AeroElectric-List Digest Archive

Wed 04/27/11


Total Messages Posted: 1



Today's Message Index:
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     1. 01:09 PM - VM1000C and Capacitive Fuel Senders (Jeff Page)
 
 
 


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    Time: 01:09:40 PM PST US
    From: Jeff Page <jpx@qenesis.com>
    Subject: VM1000C and Capacitive Fuel Senders
    A datapoint for the archives, in case someone else runs into this. I helped a friend solve a problem with his Vision Microsystems VM1000C and VMC capacitance probes. Before I was involved, one tank had been mis-wired and then corrected. That probe no longer provides a signal, so my conclusion was that the electronics in the probe are not reverse polarity protected. This is a sad state of affairs, since a repair would involve removing the wing. The fuel system in this airplane has both wing tanks plumbed together, and generally an indication derived from one tank is sufficient, along with a separate indication for the header tank. Fortunately, a probe had been installed but never wired up in the other wing tank, so we could use that. I used an oscilloscope to confirm a 5V square wave output from the probes when powered by the 5V supply of the VM1000C. I used a signal generator to confirm the VM1000C operated. The maximum frequency the VM1000C would handle was just above 14KHz. The header tank probe is very short, perhaps a foot and when empty, and generated about 11KHz empty. Frequency decreases as fuel is added, so there is no problem with the frequency being out of range of the VM1000C. So with a 5V square wave being generated by the probe, and the VM1000C confirmed to accept it, everything should work, right ? One might be forgiven if they assumed that both products, offered by the same company might be compatible. Sadly wrong. The VM1000C put too much load on the signal from the probes, and converted it into a 1 volt peak-peak triangularly shaped wave :-( JPI now owns the Vision Microsystems product line, and provided very limited support. Although the tech seemed to want to be helpful, he did not have access to the schematics or any knowledge of the input characteristics of the VM1000C. I am quite sure this was not the first non-functional installation, yet he had no knowledge of these issues. Before I was involved, the owner sent his VM1000C in for testing. The tech indicated that the unit would have been connected to the same quality test equipment as new units off the production line. He indicated that a number of wiring harnesses had been shipped that were incorrectly wired, and that new wiring harnesses have the same error. Although a new harness was promised to be shipped when the unit went for testing, it was not included, so I was not able to confirm this. I do not know if the harness in the airplane was provided by VMC or fabricated. The tech is at a different location than the factory and seemed isolated. For example, he could not look at a harness to identify if it was correct or not. My solution was buffer the signal with a transistor, and everything works fine. If anyone needs to do this, what worked well is the following (repeat for each fuel tank). Resistor values were optimized by observing the resulting waveform with the oscilloscope. The drawing is crude, so the description is: Connect the signal from the fuel probe to the base of the transistor with a 2.7K resistor. Connect the collector of the transistor as the signal input to the VM1000C. The collector is also pulled up to +5V with a 1K resistor. The emitter is connected to the ground. PROBE VM1000C Red +5V ------------------------------------------------- +5V Pin 11 or 12 | # # 1K resistor # |--------------- Fuel Signal 9 or 10 | / C 2.7K resistor | White Fuel Signal ------####----------B| 2N4401 NPN transistor | V E | | Black Ground -------------------------------------------- Ground Pin 3 Jeff Page Dream Aircraft Tundra #10




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