Voltage Leak?

JimP · #1 12-24-2006, 11:37 PM

Well, Santa's been here and I finally have a minute.

I'm in the process of getting the basic wiring structure in place per Bob's Z-13 architecture. The battery is currently showing 12.58V. With the master ON, I get the appropriate voltage across the battery contactor. With the MASTER OFF (contactor OPEN), I'm still seeing a little over .020V across the battery contactor, the Main Buss still sees .015V and the Essential Buss shows .012V.

With Master OFF and E-Buss ON (feeding the Essential Buss from Batt Buss), I see 12.48 on the Essential Buss and .132 on the Main Buss. System appears to be configured correctly although there seems to be some sort of small leak.

Is this a concern? I'm not quite sure how to track this one down.

Jim

Paul Eastham · #2 12-24-2006, 11:55 PM

Hey Jim, how's it going? Merry Christmas.

This doesn't sound right, though I have not measured mine in the all-off situation.
Are you measuring voltage relative to the negative terminal of the battery?

Do you have an always-on battery bus? If so, I would try pulling the fuses on that bus to make sure no device (like a keepalive circuit on an EFIS) is leaking power to the switched buses.

There shouldn't be too many ways that voltage can get to the main bus if the contactor is open. You should be able to track it down...

ronoc74 · #3 12-25-2006, 05:21 AM

It takes a very tiny current to show .132V on a digital multimeter. Leakage from the essential bus is probably reverse leakage through the diode connecting it to the main bus. The .02V bypassing the contactor might even be going through the insulating material.

wv4i · #4 12-25-2006, 05:57 AM

You cannot measure a voltage drop across open contacts unless something else is wired in parallel with the contacts. I assume by voltage drop you mean measuring batt ground to a given circuit point, vs batt ground to batt pos. I doubt if most voltmeters, be it handheld or aircraft installed are accurate to .020 vdc. If you touch the probes of a dc voltmeter together, it should read 0 vdc to be accurate. Measuring voltages as small as .020 vdc could be attributed to even poorly conducting probes, etc. To have a voltage drop in a series circuit, you also have to have a resistance (load) present, prior to the voltage drop. I may not be understanding the issue correctly, just trying to mention some basics which are easy to forget....

V = (R1+R2+R3+....Rn) x I

where V is a DC voltage, i.e. 12.6, Rx are individual load resistances, and I (current) is equal thru out the elements of a series DC circuit. In a series DC circuit, resistances simply are added together to find total circuit resistance (load).

The cumulative voltage drop at any point in the DC series circuit is found by adding the resistances (loads) encountered up to that point, dividing them into the total circuit resistance (load), and multiply result times dc volts applied, i.e. 12.6.

Note I am only discussing a SERIES dc circuit, as parallel dc circuits are analyzed differently, i.e resistances do not simply add and currents across loads are not equal. Also not talking about ac circuits either, where resistance, reactance, much more, must be considered.

Hope this is helpful. Off to Xmas activities!

Captain_John · #5 12-25-2006, 06:49 AM

Jim, all good advice here!

Tell me, are you experiencing a problem and are troubleshooting it or are you simply curious?

I dare say, the reading is a phantom on the instrument.

Merry Christmas!

CJ

JimP · #6 12-25-2006, 09:44 PM

I was introubleshooting mode using a Fluke digital meter.

Here's the readings I ended up with (all readings taken from power lead on the notesd buss to firewall ground:

From V Master Aux Alt
Batt+ 12.58 OFF OFF (normal)
Batt Buss 12.58 OFF OFF (normal)
Main Buss .017 OFF OFF
Ess. Buss .012 OFF OFF
Ess. Buss 12.02 ON OFF (a little low but normal)
Ess. Buss 12.48 OFF ON (normal)
Main Buss .132 OFF ON

The interesting thing is that I isolated the downstream side of the battery contactor and did manage to get a small V reading (.016). The Start contactor behaves the same way.

I'm beginning to think it's an artifact of the meter.

Jim

AltonD · #7 12-26-2006, 03:55 AM

Florescent shop lights will sometimes cause digital meters to pick up stray currents.

jdeas · #8 12-26-2006, 06:58 AM

The Fluke meters have a real high impedance and can receive signals from many sources. Bridge the leads with a 1K resistor where they enter the meter. This should eliminate a false DC reading due to lead induction.

*remember to remove the resistor when done! You do not want it there when mesuring AC around you house!

Regard,
JD

airguy · #9 12-26-2006, 11:41 AM

Use the frequency measurement on the fluke to determine if you're picking up AC hum as an induction on the test leads - I've had it happen to me, and the voltage range is about right....

ronoc74 · #10 12-27-2006, 05:58 AM

Fluke digital multimeters have an input impedance of around 10 megaohms when measuring voltage, so if you connected your multimeter to the battery through a 950 megaohm resistance it will show about .132V. If you have a diode connecting the main bus to the essential bus, it will easily conduct that much in reverse. Your meter can read .017V from the battery through 7.4 gigaohms. A good insulator can have resistance in that range!