Ground power polarity protection...

[rmartingt]

I'm putting in a ground power connection for running avionics etc. but not for jump-starting or charging. I'd like to use a simple "SAE" DC connector but there's no hard standard for polarity, so I'd like to put some form of polarity protection in place.

I've thought of two different ways of accomplishing this. The first would use a relay with a diode in the circuit that operates the coil, and wouldn't allow the relay to close unless the polarity is correct.

The second way is to use a rectifier diode, accepting the voltage drop in favor of simpler installation.

Theoretically, I could even set both of them up to operate regardless of the polarity, which could be useful.

Any thoughts from the peanut gallery?

 

[PilotjohnS]

Diode bridge

I used a diode bridge from digikey on the instrument panel. No matter which polarity ground power comes in, the plane receives correct polarity.

However, most diode bridges drop 1.5 v, so the shore power voltage must be set up at 14.5 v.

To solve this, i made a similar bridge, but used 12v relays in place of the diodes. Works great. YMMV

 

[mburch]

If you're using an SAE connector, I would follow the conventions used by battery tender devices: positive to protected terminal, ground to exposed terminal. (reference here)

 

[wirejock]

SAE

If you're using an SAE connector, I would follow the conventions used by battery tender devices: positive to protected terminal, ground to exposed terminal. (reference here)

That's how mine is set up. The ACM charger came with the pigtail. I bought another from an auto parts store for my DC Power supply. When I'm working the power supply is connected. Afterwards the charger goes back on. So far, it works well.

 

[Mich48041]

Here is a LINK to Bob Nuckolls' Z-31 ground power diagram.
Another option is to just use a polarized cord attached to the aircraft power bus.
No matter how careful one is, mistakes happen and polarity could be reversed.

 

[Desert Rat]

So I know you said that you weren't going to use this for battery charging, but what's wrong with just having a pigtail off the battery and a power supply hooked up through the oil filled door for when you want to do database updates or whatever?

I'm asking because thats the way I have mine set up and I'm wondering if I'm missing something. It seems like a dirt simple solution that doesn't require any fancy wiring or additional stuff other than the pigtail that came with the purpose built battery tender/power supply.

 

[Mike S]

So I know you said that you weren't going to use this for battery charging, but what's wrong with just having a pigtail off the battery and a power supply hooked up through the oil filled door for when you want to do database updates or whatever?

I'm asking because thats the way I have mine set up and I'm wondering if I'm missing something. It seems like a dirt simple solution that doesn't require any fancy wiring or additional stuff other than the pigtail that came with the purpose built battery tender/power supply.

Yep, done a couple planes this way.

 

[Foghorn]

So I know you said that you weren't going to use this for battery charging, but what's wrong with just having a pigtail off the battery and a power supply hooked up through the oil filled door for when you want to do database updates or whatever?

I'm asking because thats the way I have mine set up and I'm wondering if I'm missing something. It seems like a dirt simple solution that doesn't require any fancy wiring or additional stuff other than the pigtail that came with the purpose built battery tender/power supply.

Mine is this way. 200 trouble free hours. Simple is the best route for me.

 

[12vaitor]

I went with Anderson Power Products SB50 connectors (50A). They are polarized so you cannot reverse them. I mounted mine under the panel with an external power contactor wired in to activate power to the main buss. As my airplane is all electric, the secondary use is to connect a EarthX Jump Pack in-flight if the alternator and two main batteries die.

John Salak
RV-12 N896HS

 

[rmartingt]

If you're using an SAE connector, I would follow the conventions used by battery tender devices: positive to protected terminal, ground to exposed terminal. (reference here)

That's what I've used in the aircraft, but the possibility remains to accidentally hook something up backwards. A few bucks for a diode or relay(s) seems like good insurance...

So I know you said that you weren't going to use this for battery charging, but what's wrong with just having a pigtail off the battery and a power supply hooked up through the oil filled door for when you want to do database updates or whatever?

Honestly, I've run out of space over there. I'm electrically-dependent and running a dual-bus system; just about the entire right hand side of my firewall is dedicated to electrical already (contactors, batteries, relays, Hall sensors, etc). I had originally put in some battery charger wires and used them for power-on checks with dummy batteries a while back, but I could never figure out a good way to mount/secure them, especially with all the extra engine mount tubes on a taildragger (you nosedraggers have lots more room on the sides). It was just ugly.

Instead I plan to run (protected) power directly to the buses, and I have a 30 amp DC power supply I can use to power everything except the engine. The cord would just tuck into a pocket on the left sidewall. If I really have to charge on the ground, I'll pull the cowl and connect directly with alligator clips (it's a good chance to look under there anyway).

 

[Desert Rat]

Amazing how crowded it can get back there isn't it?

Not trying to persuade you one way or the other, and maybe all that electrically dependent stuff makes the difference, but I've got a tail dragger as well. Here was my solution. The pigtail came with my earthX power supply. It has ring terminals off the battery and an inline fuse built in. As long as I use the power supply that it goes with, it's pretty much impossible to hook it up backward because the plug only fits one way.

I just wrapped the plug with fusion tape to give it some grip and mounted it to an upper engine mount tube with an adel clamp. It's pretty accessible for one handed operation though the oil filler door. It's the thing with the green dust cap in the picture below.

in my case I just decided I really didn't need yet another set of wires coming into the cabin and it gives me the opportunity to switch the power supply over to maintenance mode and use it as a battery tender without having to leave the canopy open.

You way seems like it will work fine if that that's more convenient for your setup.