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LR3C and overvoltage

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l.p(at)talk21.com
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Posted: Sun Nov 14, 2010 1:48 am Post subject: LR3C and overvoltage

Bob.

I have a B and C LR3C (?) regulator watching over my electrical system battery and alternator.

I also have a Vision Micro Systems VM1000 instrumentation.

In monitoring the elec system voltage the VM uses a lead from my main bus, and normally shows between 13.6 and 14.2 volts.

I have recently, yesterday, fitted a Dynon D-10A and on its screen it shows a voltage of 15 -15.3 volts. The Dynon is powered from the main bus.

I also have two auxilliary outputs from the main bus to power a standby handheld gps and handheld radio.

The gps also monitors its input voltage and also shows 15 volts.

My questions are: Could the LR3C be faulty and should it be replaced? How can the VM1000 see a different voltage to the Dynon and standby gps? Is the VM1000 at fault?

Any help in identifying which is correct and which isn't would be much  appreciated.

Patrick C Elliott

G-LGEZ.

PS: Presently in Indonesia on our rtw trip from the UK, so getting things repaired isn't so easy!



 [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Nov 14, 2010 9:55 am Post subject: LR3C and overvoltage

At 03:44 AM 11/14/2010, you wrote:

 Bob.

 I have a B and C LR3C (?) regulator watching over my electrical system battery and alternator.

 I also have a Vision Micro Systems VM1000 instrumentation.



 In monitoring the elec system voltage the VM uses a lead from my main bus, and normally shows between 13.6 and 14.2 volts.

 I have recently, yesterday, fitted a Dynon D-10A and on its screen it shows a voltage of 15 -15.3 volts. The Dynon is powered from the main bus.



 I also have two auxilliary outputs from the main bus to power a standby handheld gps and handheld radio.

 The gps also monitors its input voltage and also shows 15 volts.



 My questions are: Could the LR3C be faulty and should it be replaced? How can the VM1000 see a different voltage to the Dynon and standby gps? Is the VM1000 at fault?



 Any help in identifying which is correct and which isn't would be much appreciated.



     You are suffering a common malady known as "too much

     data". We're all operating vehicles from airplanes

     to cars, trucks, boats and lawn tractors that feature

     DC power systems. The vast majority do not have voltmeters

     and when they do have voltmeters, their veracity is seldom

     questioned.



     Obviously, only one of the readings you've cited can

     be "most nearly correct". I say "most nearly" because

     all measurement devices have accuracy specifications

     (and perhaps drift with age) that contribute to errors

     in displayed readings.



     Just for grins, I just gathered up a flock of 5 multimeters

     from various hiding places around the shops and vehicles

     and hooked them to a power supply on the bench. I set

     the power supply to 15.0 volts output based on it's

     3-digit display. The various meters read as follows:



     Micronta (radio shack brand)  15.00

     Digital Instrumets Co         15.05

     Radio Shack                   15.07

     Fluke Model 75                14.96

     Fluke Model 8000              14.94



     If I average the total of 6 displays I get

     15.003 which is a very strong suggestion that

     the power supply is indeed putting out 15.00

     volts. This isn't a very good illustration

     because the devices I choose to keep in my

     toolboxes are of known reliability and are

     periodically checked. Nonetheless, if I

     were to split hairs, I'd have to say that

     the REAL output voltage was probably closer

     to 14.95 volts. Why? Only because of my experience

     with instruments crafted with John Fluke's

     name on them.



     Now, what's the significance of these tiny variations?

     In an airplane or any other vehicle, obviously

     none. What you're experiencing is the uncertainty

     generated by non-agreement of multiple instruments.

     This is compounded by the  fact that you don't

     know which if any of the instruments were crafted

     with the skills and quality exemplified by

     John Fluke's test equipment.



     The preponderance of evidence suggests that your

     bus voltage is indeed on the order of 15.0

     volts. Assuming that the VM1000 is not being

     "tricked" into some lower reading as a result

     of configuration (conductor voltage drops 

     throughout the system) then the major suspicion

     of error is on the VM1000.



     The best thing to do is get a multimeter of

     some known calibration and use it to referee

     the contest. A fresh Duracell AA battery is

     very close to 1.60 volts at room temperature.

     Get a Radio Shack AA battery holder for 8

     cells (Catalog #: 270-387)



 [img]cid:.0[/img]

    An populate with fresh AA cells. Measure

    the terminal voltage on the holder. If your

    meter reads 12.8 volts, it's close enough

    for government work. If it's something else,

    make note of that difference. Let's say it's

    12.5 volts. Okay, any reading in that range

    is 0.3 volts high.



    Now, go measure the bus on your airplane.

    Apply any observed calibration offset to the

    reading and THAT becomes your benchmark for

    evaluating the veracity of the other displays

    in the airplane.



    Your LR-3 senses the main bus so if your

    meter suggests some adjustment is called for,

    then there's a plug button on the side of

    the LR-3 that covers an adjusting screw.

    This reading is best taken while in flight

    and with normal loads on the system. So it

    might take several observations and subsequent

    adjustments to achieve what you want.



    On the other hand if you find that the bus

    is 14.2 to 14.6 volts under all operating

    conditions in flight, then I'd leave it alone.

    Just make note of the correction factor you

    now need to apply to any ONE instrument that

    displays voltage and ignore the rest of them.



    Many moons ago, the RM series mercury cells

    could be depended upon to offer a 1.345 volt

    calibration source over a wide range of

    temperature. Of course, we can't buy those any

    more.

 

   Bob . . .

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