Current through Start Button

There was a discussion on the Sonex forum about what size fuse to use on the Start Button circuit with the Sky-Tec AeroVee Starter. A builder was immediately blowing 15A fuses when pushing the start button. I had a vague recollection that I measured the steady state current at ~6A with a bench power supply (I now think this number is more like 8.6A), so this seemed unexpected.

I wondered if a current transient could be the problem and found an article by Bob Nuckolls that describes how the starter solenoid uses two coils so there’s an initial high current to quickly actuate it and a lower current to hold it closed.

Then the builder mentioned he didn’t have the heavy battery cable connected to the starter and another builder wondered if that might be preventing the initial current transient from decreasing.

I thought it would be interesting to know exactly what is happening, so I measured the current with and without the battery attached using a 75mV per 5A current shunt in series with the start button. Turns out, I had a 10A fuse on that circuit and ended up blowing it just like the original poster when the battery was not attached.

I found the following chart in the datasheet for Littelfuse that agrees with the ~0.15s time to blow the 10A fuse at 34A.

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