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gary(at)liming.org
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 Posted: Fri Jun 23, 2006 1:28 pm Post subject: Ammeter surge problem |
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I am helping a friend with his RV and he is experiencing a strange problem.
When he hits the PTT button with the radio on, the ammeter shows a 30
amp surge! We are trying to figure out why.
Here are some relevant facts:
1. The radio works ok - transmission is clear and normal, the antenna
is installed and hooked up properly.
2. We put a hand held ammeter in series from the battery positive
cable to the battery post, and no such real surge is occurring -
all other components (like various lights) are showing a normal
current load. The radio shows a 1.3 amp load on receive, and a 3 amp
load or so on transmit - consistent with it's specified load rating.
3. The ammeter itself is a shunt type, the standard one that Van's
sells. It requires a separate power input to run the meter - I am
guessing that there is a circuit in there to compensate for a voltage
range across the shunt, but I am guessing about that. (It is not used
for internal lighting, that is yet another lead.) The shunt appears
to be installed ok.
4. The same ammeter shows normal current consumption for other things
like strobes, pos lights, etc.
5. All of this is done using only the aircraft battery. The engine
and alternator are not operating yet.
I kind of find it hard to believe that the RF is causing it, but we
are stumped.
Any ideas?
TIA,
Gary Liming
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tigerrick(at)mindspring.c
Guest
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| Posted: Fri Jun 23, 2006 2:23 pm Post subject: Ammeter surge problem |
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Hi, Gary!
Sounds to me like the sensitive ammeter is being swamped by the RF from
the transmitter, and that the RF carrier is somehow getting rectified
enough to create a small DC voltage across the shunt (which shows up as
a surge on the ammeter). Is there a solid state rectifier or regulator
somewhere in the RV's electrical system? It could be happening there.
I suppose there's a couple of things that might "fix" it.
1). You could trying relocating the offending antenna.
2). A small filter capacitor between the DC buss and ground might be
enough to kill the induced RF.
3). You could add a small label to the "Amps" one that says "and
Carrier Output". <grin>
Rick Lindstrom
Gary Liming wrote:
| Quote: |
I am helping a friend with his RV and he is experiencing a strange
problem.
When he hits the PTT button with the radio on, the ammeter shows a 30
amp surge! We are trying to figure out why.
Here are some relevant facts:
1. The radio works ok - transmission is clear and normal, the antenna
is installed and hooked up properly.
2. We put a hand held ammeter in series from the battery positive
cable to the battery post, and no such real surge is occurring - all
other components (like various lights) are showing a normal current
load. The radio shows a 1.3 amp load on receive, and a 3 amp load or
so on transmit - consistent with it's specified load rating.
3. The ammeter itself is a shunt type, the standard one that Van's
sells. It requires a separate power input to run the meter - I am
guessing that there is a circuit in there to compensate for a voltage
range across the shunt, but I am guessing about that. (It is not used
for internal lighting, that is yet another lead.) The shunt appears to
be installed ok.
4. The same ammeter shows normal current consumption for other things
like strobes, pos lights, etc.
5. All of this is done using only the aircraft battery. The engine
and alternator are not operating yet.
I kind of find it hard to believe that the RF is causing it, but we
are stumped.
Any ideas?
TIA,
Gary Liming
http://www.matronics.com/Navigator?AeroElectric-List
http://wiki.matronics.com
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khorton01(at)rogers.com
Guest
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| Posted: Fri Jun 23, 2006 3:32 pm Post subject: Ammeter surge problem |
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On 23 Jun 2006, at 17:25, Gary Liming wrote:
| Quote: |
I am helping a friend with his RV and he is experiencing a strange
problem.
When he hits the PTT button with the radio on, the ammeter shows a
30 amp surge! We are trying to figure out why.
Here are some relevant facts:
1. The radio works ok - transmission is clear and normal, the
antenna is installed and hooked up properly.
2. We put a hand held ammeter in series from the battery positive
cable to the battery post, and no such real surge is occurring -
all other components (like various lights) are showing a normal
current load. The radio shows a 1.3 amp load on receive, and a 3
amp load or so on transmit - consistent with it's specified load
rating.
3. The ammeter itself is a shunt type, the standard one that Van's
sells. It requires a separate power input to run the meter - I am
guessing that there is a circuit in there to compensate for a
voltage range across the shunt, but I am guessing about that. (It
is not used for internal lighting, that is yet another lead.) The
shunt appears to be installed ok.
4. The same ammeter shows normal current consumption for other
things like strobes, pos lights, etc.
5. All of this is done using only the aircraft battery. The engine
and alternator are not operating yet.
I kind of find it hard to believe that the RF is causing it, but we
are stumped.
Any ideas?
|
Since you have satisfied yourself that it is not a real current
spike, only a false indication, and it only happens when you
transmit, perhaps the best solution is simply to not look at the
ammeter when you transmit.
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://www.kilohotel.com/rv8
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Mark Phillips in TN
Joined: 10 Jan 2006
Posts: 431
Location: Columbia, TN
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| Posted: Fri Jun 23, 2006 9:51 pm Post subject: Ammeter surge problem |
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In a message dated 06/23/2006 5:28:05 PM Central Daylight Time, tigerrick(at)mindspring.com writes:
| Quote: | 3). You could add a small label to the "Amps" one that says "and
Carrier Output". <grin> |
>>>
Good one! I'll suggest toss the ammeter and substitute something useful like a G-meter- problem solved! But seriously now- having heard many such reports regarding Van's gauges, my suspicion is that the plastic case does nothing to prevent stray xmit emf from doing its natural thing on the windings in the meter movement. If ya gotta have an ammeter, maybe a wrap of steel flashing material around the case might help? But then again, I built a nosedragger, so what do I know?
From The PossumWorks in TN
Mark Phillips - RV-6A N51PW Mojo do not archive
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From The PossumWorks... |
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brian
Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA
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| Posted: Sat Jun 24, 2006 12:34 pm Post subject: Ammeter surge problem |
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On Jun 23, 2006, at 5:25 PM, Gary Liming wrote:
| Quote: | I kind of find it hard to believe that the RF is causing it, but we
are stumped.
|
You are 100% correct in that it is the RF that is causing the
problem. You will probably find that the ammeter requires power
because it has an amplifier in it to amplify the very small signal
from the shunt. This amplifier is being affected by the RF coming
from the shunt or from the power lead.
Try bypassing the power lead to the ammeter indicator. That will
probably solve the problem.
Folks: when it comes to troubleshooting RF problems in the cockpit it
is hard to beat the ARRL Radio Amateur's Handbook. It has a whole
chapter on how to solve problems with RF getting into other
electronics. It tells you how to bypass power leads, how to install
chokes, etc. It also has all the information you might need to
construct your own antennas. Definitely worth the price of admission
even if it does now cost about $40. (I paid $4 for my first one in
1962.)
http://www.arrl.org/catalog/9485/
Brian Lloyd 361 Catterline Way
brian-yak AT lloyd DOT com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
— Antoine de Saint-Exupéry
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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery |
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gary(at)liming.org
Guest
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| Posted: Sat Jun 24, 2006 2:07 pm Post subject: Ammeter surge problem |
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|
I am helping a friend with his RV and he is experiencing a strange problem.
When he hits the PTT button with the radio on, the ammeter shows a 30
amp surge! We are trying to figure out why.
Here are some relevant facts:
1. The radio works ok - transmission is clear and normal, the antenna
is installed and hooked up properly.
2. We put a hand held ammeter in series from the battery positive
cable to the battery post, and no such real surge is occurring -
all other components (like various lights) are showing a normal
current load. The radio shows a 1.3 amp load on receive, and a 3 amp
load or so on transmit - consistent with it's specified load rating.
3. The ammeter itself is a shunt type, the standard one that Van's
sells. It requires a separate power input to run the meter - I am
guessing that there is a circuit in there to compensate for a voltage
range across the shunt, but I am guessing about that. (It is not used
for internal lighting, that is yet another lead.) The shunt appears
to be installed ok.
4. The same ammeter shows normal current consumption for other things
like strobes, pos lights, etc.
5. All of this is done using only the aircraft battery. The engine
and alternator are not operating yet.
I kind of find it hard to believe that the RF is causing it, but we
are stumped.
Any ideas?
TIA,
Gary Liming
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FLYaDIVE(at)aol.com
Guest
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| Posted: Sun Jun 25, 2006 5:11 am Post subject: Ammeter surge problem |
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In a message dated 6/24/06 6:10:34 PM Eastern Daylight Time, gary(at)liming.org
writes:
| Quote: | I kind of find it hard to believe that the RF is causing it, but we
are stumped.
Any ideas?
TIA,
Gary Liming
=====================
|
Gary:
A couple of questions:
1 - Is the ammeter digital or mechanical?
I'm guessing DIGITAL!
2 - When you say surge, is it in the positive or negative direction?
3 - The surge, is it a constant reading or does it fluctuate?
Does it fluctuate with modulation of the Com?
4 - How is the coax routed from the Xmiter to the Antenna? Does it go near
EITHER the ammeter or any of its wiring?
To answer your question: " I am guessing that there is a circuit in there to
compensate for a voltage range across the shunt, but I am guessing about that."
EXACTLY ... By definition that is exactly what a shunt does.
Example: It would be very impractical to manufacture an ammeter for EVERY
range of current requested. So an ammeter that reads say 1 amp is made and lets
say you wanted to measure a current of 10 Amps. Well, a SHUNT is used to
shunt, divert the excess current around/away from the 1 amp meter ... 1 Amp goes
through the meter and 9 amps go through the shunt. WHY? Because the shunt
shows a path of least resistance at ratio of 10 : 1.
Try this ... Get a fully charge Hand Held Com, one with a HIGH output. Then
turn on all items on as you had during the surge. Start with the rubber ducky
antenna on the hand held (HH). Transmit, talk and move the HH antenna around
the ammeter ... does the meter surge?
If not then the second thing to try is connect the HH to the coax of the
plane's Com. Transmit with the HH ... Does the ammeter surge?
If it does, then you have verified that it is RF getting into the ammeter.
Yup! That is my bet.
What's the cure? Reroute the wires, keep coax and ammeter wires as far away
as possible IF they have to cross, do them at 90 Deg angles. You could also
consider a shield around the ammeter and its wire.
ALSO!!! If there is a lot of stray RF floating around ... Check the SWR of
the antenna. High SWR causes stray radiation. And lots of radio problems.
Barry
"Chop'd Liver"
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gmcjetpilot(at)yahoo.com
Guest
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| Posted: Sun Jun 25, 2006 12:17 pm Post subject: Ammeter surge problem |
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| Quote: | posted by: Gary Liming <gary(at)liming.org (gary(at)liming.org)>
I am helping a friend with his RV and he is experiencing a strange
problem.
|
Take a deep breath, it is no big deal.
Let me guess you have a Van's ammeter?
Make sure the ammeter wires are ROUTED away from the
COM radio wiring and COAX. (also intercom wire separation).
Also make sure the shunt is isolated for other wires.
Consider twisting the wire PAIR between the shunt and ammeter.
You can also consider a shielded pair cable and ground one
end of the shield (either end, does not matter).
CHECK the Com radios antenna. It must be grounded
and check to make sure the coax and connectors are
good. Good means good connections and good antenna
ground.
Consider shielding the back of the gauge or relocating it.
Shield with copper tape or make a metal box or cover.
Van's knows about this and you can ask, but last I heard
they said it was normal. In deed many just ignore it and
live with it, getting use to it.
Good luck.
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