[npowell]

I would be interested in learning more about the pros/cons of going either 14v or 28v with my system. I know one advantage iof 28 volts is you have less amps in a typical circuit, therefore smaller and lighter wires needed in general. But, you need two batteries, which adds space.

Any thoughts?

 

[gmcjetpilot]

Good Question

I would be interested in learning more about the pros/cons of going either 14v or 28v with my system. I know one advantage iof 28 volts is you have less amps in a typical circuit, therefore smaller and lighter wires needed in general. But, you need two batteries, which adds space.

Any thoughts?

Great question; there are some threads in the archive. I looked one up for you:
http://www.vansairforce.com/community/showthread.php?t=9959&highlight=volt

Keep in mind 12v components are more plentiful and often cheaper to purchase. For example 12 volt AGM batteries are cheap and avaiable. 28 volts not so cheap or as avaiable.

On large jets they use 115 volts AC and 48 volts DC. However on a jet wire runs are much longer. Using smaller gauge wire saves weight. We have a little weight savings to gain, if any. By all means research it and learn as much as you can. After all that is what building is about.

 

[asav8tor]

The road less traveled

Sometimes you will find 28v stuff on ebay or barnstormers for less than the 14 volt items since most of the homebuilders bid up the 14v stuff.

2 small batteries in series could give you the ability to have a 28v DC bus and a 14v DC bus, it would open up many possibilities. There really isn't enough long high current runs to make the case for weight savings by using smaller wire. A great project of an EE major though.

 

[gmcjetpilot]

Bottom line 12 volt is technically adequate.

Sometimes you will find 28v stuff on ebay or barnstormers for less than the 14 volt items since most of the homebuilders bid up the 14v stuff.

2 small batteries in series could give you the ability to have a 28v DC bus and a 14v DC bus, it would open up many possibilities. There really isn't enough long high current runs to make the case for weight savings by using smaller wire. A great project of an EE major though.

Both are true statements but did not want to muddy the water.

Cheap 28 volt stuff. Well take a KING KY196 24 volt/16 watt transceiver, yep its cheaper used than a similar 12 volt version. However those 24 volt units have 10-16 watt transmitting output. Although having more a powerful radio transmitter sounds great, they also draw big current, despite being 24 volts. If you are trying to get by with a little 40 amp alternator a 5 watt transmitter (2.6 amp draw) fine. Unless you plan on being at FL350, you don't need big boy transmitters which draw 6-10 amps! OK you got me, one cheaper radio. Strange enough 28 volt Bendix/King stuff cost 3 times as much new. I'll guess it will cost more to repair 28 volt stuff.

I can't make an argument about avionics since most avionics now is dual volts or works from 10-30 volts but not all. There is still a lot of cool avionics that's 12 volt only. Flap motor? Yes you can always cut down 24 volt to 12 volts with a power supply, but that is more cost & weight. Most guys I know who go 24 volt, end up with 12 volt components somewhere. Some times a drop down resistor will work, sometimes you need a drop down power supply.

Two batteries in series would do it. There goes any glimmer of weight or space savings.

No conclusion but to each his own. Until the car, truck, motorcycle & recreational vehicles go to 24 volt, the economy of volume goes to 12 volt.

Bottom line 12 volt is technically adequate.

 

[N395V]

For what I thought were many good reasons I built my plane with a 28V system. Huge mistake.

First off the wire savings were less than 2 pounds, more than offset by several more pounds in battery weight.

Secondly there were some things it was just too difficult to find that are 28 volt. So now I have a 28 volt bus and a 14 volt bus.

Initially I thought of (2) 14v batterys in series tapping off the center for 14V. This is problematic however if you do not exactly balance the load on both batteries. This leads to a situation of one battery always discharging into the other and premature failure of both. So I got one big 28v battery, more weight, more cost. Then I had to add a 14 v equalizer(converter) equals more cost more weight. Then you have to switch, fuse, and distribute 2 different voltages adding complexity.

The 28V battery doesn't last longer when the alternator quits as equipment consumes Volts-x-Amps which is a constant for either system. In fact depending on how the units drop to operating voltage internally those items that can be run on 10-35 volts may consume power more quickly on a 28V system converting it to detrimental heat.

About the only benefit I get from 28V is it spins the starter quicker and I can turn on my efis before I engage the starter and it wont "brown out".

The only other benefit is my wiring is color coded as are the busses and they really look cool when you take everything apart.

Go with 14V.

 

[Norman CYYJ]

Another advantage of the 14volt system over the 28volt system is if you are caught out in the boonies with a dead battery esp. in the winter you will not be able to get a boost to get the plane going. Not too many trucks or cars operating on 28 volts.

 

[Kyle Boatright]

14V is the way to go for most people. A 28V system is technically superior, but from a maintainability standpoint (think alternator or battery when you're out in the boonies) and a cost standpoint (all that automotive stuff works on 14v), 14v is the right choice.

It reminds me of the old Beta vs VHS argument. Beta was a superior product, but VHS captured an overwhelming market share, and BETA died because it was virtually impossible to find Beta tapes for rent.

 

[npowell]

Thanks for the great responses!

I appreciate the thoughtful responses and good ideas. The manufacuturer of my EFIS recommended 28 volts, but given all the good reasons above, I think I will stick to 12 volts.
Thanks again for responding.

 

[Auburntsts]

Which EFIS manufacturer recommended 28v and did they provide any rationale (other than what's been mentioned here)? I don't have a dog in the fight for EFIS customers, just curious since it seems most avionics manufacturers don't seem to advocate a particular voltage other than the range (say 10-30 VDC) so I'm a little surpised by them coming out with a preference.

 

[w1curtis]

Which EFIS manufacturer recommended 28v and did they provide any rationale (other than what's been mentioned here)? I don't have a dog in the fight for EFIS customers, just curious since it seems most avionics manufacturers don't seem to advocate a particular voltage other than the range (say 10-30 VDC) so I'm a little surpised by them coming out with a preference.

I think those of use who have seen this topic come up a number of times can guess that one. That would probably be Greg Richter of Blue Mountain fame. He should spend his time improving his product and not recommending system voltages. I recommend doing a search on Blue Mountain to see comments from Blue Mountain users.

 

[kevinh]

I think those of use who have seen this topic come up a number of times can guess that one. That would probably be Greg Richter of Blue Mountain fame. He should spend his time improving his product and not recommending system voltages. I recommend doing a search on Blue Mountain to see comments from Blue Mountain users.

That was the best analysis ever!

 

[Auburntsts]

I recommend doing a search on Blue Mountain to see comments from Blue Mountain users.

I was also guessing BMA based upon Greg's articles, but didn't want to speculate. Concur with with your assessment as well. I think they really missed a golden opportunity with their products. I like their form factor, but in the end the total package (the product and company) isn't for me.

 

[N395V]

BMA based upon Greg's articles

In the interest of full disclosure there is a BMA banner in the sponsor section of my website. I am also an unpaid beta tester for BMA.

I am not a BMA distributor nor paid employee I am just a satisfied customer and as such provide them with free advertising.


Greg wired his own plane with 28V. It was necessary to operate the jet engine he packed into his cozy. His personal preferance is for 14v for all the reasons enumerated above. I also seem to remember an article he published something like "wiring for dummies" or some similiar title that used to be available for download on the BMA website. In the article he cited all the benefits of 28v noted above but as pointed out few of these reasons are compelling in a small plane.

BMA, as a company policy, is not recommending 28V systems be used preferentially for their products. All of their products are setup for 14 or 28V.

Below is a quote from Greg in a 2003 post from the BMA discussion board as well as a link to the thread.

Posted by Greg Richter on 3/27/2003 from the BMA Archives

Howard is right on the money. 12 volts is common, and everything is available for it. 24 volts means lower currents, smaller wires and less heat, sometimes.

All of our equipment is designed with wide input range supplies, 9-32 volts specfied, 10-30 preferred. Since the front end supplies are switchers, there's no issue with either voltage.

The autopilots can run on either voltage, just like all our other hardware, although this is one case where 24 volts generates more heat than 12 -- there's a 7805 in the controller that provides logic power. It'll get warmer on 24 volts, but we did design for that. So, I'll stand by Howard's advice: 12v for ease of maintenance and parts availability, 24v if it suits you.

FWIW: I'm rewiring my plane and it's going all 24 volt because of the starter and engine electrics. I personally prefer 12.

As evidenced by his own post he acknowledges some of the pitfalls of 28V and personally prefers 14V.

Link to the entire thread http://bluemountainavionics.com/talk/showthread.php?t=47&highlight=volt

So contrary to hearsay it appears Greg is not pushing 28V systems to his customers.

So the above advice of doing a search is sound I would however suggest a thorough search from many sources giving more credence to info from sources with actual experience relative to the issue or product under discussion. Then base your decisions on your particular needs and the info you take away from in-depth research. Remember when you follow someones advice it is you and not the adviser that has to live with the consequences.

 

[N395V]

I like their form factor, but in the end the total package (the product and company) isn't for me.

Aspen Avionics has a similiar form factor coming out that will be a certified product at a cost similiar to exp. Really looks great in the brochures and depending on your timing needs might be worth considering.

http://www.aspenavionics.com/index.php/products/evolution/

 

[w1curtis]

So contrary to hearsay it appears Greg is not pushing 28V systems to his customers.

Milt,

Actually this was not from hearsay. Direct from his document "Aircraft Wiring for Dummies." The first sentence of the section on selection of aircraft voltage was: "Airplanes are 28 volts, automobiles are 14 volts, nuff said." Maybe he has stepped back from this since then but that was his direct quote. Suffice to say with a blanket statement like that, I did not read too much more.

 

[N395V]

Maybe he has stepped back from this since then but that was his direct quote.

Actually you are right and I stand corrected. The date on his publication is 2004 my quote from above was dated 2003.