ammeter

rwarre · #1 04-28-2010, 08:02 PM

Have read everything I can on how the ammeter is suppose to work, but can't figure out the basic operation so I can troubleshoot mine. I have the ammeter connected as per Aeroelectric Z11 diagram as a loadmeter. When I turn on the master switch, without the positive lead from the shunt connected, the meter needle goes to zero. (Amp is -40-0-+40) As soon as I connect the positive lead to the ammeter terminal, the needle pegs at +40. This is without the engine started. I tried a different wire from the shunt to the meter, same thing. I would think there is short somewhere but can figure out what to test. Help!

647jc · #2 04-29-2010, 07:43 AM

I'm not familiar with the Aeroelectric Z11 diagram but some basic electrical trouble shooting would be;

First determine if you are really drawing 40+ amps of current or if your meter is reading incorrently. If you measure the voltage at the batery to ground with the master off and then on, do you see a significant voltage drop, 1 or 2 volts? If so you probably have a major power short somewhere and will need to start removing items from the bus until the short goes away. I would think if you really were drawing 40+ amps you would be smelling something or seeing smoke. If it appears you are not drawing excessive current, about the only other things that could be wrong would be a mismatch of the shunt and amp meter or a defective meter.

noelf · #3 04-29-2010, 08:05 AM

...it actually only measures a calibrated % of that current. The real current path (either battery or alternator...you choose) is from the current source (lets say the battery) through the CURRENT SHUNT, and then on to the load.

The amp meter is connected across the shunt. The shunt is nothing more than a high-precision resistor that can handle a large current load. As current flows through the shunt, a small voltage drop develops across the shunt, and the amp meter registers this voltage-current. The shunt and the amp meter need to be specified such that the full scale deflection of the meter (-40 /0 / +40) indication is consistant with the value of the shunt that will provide the voltage for that meter reading.

Bottom line...40 amps is not going through the meter, only a small, calibrated, proportional amount is.

Now, the big question. When you connected the amp meter, and it pegged, did you get a big spark?? Were all the breakers (or fuses) pulled when you tried this?

rwarre · #4 04-29-2010, 08:52 PM

I have, through the process of troubleshooting, now have two of the Vans ammeters. Both the instruments have been installed at some point, and neither have worked correctly. I took one of the ammeters to a pilot friends shop and we hooked up power and load source and it still did not operate correctly. I think I have spent enough time on a product which clearly has a poor reputation of any kind of quality. Just venting.

L.Adamson · #5 04-30-2010, 07:19 AM

Quote:

Originally Posted by rwarre

I have, through the process of troubleshooting, now have two of the Vans ammeters. Both the instruments have been installed at some point, and neither have worked correctly. I took one of the ammeters to a pilot friends shop and we hooked up power and load source and it still did not operate correctly. I think I have spent enough time on a product which clearly has a poor reputation of any kind of quality. Just venting.

I think there could be a different problem. I know of numerous Van's ammeters that have worked just fine, including mine. More than not, it's usually a mistake in the wiring process. Same goes for radios, audio panels, etc. I have most of the Van's instruments in my RV, and they've worked well for two years.

L.Adamson --- RV6A

jdiehl · #6 07-12-2011, 03:50 PM

I may be off topic a bit. I recently installed a sl-40 in my yet to fly 7A. I am using a bent whip antenna purchased through Vans and have it mounted beneath the copilot seat. All seems to work well. When I turn ON the comm, the ammeter (Vans ammeter) shows a subtle ammeter draw (needle swings to left). But, when I push the mic button to transmit, the needle swings wildly to the far right. This doesn't seem right!?? I removed the rubber gasket on the base of the antenna to assure good RF contact but that didn't help. My wings are in a stand only a few feet away. Could that be an issue.
I'm wondering if a high SWR could be the problem. I'm at a loss as to what to do and don't want to cook the new radio

Jim Diehl
RV-7A N891DD
Pennsylvania
just about done!

aerhed · #7 07-12-2011, 07:24 PM

Check the threads and you'll see others have this too. Cheap gages (not just vans), small metal airplane, big TX spike = wacky needles. It's not a real current draw. See if some of your other gages jump also.

krw5927 · #8 07-12-2011, 07:26 PM

It sounds like you may be trying to transmit while the plane is still in the hangar. That could be the problem - all that RF bouncing around the hangar off different objects. You might try pulling the plane outside and trying the radio. If you do, let us know what the result is.

rzbill · #9 07-12-2011, 09:02 PM

Rwarre,
Like noelf said, the ammeter gage by itself is really a voltmeter in disguise. The combination of voltmeter and the precision resistor shunt that is matched to the voltmeter gives a needle movement related (via ohms law) to alternator current through the shunt. Current through the shunt resistor causes a voltage difference at the two meter terminals on the shunt. Voltmeter reads that minute voltage. Viola' ammeter.

Since you say you were having trouble with the concept, think of the shunt as a fluid orifice, the alternator as a pump, and the ammeter as a pressure gage sampling both sides of the orifice. Zero fluid flow, zero pressure difference, zero on the gage. Some fluid flow from "pump", some pressure difference across the "orifice", gage will show some pressure proportional to flow.

If you are wired per Z11 rev M, disconnect the B lead from the alternator and try again. With that configuration it is impossible for current to flow in the shunt IF YOU ARE WIRED PER Z11. The gage should not move one iota. "Fluid can't flow" with B-lead off.

rapid_ascent · #10 07-13-2011, 07:18 AM

To continue with what Bill said the Ammeter is really a just a voltmeter that is setup to measure the voltage across a specific shunt. So the shunt has to be the correct shunt for a specific ammeter to read correctly. If you know the shunt resistance you can measure the voltage across the shunt with any voltmeter and then convert to current manually using "Ohms Law". So the Current = Voltage Measured / Shunt Resistance. If you are able to make this measurement then you will know if your Ammeter is defective or if you are actually getting that much current drain.

Disclaimer: My wife says I should not give advice to people early in the morning since I am not a morning person. Please use this information at your own risk. I'm not sure how Ohms Law can be my advice but I hope this all helps.

Ray