since 1940

Over-voltage induced trip offs of aircraft electrical system

 In mid-1972 Cessna added a capacitor (see Cessna Service Letter SE72-15) the overvoltage circuit on their single-engine Cessna's. This capacitor prevents nuisance trip offs of the alternator caused by voltage spikes. This 500-microfarad, 50-volt electrolytic capacitor is located behind the circuit breaker.
1. Capacitor never installed.
2. Capacitor defective.
3. Charging system voltage too high.
4. Overvoltage sensor malfunctioning.

Numerous problems have been reported with nuisance tripping of the Over Voltage (OV) circuit and oscillating or "dancing" ammeters in Cessna 14 volt alternator systems. The Cessna 14V OV protection circuit consists of an OV sensor between the master switch and pin S of the voltage regulator. If the OV sensor detects a bus voltage greater than 16 volts it turns off the voltage regulator by removing power from pin S. The OV is designed to ignore momentary voltage spikes and fluctuations, and only trip when the voltage regulator fails and allows the voltage to exceed 16 volts longer than its built-in time delay.

The best solution after checking for good grounds is to replace the OV and voltage regulator with a Zeftronics R15V00 Rev a unit.

A bad battery can create a situation where you have high voltage and high amperage output as the charging system attempts to charge the bad battery. 

Charge circuit would go to high amperage and high voltage limit (17v).  System progressed over time, with initially after 1.5hr flying, progressing to start-up and after about 5 hours/5 flights.  Alternator, regulator, wiring, condenser all checked out fine.   Took battery into auto shop and asked them to check it.   It checked normal on bench check.  I asked them to check battery under load.  Battery under load came back as \"excessive charge rate, battery overheated, replace battery\".  I replaced the battery and system works normal.
 

Copyright 2003 by Sacramento Sky Ranch Inc. All rights reserved.  Prices subject to change without notice. Not responsible for typographical or misprint.
Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.