strange voltage in circuits

[Con]

I’ve discovered that the circuits of my plane carries a small positive voltage 0.22 or 0.25 v. with everything, including master, switched off. I have a single Dynon classic installed in my under construction RV-6A. I have an EarthX battery connected to a charger (the engine is not installed) and everything connected seems to be working correctly.

Going to the master solenoid I found that there is a positive voltage across the solenoid (master off) of 0.25 v.

Disconnecting the fault signal wire (Dynon EFIS pin 10) from the battery (EXT680C) drops the voltage across the solenoid to to 0.02v.

Disconnecting the diode from the Batt + to the start terminal has no effect, the voltage remains.

Disconnecting the battery cable from the battery + terminal gives a voltage across the solenoid of minus 0.21v. (There is no backup battery for the Dynon system).

With the ohmmeter set at 2M ohms and all wires disconnected from the solenoid there is infinite resistance (1.000 on the meter) across the terminals of the solenoid.

The diode was tested at 2M ohms. 1.000 (no contact) from the red sleeve (at the BATT terminal) to the small #10 ring. 0.775 Ω the other direction.

I conclude that the solenoid is good and the diode is good.

But why the voltage in the system? Does it matter?

 

[maartenversteeg]

I also had an issue with ghost voltages in my Dynon system with a EarthX battery and Dynon monitoring the the status line from the battery. I solved it (or at least think I solved it) by adding a diode in the battery status line with the cathode pointing to the battery. Since the battery status line is supposedly an open collector signal and the Dynon sense lines have a pull-up resistor this works fine in my system.

 

[Con]

Thanks Maarenn. I tried it with the red sleeve next to the battery and it works! No more stray voltage.

 

[cbennet12]

Interesting. What value of diode did you guys use?

 

[maartenversteeg]

Just a regular 1 Amp version 1Nxxxx

 

[Con]

just a left-over from the one on the master solenoid.

 

[Jaque]

I have a standard Odyssey battery with only a volt sensor take off to the EMS but I too have about .4 volts everywhere when nothing is on.
running an HDX and UL350 with the large capacitor in parallel to the battery.
Any ideas?

 

[maartenversteeg]

Do you have the EMS voltage sensor wired before the master solenoid? If this is the case then this voltage may be the result of feedback through the EMS, just like I had when I connected the EarthX battery status wire directly to the EMS

 

[Jaque]

Yeah I do and I’m hoping that’s the source. I have a bunch of diodes. Would the same ones used for the contactors work? N5****

 

[maartenversteeg]

I am not sure that you could decouple the battery sense in a similar way. But you might just try and use a general purpose diode, but it would have to be pointing from the battery to the EMS (which is the different direction of the diode that I used for the EarthX open collector status line), otherwise the EMS coudl never sense a voltage. But this is a different condition / setup. I don't think that Dynon ever intended a user to connect the EMS voltage sensor line before the master relay. In this setup you would have a hot wire (+12V) connectted to the EMS voltage input even though you master switch was off and I don't think that is desired for any airplane power design.

 

[Jaque]

Yeah heading out to do some tracing, not sure if I do actually have it off the battery, thing more likely off the downstream contactor.

 

[Jaque]

The volt sensor is actually on the pos terminal of the battery and from what I can gather on Vans Airforce, that’s accepatable for a feed to an EMS but I would like to hear from Don, specific to Dynon.
I did mistype though, my stray voltage is .2 to .3 milliamps, not volts.
And of course you’re right, a diode for a volts sensor is not possible.

 

[maartenversteeg]

A little confused as to what you are measuring. I woudl expect that a stray voltage should be measured in Volts (measure between aircraft ground (ground bus or FOS) and the master bus), a stray current on the other hand (current leakage if the main power relay is off) should be measured in mA (measured in line between the battery plus pole and the power wire going to the master relay, for this measurement the battery plus wire needs to be detached from the battery). You would always see a leakage if you have an 'always on bus' that supplies power to for instance the aircraft clock, but it seems unlikely that you have an external clock in addition to the Dynon.

 

[Jaque]

Oops, sorry. .2 to .3 millivolts, not amps.
It’s a small potential (voltage), no current. I am measuring very close to the main contactor but the main bus itself reads 0.
No draining of battery so not to concerned at this point, just a little strange.

 

[maartenversteeg]

Voltages that small can be hard to track down, it coudl just be a contact potential (different kinds of metals). AS long as you are sure that the battery is not draining all should be good, battery drainage can be measured by disconnecting one of the battery poles and then measuring the current with the MM connected in series