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rampil
Joined: 04 May 2007
Posts: 870
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 Posted: Sun Mar 16, 2008 5:56 am Post subject: Van's Ammeter Question |
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Anyone know why Van's Ammeter gauge requires a +12V source
(independent of lighting)?
My hangar neighbor is building a -7 and noticed that when he keys
his ICOM his ammeter goes to 20-30 Amps on some frequencies but
only 2-3 Amps on others.
Externally, the ammeter looks like a standard analog mechanical
gauge using an external shunt and with no technical docs provided
by Vans.
My buddy also has RFI on his trim servo indicator (apparently quite
common) and on his Van's Manifold Pressure gauge.
Shielded cable apparently does the trick for the servo, but I'd like
to understand the Ammeter a bit better before throwing bypass
caps and ferrites at it.
Thanks!
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nuckolls.bob(at)cox.net
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| Posted: Mon Mar 17, 2008 9:16 am Post subject: Van's Ammeter Question |
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At 06:56 AM 3/16/2008 -0700, you wrote:
| Quote: |
Anyone know why Van's Ammeter gauge requires a +12V source
(independent of lighting)?
|
Don't know for sure. I've ordered one to find out.
| Quote: | My hangar neighbor is building a -7 and noticed that when he keys
his ICOM his ammeter goes to 20-30 Amps on some frequencies but
only 2-3 Amps on others.
Externally, the ammeter looks like a standard analog mechanical
gauge using an external shunt and with no technical docs provided
by Vans.
My buddy also has RFI on his trim servo indicator (apparently quite
common) and on his Van's Manifold Pressure gauge.
Shielded cable apparently does the trick for the servo, but I'd like
to understand the Ammeter a bit better before throwing bypass
caps and ferrites at it.
|
Van's ammeter appears to be enhanced with internal
electronics. The rudimentary meter movement is not
sensitive to radio frequency interference.
It's unfortunate that suppliers to the OBAM aircraft
industry are so slow on the uptake for rudimentary
design skills. It should NEVER be the responsibility
of the installer to RFI-proof the system.
On the TC side of the house, all accessories are
tested in the lab and the designer is only rarely
allowed to take credit for external shielding and/or
filters on the interconnect cables.
We see this a lot in consumer electronics where
video, data and some audio cables come with
some form of RFI mitigation enhancement on them.
In each of these cases, the cables are pre-assembled
and the end user is never expected to fabricate one
that is longer or has different connectors on it.
In aircraft, the installers are always expected to
fabricate cables and install connectors. Installations
of a product over time and many different airframes
can experience a wide variety of assaults from
EMC/RFI issues.
As I write these words, I'm assisting an OEM with
a random nuisance tripping of the fault detection
system in a product that is over 20 years old and
installed by the thousands in aircraft where this
phenomenon has never manifested itself. However,
in this new installation, a device that has been
traditionally installed in the nose has been moved
to the top of the vertical fin on a large a/c. This
is the first time (anyone knows of) that the harness
became so long and traversed so many EMC gauntlets
on its way through the a/c.
A review of schematics for the product revealed some
ill-conceived filtering on i/o pins . . . that PASSED
DO-160 conducted and radiated susceptibility tests
20+ years ago.
When root cause of this problem is deduced, I'm
betting that a few changes to components inside
the product will fix it. But changing anything
inside will raise re-certification flags with
those-who-know-more-about-airplanes-than-we-do.
We may well have to fabricate a filtered
connector to go on the end of our interface
harness. Seems the installer controls harnesses
and are not part of the cert basis for the appliance.
We can fix the problem with an external band-aid
without having to re-qualify the appliance.
The bureaucratic Catch 22 causes us to do really
dumb things to work around the do-gooders. On
the OBAM aircraft side of the house, suppliers
of devices like trim indicators and ammeters
can make indicated improvements to their products
in a heartbeat without seeking anyone's permission.
Problem is getting them to recognize shortcomings
of their design and fix them. The TC guys will go
to great lengths to avoid significant changes to
the certification effort. I've seen changes to
a 10-cent resistor generate thousands of dollars
in re-qualification testing!
(sigh)
As soon as the Van's ammeter gets here, I'll see
what makes it tick and report back to the group.
Bob . . .
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Mon Mar 17, 2008 11:52 am Post subject: Re: Van's Ammeter Question |
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Thanks Bob,
It's hard to imagine why they would need an "enhancement"
to the rudimentary meter movement since they are so widely
used in this shunt current (voltage) range without.
Also, the price of the meter from Van's suggests they are just custom
labeling a automotive movement.
Well, we will see thanks to Bob's generosity!
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ECLarsen81(at)aol.com
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| Posted: Mon Mar 17, 2008 2:12 pm Post subject: Van's Ammeter Question |
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The instruments themselves are manufactured by ISS for Van's, and now Cheif and Aircraft Spruce both sell the same item with different faceplates.
I did open the case to take a peek and it had a small circuit board with SM parts all over it. You'll have fun with it Bob.
I gleaned all this when I ordered one from spruce for a customer of mine, it didn't come with instructions but had 4 pins on the back, so I called spruce. The saleperson there gave me the number to an engineer at ISS who was very unhappy I interupted his day. I did get the instructions from van's site and a few weeks later a friend ordered one for his homebuilt from spruce. This time it came with the Van's instructions. so the engineer must have got it resolved in that direction.
So far as I know, niether installation has had problems so I'd be curious as well.
Ed Larsen
(43G) Larsen Airpark
It's Tax Time! Get tips, forms and advice on AOL Money & Finance.
[quote][b]
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nuckolls.bob(at)cox.net
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| Posted: Mon Mar 17, 2008 5:10 pm Post subject: Van's Ammeter Question |
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At 06:08 PM 3/17/2008 -0400, you wrote:
| Quote: | The instruments themselves are manufactured by ISS for Van's, and now
Cheif and Aircraft Spruce both sell the same item with different faceplates.
I did open the case to take a peek and it had a small circuit board with
SM parts all over it. You'll have fun with it Bob.
I gleaned all this when I ordered one from spruce for a customer of
mine, it didn't come with instructions but had 4 pins on the back, so I
called spruce. The saleperson there gave me the number to an engineer at
ISS who was very unhappy I interupted his day. I did get the instructions
from van's site and a few weeks later a friend ordered one for his
homebuilt from spruce. This time it came with the Van's instructions. so
the engineer must have got it resolved in that direction.
So far as I know, niether installation has had problems so I'd be
curious as well.
Ed Larsen
(43G) Larsen Airpark
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Thank you for sharing this. It's what I would have
guessed that it has some form of electronics. When
you have an instrument with this mechanical span
(approx 270 degrees) you either use a stepper motor
or a moving magnet suspended in quadrature coils.
You can buy a motor driven instrument for $40 so
it must be a moving magnet device.
There are a number of integrated circuits designed
to drive a 2-coil instrument motor . . . many transistors
capable of responding to VHF radios. Do you have
a name and phone number for the engineer at ISS?
Bob . . .
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