Geek's STi Build

[Geek]

Hi Gary,
I'm not up to where you are yet. Did the diodes for the solenoids come with the kit or are those after-market. If so, can you give a link?

Thanks,
Greg

Any of the 1N54xx series, in a DO-27/DO-201AD case size will work fine. Big enough to be physically robust and not fiddly to install.

https://www.digikey.com/short/23wqchrd

https://www.amazon.com/dp/B07PCQD8YY/

Greg - Diodes did not come with the kit and I think I read somewhere that Kitfox had agreed about using diodes on the master relay but not on the starter relay (if using the Rotax variant). Not sure why as I have always used them on everything I have ever had cause to use a relay on. Here's a link to a video of a Rotax relay with and without a diode. https://www.youtube.com/watch?v=K3_bQV3HOH4 In the case of the 912is, you just need to be sure that the ground you use for the diode install is the one isolated from the airframe ground. What Eric said is spot on. I used the 1N5401. 100V rated and as Eric points out "physically robust". Some of the smaller diodes have thinner leads which makes me think they will break over time with vibration.

Gary

[LetMeFly]

Greg - Diodes did not come with the kit and I think I read somewhere that Kitfox had agreed about using diodes on the master relay but not on the starter relay (if using the Rotax variant). Not sure why as I have always used them on everything I have ever had cause to use a relay on. Here's a link to a video of a Rotax relay with and without a diode. https://www.youtube.com/watch?v=K3_bQV3HOH4 In the case of the 912is, you just need to be sure that the ground you use for the diode install is the one isolated from the airframe ground. What Eric said is spot on. I used the 1N5401. 100V rated and as Eric points out "physically robust". Some of the smaller diodes have thinner leads which makes me think they will break over time with vibration.

Gary

Thanks for the further info, Gary!

[Eric Page]

Here's a link to a video of a Rotax relay with and without a diode. https://www.youtube.com/watch?v=K3_bQV3HOH4

The spark you see without the diode is what's happening inside the switch that controls the starter relay. Over time it will erode the contacts and shorten the life of the switch. This is why the diode is a good idea.

In the case of the 912is, you just need to be sure that the ground you use for the diode install is the one isolated from the airframe ground.

No harm in doing it that way, and it's probably easiest given that it's mounted on the engine anyway, but I don't think it makes any difference. When you start an iS engine, you first close the Start Power switch, which ties the EMS and airframe grounds together (the Battery Backup switch does the same thing).

Edited to add... Took a minute to find this Homebuilt Help video on the subject: https://www.youtube.com/watch?v=bvDuyM2e4gw

Edited again to add... Excellent article by Bob Nuckolls, complete with oscilloscope traces showing what happens with and without a diode: http://www.aeroelectric.com/articles/spikecatcher.pdf

[Delta Whisky]

Eric, I need edjumicated. Why do they want the diode install isolated from engine ground?

[Eric Page]

Yeah, I do too. Given how the Start Power switch works, it doesn't make much sense to me. Is that direction in the Rotax installation manual or is it advice from Kitfox?

[Geek]

Eric, I need edjumicated. Why do they want the diode install isolated from engine ground?

Yeah, I do too. Given how the Start Power switch works, it doesn't make much sense to me. Is that direction in the Rotax installation manual or is it advice from Kitfox?

In my case "they" is neither Kitfox or Rotax saying this. Just me looking at how I think the electrical system in the 'IS' versions of the Rotax are working. The 912is and the 915is have that 'system' (for lack of a better word) where the Lane A grounds are isolated from the airframe grounds. (Lane B is grounded to the airframe and A is not) If you look at the pic of the starter relay you can see an isolator behind the relay that totally insulates it from the firewall and that insulator is on both sides of the firewall. The engine electrical harness provides its own 'ground' connection to the relay from the Lane A side. I metered between that connection and the isolated 'grounds' on Lane A to verify that is was indeed connected there. I know that when the Start Power switch engages, it puts the airframe ground to the Lane A side which would put an airframe ground on the base of the relay as well. But when the switch is released, it goes back to an isolated system. Figured the safe bet was that if Rotax isolates the relay from the airframe, I'll connect the 'ground' side of the diode install to the insulated ground as well so that it's always at the same potential as the rest of the Lane A side. Sorry for any confusion I may have caused here.

Gary

Relay Diodes.JPG

[Eric Page]

No need to apologize! It's great to talk through these systems so all of us operating or planning to operate a 912/915iS correctly understand how they work.

Your explanation above sounds 100% correct to me. Thinking about it a bit more, I think you're right that the diode should be connected to Lane A ground. Under normal circumstances, the only time the diode will ever be called upon to do its job will be when either the Start Power or Battery Backup switch is closed (Lane A and Lane B grounds tied together). For a normal start, Start Power isn't released until well after Engine Start is released, so the diode's ground reference makes no difference.

However, if you accidentally release Start Power while Engine Start is still closed, the starter contactor will drop out because its coil will lose its ground with respect to the battery. Or, perhaps it would be more accurate to say that the Fuse Box, and the relay inside it that controls the starter contactor, will lose its ground with respect to the battery. In that case, the diode will do nothing if it's connected to Lane B ground, since it will no longer have any connection to the Lane A circuit that powered the contactor coil in the first place; it might as well not be there at all.

So, in order for the diode to protect the Fuse Box relay contacts under all conditions -- both normal starts and fumble-fingered pilots -- it must be connected to Lane A ground. Thoughts?

[jiott]

Van's Aircraft has some nice diodes for both the master and start solenoids. They come with end connectors all installed and shrink tubed, ready to install. I used them on mine.

[Geek]

No need to apologize! It's great to talk through these systems so all of us operating or planning to operate a 912/915iS correctly understand how they work.

Your explanation above sounds 100% correct to me. Thinking about it a bit more, I think you're right that the diode should be connected to Lane A ground. Under normal circumstances, the only time the diode will ever be called upon to do its job will be when either the Start Power or Battery Backup switch is closed (Lane A and Lane B grounds tied together). For a normal start, Start Power isn't released until well after Engine Start is released, so the diode's ground reference makes no difference.

However, if you accidentally release Start Power while Engine Start is still closed, the starter contactor will drop out because its coil will lose its ground with respect to the battery. Or, perhaps it would be more accurate to say that the Fuse Box, and the relay inside it that controls the starter contactor, will lose its ground with respect to the battery. In that case, the diode will do nothing if it's connected to Lane B ground, since it will no longer have any connection to the Lane A circuit that powered the contactor coil in the first place; it might as well not be there at all.

So, in order for the diode to protect the Fuse Box relay contacts under all conditions -- both normal starts and fumble-fingered pilots -- it must be connected to Lane A ground. Thoughts?

You're correct in all the above and past (probably future) comments about the schematics that Rotax provides us are a bit lacking in areas so it's sometimes hard to capture the operational dynamics of a circuit. Full disclosure, I had not thought it out to the extent that you did above and both your's and Darrel's comments made me go back to look closer to see if I was providing bad gouge. My initial wiring hook up was me simply sticking with what an old school technician taught me many moons ago about doing mods to aircraft with a lot more onboard systems than our planes and keeping potentials the same. If two circuits were working at two different potentials/references, they're not broken, don't try to make them work together. The apology was more for any confusion my comment caused to anyone who may not have the electrical/electronic foundations that I sometimes take for granted. (Doesn't everyone know this stuff?) Glad you all spoke up and provided a clearer reason for the means of doing this one.

Cheers

Gary

[Pushboy]

I try to follow along on build threads to learn -- I take delivery of my kit in a couple weeks, but when I read posts like the last several... I gotta be honest -- I think OMG... what have I done???

Maybe I'll go complete old school and have no electrical system at all.