13.6 Power Supply

This entry is part 43 of 66 in the series 13 - Electrical / Instruments

I went with a 24v electrical system. There are a couple of reasons behind this.

1) All new production aircraft being built today are 24v. 2) A 24v system is more efficient, uses lower current and requires a smaller diameter wire.
3) The engine came with a 24v starter and alternator.

Maybe not the best reasons, but they’re mine and that’s the way it is.

Once I started doing the electrical system, I learned of a drawback to 24v systems; In the experimental aircraft market, many of the suppliers are either 12v only or charge extra for 24v.  So for things like the EFIS screens, I had to pay a little more to get the 24v version.

But the area that really bit me was in batteries. The 12v guys have tons of small, lightwieght batteries to choose from. 24v batteries are pretty much unchanged. Frustrating, but I can live with that. Where it really stings is in battery backups. The EFIS supplier (Grand Rapids Avionics) have a backup battery that will keep the EFIS and AHRS screens alive for at least an hour if there’s an electrical system failure.  But it’s only available in a 12v version. That’s not a huge problem as I’ve got two alternators.

However, the EFIS screens also show engine information. And that’s nice to see when you start the engine. But when you start the engine, the battery voltage can drop low enough (under 20v) that the EFIS reboots.  So how to keep the primary screen glowing while starting the engine?  I looked at getting a couple of small 12v scooter batteries and wiring them in series. But then I would have to find a place for them. Possible, but that would add more work and I’m not looking for new work.

I thought about building a “de-slumpifier” from some plans I got from Eric Jones and Perihelion Design. Basically, it’s  a bunch of capacitors that discharge when the voltage drops to maintain 24v for a short period of time. But remember, I’m not looking for new things to do.

So then I got an email from Tim at TCW Tech. He provided my trim controllers. They also have an Intelligent Power Stabilizer (IPS). This is like the “de-slumpifier” in that it will continue to provide 24v if the input power drops down, but the IPS will continue to put out 24v all continuously as long as the input voltage is above 10v. Now it can only supply about 5amps but since the EFIS only draws about 1amp, that’s fine.

The next problem was where to put it.  My Avionics Sub-Shelf wouldn’t have enough room. So I have to expand it.

Here’s the original sub-shelf which sits over the VPX. The TCW Tech Trim Controller is on top. The empty area behind the trim controller is for the GPS.

2014-05-10 IMG_20140510_100804506 (Large)

And here’s the expanded sub-shelf with a penthouse suite. The IPS is on the lower shelf where the GPS was going to go. Now the GPS will go on the upper shelf.

2014-07-16 IMG_20140716_111956488 (Large)

Then I just added a pair of fuses to the fuse panel for the output to the primary EFIS and AHRS.

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