Over Voltage Protection - Crowbar - Voltage Spikes

·         John McGlynn

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Hi All,
John McGlynn here from St Johns Florida USA and I am building a Rans S-19 powered by a 3300 Jabiru. I am laying out the electrical schametic and using the Jabiru J series diagram as a guide. I see that the alternator one and two are wired directly into the voltage regulator. My mentor used to maintain Lockheed P3's for a living and said they had similar permanent magnet alternators that protected downstream circuits with a huge voltage protection device. The way I see the J series wired it looks like the regulator is vulnerable if an alternator fails and there is a voltage spike. Does anyone know the rationale for placing it on the downstream side of the voltage regulator instead of between voltage regulator and the alternator?

Also, can anyone advise how the Crowbar works? Some of my research seems that it also protects against Voltage Spikes
Regards
John

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·         Doug Smith

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Hi John,

That was a while ago, I'll do my best...

The regulator is a type designed for the our type of alternator and from memory has internal over-voltage protection built in. The crowbar etc. is there to protect the avionics from a regulator failure, not the regulator from an alternator issue.

From memory the crowbar uses a system of zener diodes: normally it is closed (allows no current) but when exposed to a voltage spike it opens, allows current through that circuit which pops the breaker and isolates the alternator/regulator from the aircraft power supply.

Regards,
Doug.

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·         John McGlynn

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Thanks Doug. Sorry for the late acknowledgement. I am new to this form and I suspect my profile is not set up properly as I don't recall getting any notification of the response. I also posted another question on the form regarding the circuit diagram. I'll check back soon. Thanks again, John

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·         Doug Smith

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No problem John. I had a look and could not find another question from you... you might need to send it again.
Regards,
Doug.