Alternator open-load voltage?

[JP]

Always read about the danger of damage alternator (rectifier diode or diodes) when accidentally left battery switch in OFF position and started the engine. (So we have Zap-stop alternator protector.)I never have opportunity to measure alternator output voltage in open-load condition. I do know that the regulator should cut off the field current to keep output voltage at set point. The only reason for voltage to go higher than regulator setting is the magnetic flux left in alternator core, which I guess is very small. Is this could generate enough voltage and current to damage the diode(s)? Or is there something else?

 

[Derek Rowell]

Reason for Zap Stoppers

JP, The potential for damage to your alternator is not from starting the motor with no load on the alternator, but rather from interrupting the current by switching the battery off while the engine is running, and the alternator is delivering current. While an alternator is running a magnetic field is set up around the coils, and energy is stored in the field. If the current flow is interupted (by turning off the switch), the magnetic field collapses, and the stored energy in the field generates a very high voltage spike. It is this very brief spike that can damage the diodes in the alternator. The Zap-Stop diode is designed to "break-down" and absorb the spike (without being damaged), thus protecting your alternator.Derek