may require both capacitors and TVS diodes for robust protection.

Adhering to these recommendations reduces the risk of ESC fires and extends component life. Voltage spikes are a silent menace; proper mitigation is essential for reliable, safe flight.

FAQ

Why do ESC voltage spikes happen?

Voltage spikes occur due to rapid changes in motor speed, especially during braking, causing back EMF and oscillations that exceed battery voltage.

Can I use any capacitor on my ESC?

Use low ESR electrolytic capacitors typically 470µF, designed for smoothing power supply lines. Avoid placing capacitors across motor phases.

Do TVS diodes replace capacitors?

No. TVS diodes act as backup protection, clamping extreme spikes, but capacitors provide better smoothing of voltage fluctuations.

Does capacitor leg length matter?

Shorter legs reduce resistance and improve performance, but slightly longer legs only marginally impact spike suppression.

Are voltage spikes dangerous for ESCs?

Yes. Spikes can exceed FET voltage ratings, causing damage, failure, and fires, especially if no mitigation is in place.

Takeaway Box

  • Voltage spikes can double battery voltage, risking ESC fires.
  • 470µF low ESR capacitors with short legs dramatically cut spikes.
  • TVS diodes help but don’t replace capacitors.
  • Long capacitor legs slightly reduce effectiveness; use thick wire if needed.
  • Consider TVS diodes on motor phases for extra ESC protection.

This article was based from the video Protect Yourself from ESC Voltage Spikes: Testing Capacitors and TVS Diodes