LX9xxx 12v out

[Ian Molesworth]

The LX9xxx series provide a switched 12v out ( pins 6+14 of the DB15 connector ) 

This is used to power an attached e-Vario unit ( V8 etc ) via a DB9 connector. This connector also carrries the Rs485 signals (and can bus ) so in most installations runs via an RS484 splitter board.  Usually a remote stick, the vario and perhaps a Flap position sensor. In ZS the 485 and 12v also drives the MOP sensor and a RS485-232 bridge to talk to the ACD-57. The switched 12v is also split off for use in the 'PC connector', on the binder 5 bin connector. This means the switched 12v is shared around quite extensively in some aircraft. I have now had 2 LX9xxx devices fail internally, a FET blows and it requires return to the factory for repair and re 'sealing'.

There is NO protection in place for this outgoing 12v. I traced the cause of my failures to a failed RS485 splitter board that shorted out the 12v and thus killed the switched 12v. Not before the splitter was used in 2 different aircraft and killed 2 LX's over a period of 2 years or so. Aaargh! With the 12v extending to the stick/s and the middle of the aircraft engine compartment or flap sensor, the potential for an accidental short is considerably less than zero.

LX does not suggest fusing this 12v line in the installation docs but it's well worth considering if you spend any time tinkering behind your panel or updating instruments etc.

I have replaced the faulty RS485 splitter and I have now made up a short back to back, DB9 plug to DB9 socket adapter cable with an inline 20*4 glass fuse holder to try to mitigate it happening again. I'm using a 500mA fast blow fuse which is ample for my installation.

Ian M

JS1-BJ  'ZS'

[Marc Arnold]

Ian

Thank you for taking the time to document this concern. I am in the midst of redoing my panel, so this was very timely. 

Marc Arnold

Boulder

On Nov 9, 2025, at 1:36 AM, Ian Molesworth <imole...@gmail.com> wrote:

The LX9xxx series provide a switched 12v out ( pins 6+14 of the DB15 connector ) 

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[Ian Molesworth]

Glad to help!

[John DeRosa OHM Ω http://aviation.derosaweb.net]

Ian - Thanks for your valuable research.

I caution the use of such a small value fuse (0.500A/500mA quick blow).  I can understand the logic of using a small value quick blow fuse which is to blow the fuse as "quickly" as possible to save the instrument.  I get it.  But there are a couple of other considerations beyond this view which should be taken into the decision.

- What failure mode are we trying to protect against?  You mentioned the 12Vdc was being "shorted out".  This is a very common failure mode.

- Nearly any size fuse will protect that type of failure mode.  A 10a fast-blow fuse will do the same job as a 0.500A (500mA) quick-blow fuse.  And just as quickly.

There is a problem lurking behind the use of this small size of a fuse.  Looking at the specifications for a 0.500A (500mA) fuse shows that the resistance across this fuse is 4.8Ω.  Doesn't sound like much but can have an oversized impact depending on the current going through it.  

As the current through a fuse grows the amount of voltage lost ("dropped") across the fuse also grows.  Basically, the fuse acts like what I like to call a tiny "electrical space heater" which is what melts the fuse.  These lost volts are gone forever and depending on how much you are loosing it can impact the operation of your avionics (especially power hungry devices like your transceiver when transmitting). 

Below is a chart of the lost volts across your 500ma fuse, with a 12Vdc source, at various current loads.  You can see that the lost volts really add up!

  

Documentation: The FAA recommends using 5A and larger fuses and breakers (AC 42131B Chapter 11 Table 11).  Specifications for (Eaton) glass fuses can be found at https://aviation.derosaweb.net/avionics/Power/.  See also the "Lost Volts" section of https://aviation.derosaweb.net/presentations/#wiring (Part 3 - Chapter 8).

Thanks again and best of luck.

- John (OHM)

[George Underhill]

I'll have to keep all that information in mind should I have any issues as I have the same vario, stick, MOP and flap sensor in my JS-3.  Nice work finding the cause.

George 

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