Hi All,
I am not an electronics person at all and have only but an elementary understanding. I have a very simple flywheel mounted alternator (like many of the snow mobiles have) with a stationary stator and the magnets fixed to the flywheel. The regulator has understandably, two wires to the alternator (with a link to the battery positive) and a Tachometer output which goes into a EFIS panel. The Tachometer output is 20amps at 6 pulses per rev and 10amps at 5 pulses per rev.
If my EFIS system fails for whatever reason, I wanted to have the ability of a simple backup in the form of a LED charging light. I was thinking that maybe a MOSFET might be the way to go and switch ON when the battery voltage falls below 13 volts. Can anyone advise me what MOSFET might do the job and give a simple diagram to work from?
Cheers,
HH

 

A MOSFET is a poor choice to sense a voltage. Use a comparator. I can directly drive an indicator LED.

 

In practice, it's a bit more complicated than that. First, you'll need a voltage reference into the positive input of the comparator. Use a 5.6V zener diode and a resistive potential divider such that the voltage stays at 5.6V for any battery voltage above say 10V. Then another resistive potential divider so that the input to the negative input of the comparator falls to 5.6V when the battery voltage drops to 13.0V

Of course, as soon as the alternator is charging the battery the light will go out. The simplest way to control this might be running the alternator for a fixed time every time the 13V threshold is reached.

 

The function of whatever circuit you seek is not at all clear to me. So please clarify just condition it is that you want the LED to indicate. AND, what is "an EFIS system"???

 

what is "an EFIS system"???

An Electronic Flight Information System, although I’m still not clear what that means!

 

Sorry folks,

Yeah, my bad, an EFIS is an Electronic Flight Instrument System. It displays alternator output (tapping comes off regulator) as a pulse feed (see my original post) into the EFIS system which displays the voltage as the bus voltage.

https://www.aircraftspruce.com/catalog/avpages/mgleifsdiscovery11-13105.php

Basically, I am trying to achieve a degree of basic redundancy to monitor both battery charging and oil pressure in the form of basic indicator LED’s should the EFIS develop a fault (as I lose everything). I have backup flight instruments, but until now, no backup engine indicators.

Heres the wildly detailed info I have in the engine owners manual….

Cheers,
Bruce

 

If all you are looking for is an indicator LED to light when the battery voltage drops below 13V (which presumably means the alternator is not working) then I think this should work for you:


The 5.6V zener diode will keep the inverting input of the comparator at around 5.6V even if the battery voltage drops well below 13V. The resistors R2 and R3 would make the non-inverting input to the comparator around half the supply voltage so the 1K trimpot allows you to fine tune the voltage to be 5.6V when the battery voltage is 13.0V. The 470K resistor will introduce a bit of hysteresis so you don't get the LED flickering on and off around 13V - but you can leave this out altogeher if flickering is a good way to draw attention to the problem!

 

Why not this:
No fancy rigging up an indicator. Just read battery voltage from any available switched positive line. Switched so it doesn't run the battery down when the key is off. (forgive a non-pilot for not knowing how the power is controlled)

 

In my truck I have something similar to this:

It doesn't have a low voltage warning indicator, but if you don't know that 10V on a 12V system is a low voltage then maybe you should be driving a taxi.

 

Below is the LTspice sim of a circuit that uses the inexpensive TL431 programmable voltage reference as a comparator to accurately turn on an LED when the voltage drops below about 13V:
The TL431 starts conducting to turn off the LED when its Ref voltage from the R2/R3 divider goes above 2.5V.

R2 can be varied, or be a pot for adjustment of the trip point.

Two LEDs are used in series so they turn fully off, since the ON voltage of the TL431 is about 2V.

 

I also installed something similar to Tony’s

 

If all you are looking for is an indicator LED to light when the battery voltage drops below 13V (which presumably means the alternator is not working) then I think this should work for you:
View attachment 349650

The 5.6V zener diode will keep the inverting input of the comparator at around 5.6V even if the battery voltage drops well below 13V. The resistors R2 and R3 would make the non-inverting input to the comparator around half the supply voltage so the 1K trimpot allows you to fine tune the voltage to be 5.6V when the battery voltage is 13.0V. The 470K resistor will introduce a bit of hysteresis so you don't get the LED flickering on and off around 13V - but you can leave this out altogeher if flickering is a good way to draw attention to the problem!

 

Thanks very much Jerry, much appreciated. Will cobble something up.
Cheers, Bruce