Incorrect voltage reading

[timsrv7]

My EMS-D10 voltage reading is showing 0.9 volts lower than the actual voltage. I measure 14.6 volts across the battery terminals, but the D10 shows 13.7 volts. I also measure 14.6 volts from the buss and ground where the D10 is connected. I even measured the voltage across the D10 buss and one of the extra unused ground wires out of the D10 which is used for other sensors. All readings show 14.6 volts while the D10 shows 13.7 volts. By the way, I have confirmed the 14.6 volt reading with two different meters.

Is this normal? Will this interfere with the sensor readings? All the latest software upgrades have been done. I have not noticed any other problems.

Thanks!

Tim

 

[dynonsupport]

This voltage being inaccurate shouldn't affect other sensors, but it also shouldn't have passed our calibration. If you're 100% sure that the voltage at the back of the EMS is that much higher than we read, you should probably send it in to be checked. That's pretty far off, and who knows what might be up inside the unit.

Customers usually see this difference when they have a diode drop for an emergency bus, though. Your D10 isn't powered via a diode anywhere, is it?

 

[WestCoastOz]

I also have a variation in voltage readings......not sure if it is of any significance.

Across the battery (Master SW "on" D100 & D120 powered) 12.4V

EMS D120 voltage shows 11.8V
EFIS D100 voltage shows 12.2V

 

[dynonsupport]

The .2 difference between the EFIS and the battery is about normal. The EMS is a bit more of a difference than we're expecting to see.

There are a few things to look into. First, if you can get a voltage reading right at the EMS, you can determine whether or not the EMS is underreporting voltage or no. Also, insufficient grounding of the instrument can play a role here too.

 

[danburdette]

I have a D180. I am seeing buss voltage initally when I turn the unit on. Voltage indication then drifts to about one volt lower than voltage on the battery buss over about 30 seconds. I can switch aircraft power off, where backup battery powers the D180. Then turn aircraft power back on. Indicated voltage agrees with buss voltage, then drifs down to about about one volt lower. I have seen D180 voltage as low as 12.5 with buss voltage at 14.3. any suggestions? The unit has already been back to your shop once. 85 hours now on the plane.
Dan Burdette
Edmond, OK

 

[dynonsupport]

So given the way that the EMS works inside, we can't think of a failure mode that would cause it to do this, even when broken. So we think that the voltage is real. A few thoughts:

Some components increase resistance as they heat up. If you have self-resetting fuse on this circuit, for example, that could cause this effect.

One surefire check, if you have the capability, is to run the EMS directly from a regulated benchtop power supply, measuring the voltage at the power supply and comparing it against the display's reading. We're happy to test your unit here, too, if you want to send it in.