AMP meter goofyness

jeff beckley · #1 08-22-2006, 07:01 AM

I installed Van's AMP meter in a project that is not yet flying. I originaly asked about a problem I was seeing when I keyed the mic on com1 and com2. I found that I had wired the leads to the shunt backwards. I reversed them to proper.
Now I get a reading of positive 10 amps. When I key the mic I get neg 40amps.
This is without the engine running. Is this the correct readings? Seems a little high.

Mike S · #2 08-22-2006, 08:44 AM

You should never see a positive amp with the engine off.

Your radio is protected by a circut breader of probably 7 to 10 amps----

A 40 amp drop aint right for keying the mic.

Mike

az_gila · #3 08-22-2006, 09:25 AM

Jeff ... two easy questions....

Where in the circuit is the shunt located? Is it in the main line to the battery?

Is the shunt the correct value... they all look alike, but can scale your readings if the wrong value.

Can you check these two items?

gil in Tucson

jeff beckley · #4 08-22-2006, 09:51 PM

Ok I did some more work. I checked everything and found that the instructions said to use 18ga wire to the shunt. I was using 22 ga. I ran some 18ga right to the shunt outside of the plane to the gauge. Better results! Keying the mic on both com radios results in a 5 amp drop. But I start with the gauge showing positive 8 amps. Just a battery connected and no charger or supply attached.
Every additional device I turn on results in an additional negitive reading so I know I have the shunt hooked up correctly. I just cant figure out why I am starting with a positive 8 amps. If I turn off every device I am still showing plus 4 amps until I remove all power then I get 0 amps.

The shunt is marked 40 amps as well as the guage.
The high load is routed from the switched side of the master relay up to the shunt, from the shunt to the master bus. So I have the relay in between the shunt and the battery

az_gila · #5 08-22-2006, 10:28 PM

Jeff... all might be OK.

Sounds like the 4 amps is the power consumed by the Master Relay being ON.

When you say "remove all power", I presume you mean switching the master switch off.... Is that correct?

gil in Tucson

jeff beckley · #6 08-23-2006, 06:37 AM

Yes switching all power off I get a zero reading. I forgot about the relay consuming power. But why would I see the readings on the positive side?

az_gila · #7 08-23-2006, 09:01 AM

Jeff ... I'm guessing that your shunt is not in the correct location in the circuit.

Can you sketch out the power portion of your electrical system? ...just the battery, shunt, master relay and ground connections...

And are these readings using the real battery, or are you "injecting" power from a supply/charger for these tests? If so, where is the supply/charger clipped into the system?

gil in Tucson

jeff beckley · #8 08-23-2006, 09:02 PM

I drew up an image but I dont know how to insert. Anyway I removed the amp meter and hooked the meter up outside of the plane.
I figured I was getting some RF noise and I wanted to option that out.
I keyed the mic and the gauge still was acting badly. While holding the mic open and moving the signal leads from the shunt back and forth I could get the gauge to swing from -40 to +40. Kind of like those tone genorators at sicence labs that change tones as you move your hand closer and farther away.
So if I get the wires to the shunt in just the right position I can get it to work but just a fraction of an inch movement then no joy.
I called the company that makes the gauges for Van's and they are sending a new gauge and shunt next day air. What service! With a new meter tomarrow I can get some different perspective

az_gila · #9 08-23-2006, 09:22 PM

Jeff... let us know the new results.

It would also be a good practise to lightly twist together the two wires that go to the shunt.

This would cut down on RF pick up. Also check that you have good connections (crimps, I presume) at the terminals at the shunt and the meter.

gil in Tucson

jeff beckley · #10 08-24-2006, 01:53 PM

Well the new amp meter showed up today. That was the problem. The old one was some how spewing so much RF that I was getting bad readings from other gauges as well. The trim tab L.E.D.s would go berserk when I keyed the mic. The fuel pressure gauge would show 4lbs without the engine running. The EGT gauge would start to climb when the mic key was pressed.
After conferming that the gauge worked I decided to rewire the path from the shunt with shelded cable. Connecting the grounded end at the gauge.
Everything is working as far as I can tell.
I hate this part of the process. Debuging the electrical and electronics is a pain.
RF and ground loop noise problems are the hardest to locate.