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Werd issue with transponder tripping when LED lights put on with VPX Pro

david.perl

Well Known Member
hi

Ive been flying my RV7 with a vpx pro with no issues for 300 hours

after a 2month lockdown hiatus i flew the aircraft a couple of times

ive got an odd condition developed

the transponder shows a short fault when the engine is below 1000rpm and when i switch on the LED nav/strobe and landing lights

if i reset in the air and the turn the lights on, the TX stays on

on landing, when cutting the engine to below 1000rpm it trips again

voltage on alternator is steady at 14.5volts

only thing new in last few months is i installed an ADSB conspicuity device

any suggestions?
 
TXP issue

You need to tell us more about the installation.
Please be specific about equipment txp/ adsb/ nav.
The more details we have the better.
 
Its a VPX Pro

Transponder is a dynon - its a rebadged trig transponder Dynon SV-XPNDR-26x

ADSB installed is a Pilot Aware - its a uk sold device https://www.pilotaware.com/

LED lights are from AeroLED's

let me know what other info would be useful

Other thing i think ive noticed is slight alternator whine when engine is on tickover - maybe its been there longer, im not sure
 
What size alternator do you have? 35A, 60A, 70A? How much total current (amps) is being drawn below 1000rpms?
 
60 amp plane power

on the VPX display on the dynon the total power consumption can be no more than 30 amps ,

if the current consumption was over the capacity of the alternator, why would the transponder trip showing a short circuit?
 
Last edited:
Hi David

First thing I would be doing would be thinking about what has changed. Maybe disconnect the new adsb device and retest.

Also maybe cut power at altitude and see if the same fault occurs, so is it tied to the rpm of the engine.

On a different note I see you are based near Watford, 20 years ago I used to live near there and flew out of Elstree..happy memories 😊
 
Grounds

First thing I would do is disconnect the last install and check the grounds. Sounds like a ground problem to me.
 
Regulator Spike

I and several friends suffered the same. We all had Plane Power 60amp Alternators. Connect an oscilloscope and you will note a dirty spike in the regulator charge rate. The VPX interprets the dirty spike as an overload and trips the transponder to protect it.
Replace the regulator or alternator, we are all using B & K alternators now with no issues.
 
thanks aussie flyer, sounds like that might be the issue

Ive noticed other devices are also tripping when the load is increased

if the alternator field wire is turned off, everything stays on even with every device turned on
 
replaced the alternator with the same model and issue has been fixed

just had the old one back from repair, it had a faulty diode in the VR which caused it to intermittently spike at 15 volts, not sure why this would cause the transponder to trip, perhap[s it was being picked up by the VPX?

Repair cost GB£60 including shipping
 
Must have missed your post earlier otherwise I would have responded... .

Also had symptoms of transponder triggering the VP-X when the alternator was broken. I guess somehow the interaction between the broken alternator, power pulses of the transponder combined with much faster trigger times (compared to mechanical fuses) of the VP-X causes this.

So I consider that a feature now.... . If the transponder triggers again on the VP-X it's an early warning of an impending alternator issue.

Oliver
 
I had a similar issues with the VPX, different components, but same results. To solve my issues, I disabled most of the current fault detection options. Only left them on items that had a continuous current draw. Solved a lot of nuisance trips.
 
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