Solid state master relay on an experimental

So, the title pretty much says it, Is there a compelling reason that you cannot use a solid state master relay on an experimental aircraft? It would seem that Vertical Power uses something like this in the magic boxes. I was irritated to learn that the master relay that I have draws 1.8A to simply close the relay. Solid state relays use a miniscule amount of current, typically less than 20mA. I am interested in others opinions on using an SSR like this one:
https://www.amazon.com/gp/product/B0B11YM8RH
(I should mention that my wiring layout does not have the starter current running through the master relay, so that is not a concern.)

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Originally Posted by pogcarr

So, the title pretty much says it, Is there a compelling reason that you cannot use a solid state master relay on an experimental aircraft? It would seem that Vertical Power uses something like this in the magic boxes. I was irritated to learn that the master relay that I have draws 1.8A to simply close the relay. Solid state relays use a miniscule amount of current, typically less than 20mA. I am interested in others opinions on using an SSR like this one:
https://www.amazon.com/gp/product/B0B11YM8RH
(I should mention that my wiring layout does not have the starter current running through the master relay, so that is not a concern.)

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I don't think it'd be a problem....when I bought my Fly Baby, it used an ordinary DC rocker switch as the "master"...no relay.

However, there were three downsides, one of which doesn't affect you, but the others might.

The first is that, as a cockpit switch, it meant there was the long stretch of wire that carried the full electrical load to the cockpit. I was happy to replace that switch with a conventional master relay near the firewall.

That one doesn't affect you, of course. However, my system also didn't run the starter current through the switch (like you propose). This meant that the starter switch was live *all the time*...just needed a tug on a handle to start the prop turning, whether or not the master was on.

Since the starter switch itself was located atop the starter (C85-12) that meant that there was live power in the engine compartment unless the battery was disconnected. I had a rubber boot on the terminal, of course. But one time I was had a long piece of safety wire I was trying to lead into an awkward hole when the end of the wire swung UNDERNEATH the boot and shorted it out. I grabbed the wire and jerked it away as it self-welded in place....but it did kind of sear my finger.
Attachment 9624

Now, far-fetched failure mode, true. However, the other drawback here is that the starter handle in the cockpit was ALWAYS LIVE. With the master off, mags, off, tugging on the handle would always get the prop turning. This didn't affect me that much (since I knew about it) but always worried about the A&Ps doing the yearly condition inspection.

I also tend to plop young kids into the cockpit to let their parents shoot a picture, and some of the little brats....uhhh, "charmers" treated the panel like a Busy Box....turning every dial, pressing every button, etc. I had previously re-positioned the shiny silver T-handle out of sight, but did worry about one of them finding it.

Finally, the other drawback is that there's no way to totally kill the power in an emergency. If I smell smoke, I want ALL the power removed...not just the juice to the radios. That big 'ol 4-gauge cable leading from the battery through the firewall to the back of the engine with no way to interrupt it did prey on my mind a bit.

Rebuilt my electrics ~15 years ago, adding a conventional master solenoid that totally cuts off power.

I don't see your proposal as being especially dangerous. After all, my plane was built that way (by someone else) and didn't burn away in the 25 years it took before I rewired it. Just keep it in mind.

Ron Wanttaja

I am using a battery disconnect switch, mechanically actuated through the firewall with a torque tube. Overall it was a little lighter than a normal solenoid/master combination and it uses zero amps. The Sonex/AeroVee only has 20A available so an amp (or more!) is a killer.

I am not sure if you can see the pictures without being a forum member but here is the thread with my solution:
https://www.sonexbuilders.net/viewto...=2009&start=20

Edit: I opened in incognito mode and can still see the pictures. There is one missing picture that was on postimg but the rest are there.

Quote:

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Originally Posted by pogcarr

So, the title pretty much says it, Is there a compelling reason that you cannot use a solid state master relay on an experimental aircraft? It would seem that Vertical Power uses something like this in the magic boxes. I was irritated to learn that the master relay that I have draws 1.8A to simply close the relay. Solid state relays use a miniscule amount of current, typically less than 20mA. I am interested in others opinions on using an SSR like this one:
https://www.amazon.com/gp/product/B0B11YM8RH
(I should mention that my wiring layout does not have the starter current running through the master relay, so that is not a concern.)

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I would have two concerns with that part - 2ma leakage current, plus the poor reviews, and the third of two concerns would be that everything on Amazon seems to be from China. If I was going to hunt for an alternative relay, solid state or other, I'd at least look at quality electronics suppliers like Newark, Digikey, or so on.

I'd want to know and consider the possible failure modes. A reputable electronics house should have that information. Chinesium on Amazon, not so likely.

Sadly, the parts from Newark and Digikey are also very often from China...

CAP checked me out in the Cessna O-1 Bird Dog, waaaaay back when. I was always impressed by its power management. Instead of a big relay, they had a big, insulated bus bar that fit at the very base of the panel. When you took the bar out, it physically disconnected the battery, and all you had to do what turn a big knob.

I don't see it in photos of modern, restored Bird Dogs.

I actually found a similar component at Bayside Shipping about 15 years ago. Almost put it in my Fly Baby, but I was a tad worried about its antecedents......

Ron Wanttaja

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Originally Posted by pogcarr

Sadly, the parts from Newark and Digikey are also very often from China...

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That is correct; a lot of production for reputable American, Japanese, Korean, and European companies gets done in China. The difference is often the design and the QA. I've also seen a lot of no-name Chinese stuff with specs that FAR exceed the actual limits of the device. I would not use a no-name Amazon part for something like that, but I would use something from Credo. One I'd trust in my airplane... the other I'd trust to hold down a stack of papers, as long as the breeze wasn't too strong. OK, maybe that's a bit unfair; I'd maybe be OK with using it to control 8 or 10 amps of stuff that doesn't HAVE to work all the time or someone has a really, really bad day. Who knows? It might be OK, but I have seen some really abysmally horrible parts come from sources with names that look more like a random letter selection. than an actual reputable company.

"Chinese made" doesn't necessarily mean "garbage". There are Chinese manufacturers that produce top quality stuff. There is also a lot of absolute crap coming from there, stuff that's not worth the postage you pay to get it to your doorstep. Most of it looks like it would work... but a lot of it simply doesn't.

There's a very good reason you'll pay 10x that cost for a real SSR with those ratings.

I'll add another voice to the chorus advising against using any SSR sourced from Amazon, eBay, AliExpress, Temu, etc. They're well known -- as are a great many electronic components from China -- for being wildly over-rated. There is zero chance that a $14 SSR from anywhere can carry 80 amps.

For installation in an aircraft, buy brand name parts from distributors who have a reputation to protect. Here's a search at Digi-Key for 80A+ DC-output SSRs. As you'll see from the prices, parts that can actually carry their name plate rated current are not​ cheap (TLDR: the cheapest one is $120).

If high coil current is a concern, there are automotive relays available with very high current ratings that have much lower coil currents than whisky-barrel contactors. Here's a $3.83 80A automotive relay that draws 150mA coil current and even has an integral back-EMF diode across the coil (put +12V on pin 86 and switch pin 85 to ground). Here's the search that produced that part.

Interesting thread. I used a normal solenoid for the master switch but I also added another 70A relay for the second bus in my Hatz. The second bus is because of the electronic ignition (Rotec R3600). Plus all the avionics will accept input from two buses.

Attachment 9627

What peak my interest was the idea of the starter connected directly to the battery rather than going through the master solenoid. I suspect that most automobiles have the battery connected directly to the starter. Could be wrong.

So what is the benefit? For one the master solenoid doesn't have to support the 100's of amps required by the starter. I could get away with a couple relays, one for each bus. What could go wrong? As Ron mentioned the starter is live. But that would only be if it was manual pull type (like my Luscombe). Normally with the master off there is no power to activate the starter solenoid. With the keys out, the kids can't inadvertently active the starter.

I think if I had seen this thread sooner I would have skipped the master solenoid and went with a couple of relays. The 70A relay is probably overkill as I believe the full draw will be less than 15A with everything ON.

As far as products from China, my build has plenty of parts from Ali-Express. When I compare them with the parts I get from A/S there is no visual difference and I use them interchangeably. I also use auto parts if they fit the need. Have a look at the Chrysler alternator in your Piper aircraft.

I did go with a master solenoid from A/S. I couldn't find one online with the same specs. That was a tough purchase dollar wise when I could have gotten away with a $5 relay.

If the battery is behind the cockpit as it was in my Hatz, running everything from a master solenoid near the battery (mine was next to my seat) makes sense as it allows you to isolate that long heavy cable running from the battery to the starter solenoid. If the battery is up front, I think it'd be better to have the starter circuit not run through the master solenoid. In that case you'd want the starter solenoid coil fed by the master so you can't crank the engine with the master off.

My original design was to have the battery behind the pilot. It was too close to the magnetometer so I moved it up to the firewall. I have a bunch of expensive 4 AWD cable left over from that change.

I guess the original question on this thread was about solid state relays. I have some solid state relays that I bought off Amazon. I used a pair for the pitch trim. These are only good for 10A but use an optocoupler for switching so they are faster then mechanical relays. They also are smaller and lighter than regular Bosch type relays. Plus no moving parts.

Attachment 9629

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Originally Posted by mmarien

I have some solid state relays that I bought off Amazon.

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This makes my point. Those "solid state relays" from Amazon aren't SSRs at all; they're standard electro-mechanical relays. I'm sure they work fine for the application, but whoever put them in that two-relay module lied about it on Amazon.

Here's the manufacturer's website listing for the relays, and here's the datasheet.

Ah. So I didn't get what I ordered at all. This is the Amazon page:

https://www.amazon.ca/dp/B085N49S79?...title_0_0&th=1

No, I think you did; the listing doesn't mention SSRs.

The optocouplers on the board's input are there to provide electrical isolation between the control signals and the switched circuits. That's only important if you use the board to switch high voltage, but it doesn't hurt anything in a low voltage application. It's kind of a belt-and-suspenders design, as the relays provide 1,500V of isolation by themselves.

The relay switching speed isn't affected, but the relays close in 10mS and open in 5mS -- significantly faster than human perception -- so it doesn't really matter.

Looks like a good choice despite the confusion, and $45 cheaper than a similar board at Aircraft Spruce.

Sonex is using a SSR in their high wing. It's in the story here:
https://www.sonexaircraft.com/highwing-update-040725/

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The Sonex team is also experimenting with the use of a solid-state master relay, saving weight, eliminating moving parts, and reducing relay power consumption to a maximum of 13mA (0.013 Amps). The solid-state relay is performing great so-far and we’ll continue to monitor its performance as we begin engine runs and start flying.

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I would not be surprised if they offer it for sale on the Sonex Web store if it works out.