912 ULS starter solenoid control circuit current requirement

[FredG]

I have been looking and can't find anywhere... What is the Rotax 912 ULS starter solenoid control circuit current requirement?  On my 2006 it appears that Rotax installed a 20 AWG wire to the solenoid control terminal, but any inference from that observation is just a guess.  I suppose I could put an ammeter in series and just measure it...

Also, when reviewing the CTsw wiring diagram, I am unable to find any circuit protection for the starter solenoid control circuit (no breaker to trip if this wire shorts to ground).  Is that a correct observation?  

Thanks,

Fred

[Anticept]

Starter wires are the single exception to circuit protection specifically spelled out in regulation, probably the same in the ASTM.

The control wire IS protected, though not to any significant degree...

The solenoids take hardly any power at all. Just put an ohmmeter on it (case grounded) and solve for I: 12 = I R, where R is your reading from the ohmmeter, I is in amps.

[FredG]

Corey, I am not sure what you mean by the "starter wire".  When you say the control wire is protected, how is that achieved?  I am talking about protection with a circuit breaker or fuse, which do not appear on the CTsw wiring diagram.  How would a short to ground not result in a burned control wire?

Your method of measuring for current required by using ohm's law makes sense.  Any reason not to take a solenoid (on the bench) and just measure actual current using an ammeter and a 12 V battery?  

[Anticept]

You could, but it's not necessary. Typically, bench testing is for circuits that are more complex than simple resistive circuits. Since you're dealing with DC when you test with an ohmmeter, the solenoid coil will just act like a long piece of wire.

The starter wire is the wire to the starter. They carry a very temporary, but ENORMOUS load. So therefore, it doesn't make sense to fuse it. Proper design calls out for the starter solenoid to be as close to the battery as practical, as the solenoid itself is the switch for the positive polarity.

Circuit protection for the starter control wire is basically the ignition switch and master breaker. You as well as I would prefer a fuse or something on it appropriate for the gauge, but that's not often found in many aircraft designs. but that's not something I personally find very often in other aircraft designs.

EDIT: edited for clarity.

[FredG]

Corey, thanks. I do know that the (very large) wire to the starter is never protected (due to the enormous and intermittent current carried).  I am a bit confused about the absence of circuit protection for the starter solenoid control circuit, however.  The master breaker won't protect the 20 AWG wire that controls the starter solenoid.  You are right, I would prefer a breaker (or fuse) on that wire.  Odd that virtually every other circuit in the CT is protected by a breaker sized to the wire but that circuit/wire isn't.  Also odd (to me) that protection of that circuit isn't common in other designs.

[Anticept]

The control wire is very temporarily switched. I guess it's assumed that you'll never have it on long enough to be of any real threat. Doesn't mean I like it... but unless you have a very weird switch failure AND short failure down the line...

Also, I don't want to say if it's common or not, I am not an authority to say that. I just know I personally haven't seen it too often. Edited for clarity.