For all interested,
     Issue 46 of "Contact Magazine" has excellent article
on converting the Mitsubishi alternators commonly used
on 86-91 RX7(and perhaps 93+ RX7 and Cosmo 20B)to either
stock internal reg with external shutdown or external reg
and shutdown.The article also includes the same info for
some Nippondenso alternators.The Mitsubishi is said to
be the better design and easiest to convert.

   Kelly Troyer

Mark Steitle wrote:

I was just looking at my 90A Mazda alternator and feel I have discovered how to disable the internal regulator in order to convert to an external regulator.  However, it would be reassuring to read up on it before I take the wire cutters to things.  Can someone summarize the Contact article, or tell me how to get a copy of the appropriate issue?

I read up on Bob Nuckolls' Aero-Electric book to find out why I would want to convert to an external regulator system.  Bob states that the benefit in switching to the external regulator is to replace the "switching" regulator with a "linear" regulator.  He states that he is unaware of any automotive alternator/regulator that is of a linear design.  He sells a linear a/c regulator with a "crowbar overvoltage protection" circuit that will cut out within 5 miliseconds in case of an overvoltage type failure.  That should prevent one frying their expensive avionics.  Sounds logical to me.
Mark Steitle

I might add the drawback to a linear regulator is less efficiency and as a result
increased heat dissipation in the regulator. Make sure you have a big enough
heat sink on the regulator transistor and it is placed in a cool spot.
The up side to a linear regulator is less electrical noise on the power buss.

Also BTW most modern avionics use low cost linear regulators on the input that have
a capability of probably handling as much as 35 volts input with out blowing.
They also probably have a diode to prevent being hooked up to the battery
backwards. If not they should.

I am sure Tracy uses such low cost regulators in his EFI/IGN computer.
What is max input voltage Tracy?

Paul Lamar

Finn Lassen wrote:

As discussed extensively on Matronics.com's Aero-Electric list, a run-away  alternator can easily put out more than 100 volts! By run-away I mean a failed voltage regulator feeding full 12 volts into the field winding (B+ lead).

I don't think that the field lead amperage (from the voltage regulator) is very high, so efficiency in the voltage regulator is not a major factor.

Having said all that, I've still not modified my Mitsubishi (?) alternator to include the crow-bar over-voltage protector. But if I had a $10,000 avionics stack in my plane, you can be sure I would.

Finn

Somehow I doubt that 100 volt number. 
Sounds like a good way to make a buck.

Must be a very rare failure mode or car radios and EFI/IGN  computers 
would be dying all over the place. With 12 volts applied to the 
field how long before the wires feeding it fused? 

With a battery across the output from the alternator the connection 
to the battery must also somehow disappear as the battery
would look like a giant short to 100 volts.

Further more 35 volt zeners on the input to most electronic equipment 
would do the job. Perhaps there could be a 100 volt spike but a 
capacitor could handle that and knock it down to way less than 35 volts.

Color me skeptical.

Paul Lamar
 
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