kmjone2(at)comcast.net
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 Posted: Mon May 11, 2009 6:47 pm Post subject: Wonky Ammeter Redux |
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Hello Folks,
I have a problem that is very similar to the Aeroelectric Connection issue of the same subject from 2001. I've got a Vans -40,0+40 ammeter (load meter) connected to a 40mV (at)40A shunt wired exactly (minus the fuselinks) as Figure Z-13 in the Connection. I also have a 10mV shunt wired to read the aux SD-8 alternator loads. The signal wires from the two shunts are controlled by a switch on the panel. Main is wired to show negative, aux is positive ammeter deflection. The following is my scenario:
1. Ground operation with engine off; power supplied to main alternator feed through a radio shack 13.8V 3A supply.
2. Ammeter power is on the E-Bus, with alternate feed switch off.
3. Power supply is providing ~3A and I measure 3.8mV across the shunt. Shunt voltage to ground is 13.08V.
4. Instrument reads ~3.5mV at the sense wire connections
5. Instrument power voltage is ~12.2 volts
6. Ammeter shows full-scale discharge (negative). If I turn power supply off, this doesn't change???
If I switch on the E-Bus alternate feed, the ammeter returns to zero. Instrument voltage increases to 12.6V.
If I power the alternate alternator feed, ammeter shows slight positive deflection, but nothing near the 35% deflection expected.
I first started running this problem down after seeing large ammeter deflections with the strobe on without any load applied to the shunt.
How robust are the Vans ammeters and should I invest in a new one or re-wire the system? Why would opening the E-Bus alternate path cause a change in ammeter reading? Do I need to feed the ammeter with the same voltage as the shunt?
Many thanks.
KJ
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