Almost every light aircraft electrical diagram I've seen shows the starter solenoid on the output (i.e. switched side) of the master relay/contactor. In this configuration, starter current flows from the battery positive terminal through the master relay/contactor and then through the starter relay/contactor to the starter. If you look at the Rotax 912ULS Installation manual (page 20 of section 24-00-00), you will see that the starter relay/contactor is connected directly to the battery positive terminal and the control voltage to energize the starter relay/contactor comes from the main 12 volt bus (which is switched by the master). Both of these approaches require the master switch to be on in order for the starter to operate. I am leaning toward the Rotax approach because there will be fewer connections and less voltage drop when the high starting current flows from the battery to the starter. In the common approach there are connections (i.e. small voltage drops when the high starter current flows) at both the master relay/contactor (2 connections) and the starter relay/contactor (2 connections) as well as the 2 contacts in each relay/contactor. Is there any reason to wire these relays/contactors differently than as shown in the Rotax installation manual?
Thanks in advance for your comments.
Mike
Thanks in advance for your comments.
Mike