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  #21  
Old 01-03-2010, 10:27 PM
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jjconstant jjconstant is offline
 
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Default great responses

Thanks guys! I think in the end I'm going with simple, cheap, light and easy: an on/off switch.

One of the deciding factors was that I don't have an endurance bus, just 2 batteries and 2 master solenoids, both or either powering the bus.

Jeremy
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  #22  
Old 01-05-2010, 11:40 AM
the_other_dougreeves the_other_dougreeves is offline
 
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Default

Quote:
Originally Posted by Ironflight View Post
I power the Display Heads and AHRS from the Aux Battery feed, and keep the Aux battery on from before start to after shutdown - no reason to turn it off, and i you have a main power problem in flight, you don't have to take a step to turn the Aux on. Simple is your friend!
Agreed. Using the "OR"ed power inputs and a standby battery allows for engine start with the EFIS/EIS on, automatic and uninterrupted feed from the aux battery if the main power feed goes out (for any reason) and requires no pilot input. No having to flip a switch and reboot the units.

Simple is good.

TODR
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  #23  
Old 01-07-2010, 12:03 PM
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carlrai carlrai is offline
 
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Default A little more detail?

Quote:
Originally Posted by Ironflight View Post
Mine comes off the battery bus with a diode and resistor (to limit current in case of a problem). Also have a circuit breaker/fuse in there for safety. It charges anytime the smaller battery is at a lowr potential than the main battery bus. I put in a new 5 Ah Aux battery about every two years.

Paul
Understand the need for a diode to protect the aux battery from discharging into the main battery when capacity has diminished. A bit rusty on current limiting though. Assuming a 5 - 7 AH gel cell, what value and how is the current limiting resistor wired in?

I also understand a question about our airplane wiring pales when you're getting ready for a launch. Whenever you have a moment - the input is more than appreciated.

Thanks
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  #24  
Old 01-07-2010, 02:09 PM
nucleus nucleus is offline
 
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Location: Bozeman, Montana
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Talking Thanks for the Input Guys

I think I am going to emulate Paul's set up, it will be inexpensive and do what I want -freedom to keep my EFII and AHARS "spun up" as well as a back-up system in case something goes haywire.

Hans
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  #25  
Old 01-07-2010, 08:07 PM
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Ironflight Ironflight is offline
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Default

Quote:
Originally Posted by carlrai View Post
Understand the need for a diode to protect the aux battery from discharging into the main battery when capacity has diminished. A bit rusty on current limiting though. Assuming a 5 - 7 AH gel cell, what value and how is the current limiting resistor wired in?
The current limiting resistor is in series with, and upstream of, the diode. It is simply to limit the current if, for instance, the Aux battery is drained very low. In this case, you have a large voltage difference between the main battery and the Aux, and the current can get pretty high, turning your diode into a NED (Noise Emitting Diode) or a SED (Smoke Emitting Diode) - both are "one-time-use" by the way...

My chicken scratchings say that I used a 50 Ohm resistor, 2 watt rating, and a 1/2 watt, 0.7 volt drop diode. I will caveat this with the usual notes - I am not a EE, I have friends who are, and they gave me this design. In addition, the Aux battery is there primarily to provide a stable voltage during start, not to give me a long-life backup to get me to the ground (I have a standby alternator for that). Because of the voltage drop across the diode, you are never going to get a full charge on the Aux battery unless you are running for a long time at alternator voltage, so don't depend on it to run your EFIS for 30 minutes. It might....but it might not.

Paul
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  #26  
Old 01-08-2010, 12:53 AM
nucleus nucleus is offline
 
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Default Diode Selection

I was thinking about using one of these diodes.

Hans
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  #27  
Old 01-08-2010, 07:01 AM
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carlrai carlrai is offline
 
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Default Digging through the miscellaneous parts boxes....

Quote:
Originally Posted by Ironflight View Post
The current limiting resistor is in series with, and upstream of, the diode. It is simply to limit the current if, for instance, the Aux battery is drained very low. In this case, you have a large voltage difference between the main battery and the Aux, and the current can get pretty high, turning your diode into a NED (Noise Emitting Diode) or a SED (Smoke Emitting Diode) - both are "one-time-use" by the way...

My chicken scratchings say that I used a 50 Ohm resistor, 2 watt rating, and a 1/2 watt, 0.7 volt drop diode. I will caveat this with the usual notes - I am not a EE, I have friends who are, and they gave me this design. In addition, the Aux battery is there primarily to provide a stable voltage during start, not to give me a long-life backup to get me to the ground (I have a standby alternator for that). Because of the voltage drop across the diode, you are never going to get a full charge on the Aux battery unless you are running for a long time at alternator voltage, so don't depend on it to run your EFIS for 30 minutes. It might....but it might not.

Paul
Paul,

Many thanks for the response. I'll test for the SED effect prior to installation!
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