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Voltage: 12, 14.4, or something in between?

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alpinelakespilot2000

Well Known Member
One of those dumb questions...
I'm trying to figure out what size resistors to use for some nav lights I'm putting together and, to figure that out, I need to know what kind of voltage I should be able to count on. Surprisingly (to me) it makes quite a bit of difference in resistor size depending on the voltage. We regularly talk about a 12V system, but a lot of times I see 14.4 on diagrams. I think my Odyssey battery is 12V and my Plane Power alternator should put out close to 14.4V, correct? Given a healthy (plane power) alternator and healthy battery, what would be a reasonable expectation for voltage at the main buss?
 
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how much current (wattage of the bulb), how long is the wire run and what size wire?
Curious why you are trying to drop the voltage?
 
Thanks Alton. I'm not talking about voltage drop in the wires to the nav lights. I need to use actual resistors for my LED lights because you need some form of current limiting to LED's, otherwise they will quickly burn out. To pick resistor size, however, you need to know how much voltage your power supply is providing. Thus, my question, how much voltage should normally be at my main buss?
 
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alpinelakespilot2000 said:
One of those dumb questions...
I'm trying to figure out what size resistors to use for some nav lights I'm putting together and, to figure that out, I need to know what kind of voltage I should be able to count on. Surprisingly (to me) it makes quite a bit of difference in resistor size depending on the voltage. We regularly talk about a 12V system, but a lot of times I see 14.4 on diagrams. I think my Odyssey battery is 12V and my Plane Power alternator should put out close to 14.4V, correct? Given a healthy (plane power) alternator and healthy battery, what would be a reasonable expectation for voltage at the main buss?
Click to expand...

I have a Plane Power alternator, and it does put out 14.2 volts at the buss.

The alternator determines the useful voltage during operations with the motor running. 12 volt is just a name of a system. If you read 12 volts on your buss, that means that your battery is less than 50% charged. A fully charged 12 volt battery will read 12.8 volts. But in an operating system, the alternator will produce a constant (if it is a good system) 13.8 to 14.2 volts. This is what your nav light will see.
 
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Remember those numbers. I had an alternator wire break in flight. After a period of static in the headset, I noticed that the amps were negative. Push a button to check voltage and it was in the 12s which meant that I was not getting alternator output. The value should have been low 14s volts.
 
Here's a blurb from our install manual:

Today?s experimental aircraft are powered by either 14 volt or 28 volts systems. Often you may hear 12 volt or 24 volt systems. Why the difference? The reason is because the batteries are rated at either 12 or 24 volts. When the engine is running and the alternator is turned on, the alternator generates 14 volts or 28 volts, slightly higher than the battery voltage so it will keep the battery charged.
 
voltage

Happily most modern EFIS's will alarm for voltage below a set threshold, say 13.5 volts or so, allowing you to know immediately if the alternator is not putting out and that you're depending on the battery. This is great as it lets you take steps to conserve battery power immediately and plan the remainder of your flight better. Over voltage can be alarmed as well. Modern avionics are truly useful!
 
How about a current source?

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LED devices are controlled by current not voltage. Using an LM317 and a resistor you can set the current and the LED intensity will remain constant regardless of voltage. I.E. Depending on the LED type I could string several LEDs in series and use a single LM317 to set the proper current. With a variable resistor I can even change the level. Using a 100 Ohm resistor you would supply 12ma of current to the LEDs independent of the 12-14v variations between batt and Alt.
And to boot, you could series the LEDs together to light larger areas. Just don't put all the LEDs on one series chain as if one fails they all go out!




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