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Thread: Volts constant but amps will fall off and run negative

  1. #1

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    Volts constant but amps will fall off and run negative

    Plane; CC EX-2, 60 hours on it, Dynon instruments.

    This occurs without any pattern (intermittently) and I cannot duplicate the fault, but always happens when airborne. If I where to average the fault it would be roughly every 6 hours???

    On the EMS display the amps will fail off and go negative; ~10 to 12 amps, voltage will always remain constant. I have a 50 amp breaker between the alternator (40 amp) and shunt. the breaker does not 'pop'. When the fault occurs, I will turn off the 50 amp breaker, then turn it on and things return to normal; ~6 to 8 amps. I have checked connections, looked over everything and can find nothing.

    Any ideas and/or help would be appreciated.
    Last edited by aeroaddict; 03-14-2020 at 06:50 PM.

  2. #2
    www.SkupTech.com mike mcs repair's Avatar
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    loose screw on 50 amp breaker??(or at other ends of those wires) had a noise issue we chased for years that was this... it would "weld" tight for a while, then become loose and 'noisy'

  3. #3

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    The only time I experienced negative amps with normal volts was when my shunt was wired backwards. Mine wasn't intermittent.
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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by stewartb View Post
    The only time I experienced negative amps with normal volts was when my shunt was wired backwards. Mine wasn't intermittent.
    actually that kills my theory.... because no flow on big line would just be zero amps... not negative...

  5. #5
    Gordon Misch's Avatar
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    Just guessing - Bad breaker, that opens when it shouldn't but doesn't pop - dunno if that's possible. Voltage taken from "upstream" of the breaker, and amps showing flow from battery to loads. Maybe???
    Gordon

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  6. #6
    wireweinie's Avatar
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    What charging system? If it's an alternator system, you should never have a negative current flow due to the rectifier diodes. Sounds like a problem with one of those diodes. Loose connection or possible diode break down when it gets hot.

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  7. #7
    Farmboy's Avatar
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    I’d look at the shunt anyhow. Just in case - it might not be backwards, but may be mis-wired.


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    What is the voltage and is there an over voltage control in the system?

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    Great replies, thanks.

    It is an alternator and the voltage remains constant; 14.2V. Yes there is an OV relay. I have also verified the voltage readout on the screen to the battery, they are the same.

  10. #10
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    Your OVP is activating erroneously. It gets reset when you cycle the breaker. I had this same issue in a Comanche after an alternator conversion. SOMETIMES when the flaps were moved, it would trip. SOMETIMES when the gear was moved it would trip. The manufacturer solution was to tell me to have my gear and flap motor rebuilt because they were “noisy “. That didn’t work. The problem is these circuits are generally not designed to ignore short duration voltage spikes which you could see on an oscilloscope but never on a volt meter. My solution: I pulled the breaker that powered the OVP and flew for another ten years with no problem. Yes, I know what could have happened, but it didn’t.

    If you have a separate breaker for the OVP try pulling that rather than the big breaker next time that should also reset it. If that’s the problem.

  11. #11

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    Thanks, I'll try the OV Relay breaker next time. I have pulled the OV Relay breaker on the ground and the results mimic as if pulling the 50 amp breaker; amps drop then return on reset.

  12. #12
    wireweinie's Avatar
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    What model alternator and regulator?

    It's intermittent, so that rules out miswired components. Over voltage relays cut off regulator/field power, which would not cause a reverse current condition. The rectifier diodes in all alternators physically prevent reverse current conditions. Therefore, the only reason to have reverse current flow would be a diode failure or a short to ground on the output circuit. And since the voltage shows correct and the shunt acts in concert with the ammeter to show current value and flow, any short circuit has to be between the output terminal of the alternator and the shunt.

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  13. #13

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    That makes a lot of sense. Alternator is a Denso and the voltage regulator I assumed was the shunt. From the B pole of the alternator, there is a alternator noise filter and a transient suppressor, then a OV relay between that and the 50 amp breaker, then off to the shunt. Could a faulty circuit breaker or OV Relay cause the intermittant amp drop? In other words, when a relay or breaker fail, would they just fail and not be able to reset?
    Last edited by aeroaddict; 03-15-2020 at 01:32 PM.

  14. #14
    wireweinie's Avatar
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    An open circuit breaker or OV relay would just show as 0 amps current flow. Not a negative flow. And if the OV relay opened, you would also have no voltage or, depending on how it's wired, possibly battery voltage.

    When circuit breakers pop, it takes a few seconds for them to cool down enough to reset. If you try to reset when they are still hot, the button just won't reset.

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  15. #15

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    I just saw your response while editing my last comments. Begining to sound like a short somewhere.

  16. #16
    wireweinie's Avatar
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    Don't discount the diode breakdown. They fail when hot, so when you recycle the breaker, they have a few seconds to cool down.

    Do you know what model alternator and regulator are installed? This info is important for troubleshooting.

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  17. #17
    BC12D-4-85's Avatar
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    Sometimes when diodes fail they create AC ripple current...causes noise/whine on the radio more than normal. My experience.

    Gary
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  18. #18

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    I believe it is a Denso 021080-0760 with an built in regulator.

  19. #19
    wireweinie's Avatar
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    Sorry. Our posts crossed paths about the alternator model.

    It's an automotive alternator (I'm not a fan of them on aircraft, but we can argue that later) so pull it off and take it in to an automotive shop that has a test bench for them. Tell them you suspect an intermittent diode failure so that they leave a load on it for a while. If you don't want to screw with this, these alternators are relatively inexpensive so you could just install a NEW alternator. While the alternator is out, physically trace the large, output wire back to the shunt location, checking for evidence of a short to the airframe.

    If you replace the alternator, get a new one. Not an ovehauled/rebuilt one.

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  20. #20
    www.SkupTech.com mike mcs repair's Avatar
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    bad/loose connection on shunt small wire(s) going to gauge? maybe its just lying to you.....

  21. #21

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    Yup, that is what I was thinking. I will remove the alternator and get it tested. As this has happened previously I will take the opportunity to trace out (again) the whole system.

    And also check the small wires and pinout of those wires.

    Thanks

  22. #22
    wireweinie's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    bad/loose connection on shunt small wire(s) going to gauge? maybe its just lying to you.....
    The shunt will have the two fat wires from the alternator output circuit and two small (maybe 22 gauge?) wires that go from the shunt to the amp/volt gauge. Not sure what would have to happen for the reverse current indication to come from poor connections alone. Both volts and amps should go to zero or battery voltage. Also, the two small wires are used by the gauge to sense both voltage and amps. If they shorted or opened it should again indicate zero.

    Under normal conditions, current flow would be from the alternator, through the shunt, to the bus. For the current to flow from the bus and back through the shunt would need something unusual to happen.

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  23. #23
    skywagon8a's Avatar
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    Is your circuit breaker one of this type? https://www.sager.com/w31-x2m1g-5-24...iABEgKeTvD_BwE
    It's possible that it is tripping internally without moving the toggle. So you do not realize it is actually tripped. Without knowing anything about your wiring schematic it is not possible to know where the meter readings are coming from. If the breaker is tripped, it will feel differently when you cycle it off then on than when you normally turn it off and on. The off direction will be softer if it had tripped. Maybe the breaker needs to be replaced? Perhaps it is rated at too low an amperage for your system? This sounds similar to an issue I had with mine. Changing the main breaker from a 15 to 20 amp solved the issue.
    N1PA

  24. #24
    Gordon Misch's Avatar
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    It's possible that it is tripping internally without moving the toggle. So you do not realize it is actually tripped. Without knowing anything about your wiring schematic it is not possible to know where the meter readings are coming from. If the breaker is tripped, it will feel differently when you cycle it off then on than when you normally turn it off and on. The off direction will be softer if it had tripped. Maybe the breaker needs to be replaced? Perhaps it is rated at too low an amperage for your system? This sounds similar to an issue I had with mine. Changing the main breaker from a 15 to 20 amp solved the issue.
    What I was guessing, but you said it better.
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  25. #25
    wireweinie's Avatar
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    If the either the output or field breaker tripped, it would result in a 0 amps reading, NOT a negative reading.

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  26. #26
    skywagon8a's Avatar
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    Quote Originally Posted by wireweinie View Post
    If the either the output or field breaker tripped, it would result in a 0 amps reading, NOT a negative reading.

    Web
    If the ammeter was wired to the battery side it could show the discharge from the battery. That would be negative. Mine does this.
    N1PA
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  27. #27
    wireweinie's Avatar
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    Aeroadditct

    Did you wire this alternator or do you have the diagrams that show the wiring? Tell us if the shunt has been installed on the alternator output or on the battery feed to the bus.

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  28. #28

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    Shunt is wired on the alternator side/output.
    Last edited by aeroaddict; 03-15-2020 at 05:49 PM.
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  29. #29

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    If your voltage is steady, I would think it's a fault in the Dynon. Have you verified voltage with a portable meter?

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    Yes, I have verified the Dynon voltage reading across the battery poles with the engine running.

  31. #31
    wireweinie's Avatar
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    Let us know what you find.

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  32. #32

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    I will. Should have the alternator removed tomorrow and will start tracing wires for shorts, connections, etc.

    Thanks for the help everyone.

  33. #33

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    What happens when a starter is fully engaged and being spun by the engine. I remember this causing something similar.

  34. #34
    wireweinie's Avatar
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    Quote Originally Posted by Fortysix12 View Post
    What happens when a starter is fully engaged and being spun by the engine. I remember this causing something similar.
    Drags the voltage down due to excessive current draw. Can cause a fire. But still no reverse current flow.

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  35. #35
    Gordon Misch's Avatar
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    excessive current draw
    Could you please explain this? I was thinking that the starter motor would be acting as a too-fast generator, creating a reverse EMF on the system.
    Gordon

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  36. #36
    wireweinie's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    Could you please explain this? I was thinking that the starter motor would be acting as a too-fast generator, creating a reverse EMF on the system.
    Actually there are two scenarios of this. If the start relay stays engaged, the starter just keeps spinning. When the engine starts, it tries to spin even faster. It has no magnets or the like to generate a voltage and current. But it is extremely low resistance so the high current flow just keeps flowing. It can cause a fire in some cases. The other scenario is when the start relay opens, stopping the current flow, but the drive stays engaged with the flywheel or drive gears. Not as prone to starting a fire but can still 'disassemble' the starter.

    Starter/generators used on items like turbine engines use a switching relay when changing from the start mode to the generate mode.

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  37. #37
    wireweinie's Avatar
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    Oh. And even if it did generate power, it wouldn't be able to force a current back through the shunt and into the alternator to cause a reverse current reading.

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  38. #38
    Farmboy's Avatar
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    It doesn’t sound like the shunt is the issue, but I’ll share something to be aware of.

    If a builder mis-wires the shunt, and attaches the sense wires to the buss(es), the display you are using to indicate both voltage and amps can interpolate that value in strange ways.

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    Rest assured, I corrected the issue.

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    Transmitted from my FlightPhone on fingers...

  39. #39

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    Update: had the alternator tested by AutoZone. They used testing parameters from a similar alternator (as they do not sell this particular alternator): 12V 40A. The alternator passed, tested 3 times.

    Talked with Dynon, shunt is not current flow directional, can be installed either way.

    Started up the plane before all this just to check; when either alternator breaker is pulled (field and main) the volts drop to the battery voltage (~13V) and amps go to zero. Re-engage breaker and volts go to ~14.5V and ~7amps.

    Now tracing wires.

  40. #40
    wireweinie's Avatar
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    Yes, the shunt is bi directional. Works well connected either way. It's just a resistor with a very small but very precise value. As current flow across a resistance creates a specific voltage drop, this voltage drop is used by the ammeter to show the amount of current flow on the output circuit. Volts = Amps x Resistance (in ohms). Resistance is fixed so any change in current flow will result in a change in voltage drop across the shunt resistance.

    But, again, the diodes in the alternator rectifier (output) only allow current to flow OUT of the alternator, never IN. And the shunt is connected in series with the output (meaning there is only a single path of current flow through the shunt and alternator output. Therefore, there should never be a reverse current flow on the alternator output circuit.

    I'm a little suspicious of the testing done. I'd have left it at max current out for a good 20 to 30 minutes. If a diode is breaking down it will need to get hot first. Let us know if you find any shakey wiring on the alternator output.

    Web
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