SonexBuilders.net

Area 51% wrote:

Rynoth wrote:You had the starter circuit open while engaging the starter solenoid? Makes me wonder if the solenoid circuit might draw more amps for some reason if the starter doesn't actually have its own power.

Never gave that a passing thought. Didn't think it would matter if there was power flowing through the starter motor contacts or not.
It seems the second set of windings in the solenoid someone mentioned would have to get it's power from the main cable somehow.
I'll do a bench test to check though.

Yea that's what I'm wondering, maybe the initial draw or "spike" for the solenoid is reduced when the main starter power takes over from there, and without the power on the starter the solenoid circuit has to try "extra hard" to keep pushing on the solenoid. That, and/or the starter motor is trying to pull amps through the start-button circuit in the absence of its own main battery circuit. I'm curious what you find!

Rynoth wrote:Yea that's what I'm wondering, maybe the initial draw or "spike" for the solenoid is reduced when the main starter power takes over from there, and without the power on the starter the solenoid circuit has to try "extra hard" to keep pushing on the solenoid. That, and/or the starter motor is trying to pull amps through the start-button circuit in the absence of its own main battery circuit. I'm curious what you find!

With the starter on the floor under foot, a main power cable attached, and a short (20in) wire with a fuse holder going to the solenoid, a 15amp fuse is reusable after the test. The next smallest fuse I have (had) in my inventory was a 5amp. It didn't fare as well as the 15. A 10 will be tried before the starter is remounted.
I then disconnected the main cable and repeated the test. The 15amp fuse blew with vigor. A 20amp suffered the same fate. A 30 held just as it had in the plane.
I don't expect different results with the starter turning the engine over.
Thanks for the slap Ryan!

Working on my apology speech to Kerry here @Area 51%

Here's the rest of what I know.
I couldn't get more that 1 second of employment out of a 7.5amp fuse. It did last longer than a 20 without the main cable attached though. A 10amp fuse was happy in it's work.
As long as I've already wired in a 40amp relay and 12ga wire, I'm going to use a 20amp fuse.

It took some web searches but I found a reference for the solenoid current to be 30 amps. I measured my my Skytech starter winding resistance to be 0.3 ohm so 30amp is about correct considering the battery voltage to be about 11volts during cranking. I used a remote solenoid on the firewall so I did not need to have this high current at the panel and I could use a lower current rated start push button. Using an under rated switch could result in the contacts welding.

I used an oscilloscope and a current shunt to see exactly what's happening on the start button circuit.

Looks like the steady state current is close to 8.6A after the initial ~30A transient. Without the heavy gauge battery cable directly attached to the battery, it is close to 35A. Funny enough, it turns out I had a 10A fuse on that circuit that I forgot about and ended up blowing it after about ~0.15 seconds, just like Area 51%'s experience.

Until this discussion, I would not have expected there would be that much current with the battery disconnected. It's also interesting that the puny 10A fuse I've been using has just barely been enough all this time. The initial 30A transient is apparently too brief to blow it.

Awesome data Chris! Question, if the battery is not attached, what is the power source?

I didn't word it very clearly, the battery wasn't completely disconnected. The starter has a heavy gauge terminal that runs directly to the battery and provides the current to actually spin the start motor. I disconnected this one where it connects to the starter for the "without battery" part of the test. There's also the blade terminal that goes through a fuse, the start button, and back to the battery. I inserted the shunt resistor on this circuit and measured the voltage drop across it to calculate current. This was always connected until the fuse blew :-)

on the post dated / timed ""gammaxy » Sat Mar 07, 2020 9:59 pm""".... nice (( currant / time graph))
......and the specs on a fuse ""a 10amp fuse"" should hold 10 amp for 1 to 5 sec. and then blow
........most bulk parts suppliers manual's have the specs listed .....please look it up ,in case , my gray brain cells ,are miss firing on the hold time ,it my be 1 to 10 sec hold rating....

& with fuse blow time of .15 sec...i am thinking you have something of large currant drain........
...good luck / have fun /good day / rick

Good stuff Chris.
It's nice knowing; If mine isn't right, at least I'm not alone.

Funny thing....SkyTec's answer to my required wire size and fuse rating query last week was "you need to check with Aerovee".

rick9mjn wrote:the specs on a fuse ""a 10amp fuse"" should hold 10 amp for 1 to 5 sec. and then blow
........most bulk parts suppliers manual's have the specs listed .....please look it up ,in case , my gray brain cells ,are miss firing on the hold time ,it my be 1 to 10 sec hold rating....
& with fuse blow time of .15 sec...i am thinking you have something of large currant drain........

I found this chart in the datasheet for the Littelfuse 32V 10A fuse I was using. It confirms that 0.15sec is about right for the 34A I saw. The datasheet also shows that it is likely to last indefinitely at 10% over its rated current. Even at 35% over, it could take as long as 10 minutes to blow.