Alternator current limiter.

I always assumed the alternator did not need a current limiter because automotive alternators don,t have one. They have a limited amount of iron in their stator which magnetically saturates if the alternator is asked to supply too much current.

Generators in earlier days did not have this feature so the mechanical voltage regulators they used had voltage regulation and current limit. They mostly had 3 relays 1 for voltage control one for over current control and one to act as a cut out so the battery did not discharge back through the generator when the engine was stopped. They were very reliable despite marketing people telling us otherwise.

After reading the latest alert from the Jabiru factory

Service letter JSL021-1

It appears our Jabiru alternators also do not have this feature because a few owners have managed to burn out the coils in the stator.


A very simple way would be to add a 1.5 ohm resistor in series with the DC charging circuit which would limit the current to about 15 amps. BUT there is a small problem with this. Lets say you had a 1.5 ohm resistor passing 15 amps it would have to dissipate 337.5 watts. It would make a great cabin heater in winter but not so good in summer.

Power = Current squared times Resistance 15 x 15 x 1.5 = 337.5.

There is a much better way which will dissipate almost no heat and achieve a better result than a 1.5 ohm resistor.

Use a 1 mH inductor that can pass 15 amps and install it in the AC line coming from the alternator. With the engine running at 3000 RPM the alternator output is 40 volts RMS at 300 HZ , so with a 1 mh inductor the Inductive reactance XL = 2 times pi times frequency times the inductance = 2 x 3.14 x 300 x 0.001 = 1.884 ohms. The best part is as the RPM drop so does the frequency therefore so does the reactance , at low RPM it has much less influence where it is not needed. The DC resistance is less than your wiring from the alternator to the regulator so very little heat would be produced.

Using a temporary ammeter in my charging circuit without the inductor the highest reading was 5 amps on take off and it dropped to 1 amp after about 15 minutes at cruising RPM.

This is with a fully charged battery to start with (It has a solar cell on it all the time) and my engine starts first time every time. So I don,t need any current restriction under these conditions. When you might need it is if you have a lot of trouble starting and then take off with a very discharged battery.

Plus if you have a lot of electrical load , landing lights, flashing beacons , etc etc. , My Sonex has none of these things so my average total load including the auto pilot is less than 1 amp.

This is my home made inductor on the left , if you don't have a ferrite torroid in the junk draw you can buy a 1 mH 15 amp inductor/choke for about $20 from here

Element 14

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