High amp draw question.

Discussion in 'Maintenance Bay' started by imwithtuxedo, Jul 8, 2019.

  1. imwithtuxedo

    imwithtuxedo Filing Flight Plan

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    My Cherokee 180E generally draws about 15-20 amps with the LED landing light, LED beacon, and fuel pump on. I recently noticed that since it got over 90 degrees outside I'm drawing 35+ amps with just the beacon on in cruise flight. This doesn't seem right to me. Any thoughts on what could be causing this spike in amp draw?
     

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  2. asicer

    asicer En-Route

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    Don't you also have a turn coordinator, transponder, radios, GPS, and audio panel also drawing electricity? Or is the 35+ amps with those circuit breakers pulled?
     
  3. imwithtuxedo

    imwithtuxedo Filing Flight Plan

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    I generally only pull 15-20 amps with everything on. I'm now pulling 35+ amps.
     
  4. flyingron

    flyingron Touchdown! Greaser!

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    Never accept a change in your aircraft's behavior as the new normal. What happens when you shut off the beacon?
     
  5. asicer

    asicer En-Route

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    If you have pullable breakers then I would pull and reset each breaker, one at a time until I see a jump. If the breakers are not pullable then I would turn off each electrical item.
     
  6. pburger

    pburger Pre-takeoff checklist

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    Could be a bad shunt. My amp draw went up after my TruTrak autopilot installation last year. I did a lot of troubleshooting -- checked, cleaned and tightened ALL connections. I used a clamp-on DC ammeter to verify that the alternator output was correct and the meter was in error. We finally pulled to meter and sent to Air Parts of Lockhaven to have them calibrate it. We sent it in with the shunt. The shunt tested bad. They sold me a new shunt, and the meter reads properly now! The shunt is a coiled up piece of #6 wire. How does that go bad? I don't know. Maybe oxidation over 30 years. I suspect that when they were under the panel, they disturbed the shunt and maybe somehow the resistance changed enough to throw the readings on the meter off.

    shunt.jpeg
     
  7. Dan Thomas

    Dan Thomas Final Approach

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    The crimp terminals on the shunt would be the problem. Oxide forms between the wire and terminal.

    Some shunts were a metal strip on a plastic base. Same issue: oxidation between the mounting studs/nuts and the strip.
     
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  8. weirdjim

    weirdjim En-Route

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    2, maybe 3 amps for all this stuff at max output. Not a factor.

    Jim
     
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  9. weirdjim

    weirdjim En-Route

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    Simple test. Measure length of #6 wire. Confirm (look down inside the crimp) that it is copper, go down to Home Depot electrical department, buy a length of #6 copper wire, bend each end into a small circle, temporarily connect it to the shunt terminals, turn on the master and look at the ammeter. Normal? Shunt bad. Make your own or spend $$ to have somebody else make one. I prefer to get UNinsulated crimp terminals and solder the wire to the terminal. Prevents terminal-wire corrosion, don'cha know?

    Jim
     
  10. imwithtuxedo

    imwithtuxedo Filing Flight Plan

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    So I got out to the plane today and did some checks. Cleaned and tightened all my solenoid, resistor and battery ground connections. Cigarette lighter was showing 12.4 V with engine off and 13.7 v with engine running at idle. Found the #6 AWG shunt wire running from the ammeter which was wound up behind the side panel. With the engine running and everything on (landing light, beacon, fuel pump, radios, DME, pitot heat, dome light) I gave the shunt a few wiggles and the amp draw settled down the regular 20-25 amps with everything running and 15-20 amps when I turned off the archaic DME and pitot heat. I have a Concord battery. I removed the battery cover before doing this test and the battery was not hot to the touch after running the plane. I've got my annual coming up shortly and I'll pull the panel and reground the shunt wire and/or replace it. Thanks for everyones advice and help.
     
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  11. flyingron

    flyingron Touchdown! Greaser!

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    Even a clunky DME shouldn't draw too much, but Pitot heat is a big power sink.
    I suspect that since wiggling the shut made a change that the previous suggestion issue of corrosion on the terminals may indeed be the problem.
     
  12. Clip4

    Clip4 En-Route

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    Kinda of a novel idea, but maybe the gauge is bad.
     
  13. pburger

    pburger Pre-takeoff checklist

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    The shunt isn’t and shouldn’t be grounded!!! All power from the alternator to the bus goes through that wire. Don’t ground either end!!
     
  14. Checkout_my_Six

    Checkout_my_Six Final Approach

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    if the wire/connector is bad...you should be able to "see" the heat. The insulator will be brown where the heat is....
     
  15. flyingron

    flyingron Touchdown! Greaser!

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    If there is an insulator on it and it's not a high heat insulation (e.g., teflon).
     
  16. Checkout_my_Six

    Checkout_my_Six Final Approach

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    how would you know....on a 50 yo plane?
     
  17. Skip Miller

    Skip Miller En-Route

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    This is very true. I suspect that grounding either end of the ammeter shunt would melt wires and possibly start a fire. @pburger is right, DON'T GROUND THE SHUNT!

    -Skip
     
  18. imwithtuxedo

    imwithtuxedo Filing Flight Plan

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    The majority of the shunt is circled up behind the side panel. I assumed it was grounded because when I wiggled the exposed part the amps dropped. Of course my mechanic won't ground the new shunt wire if the old one wasn't/isn't when we pull the panel at annual.
     
  19. Dan Thomas

    Dan Thomas Final Approach

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    I think the OP's term "grounding" was a simple misapprehension. The shunt is not grounded, as is nothing else in the positive side of the airplane's circuitry. Only the negative sides of any loads are grounded, including filter capacitors. A ground in the positive creates a short, and that will pop a breaker or create a fire. Nasty.

    The bad gauge comment: the ammeter in this airplane is a milliammeter, a device that measures in thousandths of amps. Quite sensitive. Almost all ammeters are built this way, since to make a meter that carries the entire current through the meter's needle movement would require HUGE components and the ammeter would be the size of a bucket. The shunt carries the actual heavy current, and has a tiny, caibrated resistance (in this case the defined length of a specified type and size of electrical cable), and the meter is connected across the ends of the shunt, in parallel with it. The current flows through the shunt, but the shunt's resistance makes the electricity try to find another path, and it does--through the meter. If that resistance in the shunt is too high, or its connections are loose or corroded, more current is forced through the meter and it overreads.

    And the resistance doesn't have to increase to the point it heats up and burns the terminal insulators. By the time that happens, the meter itself would be roasted. It takes some serious resistance to do that. The shunt's overall resistance will be a tiny fraction of an ohm, and it only has to increase a tiny bit to make the meter peg out.

    [​IMG]
     
  20. 3393RP

    3393RP Pattern Altitude

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    Your description varies from what I know to be true about DC shunts.

    The meter actually reads the voltage drop across the shunt wire loop or brass bridge, not resistance or current. The small wires and terminals that are colocated on the shunt end connectors sense the voltage drop in millivolts across the shunt, and are routed to the meter.

    Note there is no positive or negative connection labeling. Both ends of the shunt are positive, and both terminals of the meter are positive. They are being used solely to measure forward voltage drop across a positive circuit.

    While the meter scale is graduated in amperes, it is actually measuring millivolts.

    I think single engine aircraft shunts are usually 30 or 50 amps on the meter scale, and the millivolt range is 50 mV.

    This is a typical installation:

    [​IMG]


    This is a brass bridge shunt used for higher currents, it's a 200 amp capacity 50 millivolt indicating shunt. The big bolts carry the current, the small screws feed the meter. It has to be paired with the correct meter, one with a 50 mV sweep and 200 amp scale.

    Meters with a zero center and a charge/discharge scale are set up to swing either direction depending on the direction of the current flow. The millivolt signal detects the polarity and moves the meter needle from the center toward the current flow.

    [​IMG]
     
  21. flyingron

    flyingron Touchdown! Greaser!

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    I agree with 3393RP. Voltage only exists as a differential. Positive and negative only mean something when you designated a reference (such as the ground in the airplane). As far as the meter is concerned, the upper terminal in the picture is more positive than the lower terminal.

    Dan's picture can't work. An ammeter would like to have as close to zero resistance. That means no current would likely go through the shunt. In fact, the shunt (as you can tell by it's beefy appearance) is designed to handle nearly all the current. The voltmeter has a very high resistance and very little current flows through it.
     
  22. Dan Thomas

    Dan Thomas Final Approach

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    You fellows must remember that all meter movements measure current, not voltage. The magnetic field used to move the needle has a strength entirely determined by current flow.
    A sensitive meter has many turns of very fine wire, making a large resistance and responding to small current flows. Sure, it takes voltage to increase current flow, but it's the current that moves the needle. All analog voltmeters use sensitive milliammeters or microammeters.
     
  23. flyingbrit

    flyingbrit Pre-takeoff checklist

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    Actually, both the above explanations are correct. Suppose the meter has a 50 ohm coil and is full scale when 50mV is applied. There will be 1mA of current in the meter. So is it measuring voltage or current? I'd say both. When the shunt is connected you could either say the current is split unevenly so that full scale indication is >>> 1mA, or you could say the shunt creates a voltage drop which the meter responds to.

    Suppose you dump the shunt and connect a 49,950 ohm resistor in series with the meter. Now when 50V is applied there will be 1mA of current flowing (and also 50mV drop across the meter). You have created a 50V voltmeter.

    This is what classic analog multimeters like the Simpson 260 do. To measure current, low resistance shunts are connected in parallel with the meter movement. To measure voltage, high resistances are connected in series with the meter movement.
     
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  24. Dan Thomas

    Dan Thomas Final Approach

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    Most ammeters don't even use an external shunt. The shunt is built into the ammeter's case. It's just a small metal bar. The external shunt setup doesn't use such an ammeter.
     
    Last edited: Jul 11, 2019
  25. Kenny Phillips

    Kenny Phillips Pattern Altitude

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    They will discover that—instantly—if they try it.
     
  26. Dan Thomas

    Dan Thomas Final Approach

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  27. flyingron

    flyingron Touchdown! Greaser!

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    That still makes no sense. If it has a shunt, the internal meter is still measuring voltage.
     
  28. Dan Thomas

    Dan Thomas Final Approach

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    Study basic electricity. You will be taught that current is what generates a magnetic field, not voltage. As has been said, all analog voltmeters are simply milliammeters with resistances in series to enable them to measure voltage.
    Voltage causes current flow. More voltage causes more current flow through a defined resistance. But the meter's movement is an electromagnetic affair that responds ONLY to current flow. It measures voltage INDIRECTLY because of that.
     
  29. 455 Bravo Uniform

    455 Bravo Uniform En-Route PoA Supporter

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    Ok, for those of us not smart enough to engage in the last few posts above, this is my takeaway:

    If the shunt has problems conveying “juice”, the meter will read wrong (higher discharge). If the shunt is really bad or disconnected, the meter will fry. Close enough for pilot knowledge?
     
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  30. Checkout_my_Six

    Checkout_my_Six Final Approach

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    so...if you don't know what it is....you shunt do it. o_O
     
  31. Dan Thomas

    Dan Thomas Final Approach

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    For those wanting to understand a little more; an article on ammeters:
    https://en.wikipedia.org/wiki/Ammeter

    The hangup over voltage drop measurements is a valid one. Many guys know how to measure voltage, but fewer understand how to measure amperage (current) or Ohms (resistance). And mechanics use voltage drop measurements all the time (or they should be) to diagnose amperage problems. The analog multimeter is measuring voltage by inferring its potential by the amount of current flowing through its meter movement and some resistors in series with the movement. (A digital meter works differently, with voltage playing a much larger role. It has no electromagnetic movement that demands current to generate a magnetic field.) With voltage drop measurements we can identify the problem when the starter doesn't want to to crank too well; instead of starting to throw money at the problem by immediately blaming the battery, we can take measurements across the battery connections, ground connections, master and starter contactor terminals, and so forth, and find the real reason the starter is lazy. Very often it's not the battery, but some other component in the circuit that is presenting resistance where none belongs. Bad contactors and corroded connections are most common.

    But we must remember that, with the analog multimeter and the airplane's steam-gauge ammeter, we're making mechanical meter movement with a magnetic device, which relies on amperage to make it move. If we say that the shunt is generating a voltage drop and that is what the meter is measuring, we're not really wrong but we're muddying the water by bringing voltage into it. Amperage is flowing through the shunt, which forces a much smaller amperage through a meter movement that has a high resistance, which uses it to move the needle. And that's what a voltmeter does when we take a voltage drop measurement across a corroded connection: that connection becomes a shunt that should have practically zero resistance but has more than zero, and it's forcing some current through the "voltmeter" that uses a milliammeter to measure that current and read it out to you on a scale calibrated in volts. The "voltage" is inferred from the current measurement.

    Ohm's Law says that E=IxR, or voltage is equal to current times resistance. If we know two of the values we can calculate the third. E/R=I, and E/I=R. When we talk about power, we need E and I: ExI=P, or Watts.

    For my students that had a hard time with this, I talked about the garden hose. The pressure that forces the water through the hose is like voltage. Without it, nothing happens. The rate of water flowing in the hose is like amperage (current); more flow gets more work done. It waters the garden faster, It puts the fire out faster. It gets your sister wetter faster. The sprinkler or a nozzle is like resistance; it represents a load on the system. Pretty hard to get any real work done without at least some resistance. All heaters or lights or motors or radios present resistance, and their resistance is what limits the current flow. If your sister kinks the garden hose so you can't squirt her, she is putting a massive resistance into the system and the water stops, or nearly so.

    If we raise the water pressure, we get a higher rate of flow. If we raise voltage, we get more current flow. If we raise the hose's resistance, we get a lower rate of flow, same as a raised resistance in an electrical circuit. If the hose breaks, the resistance gets really low and all that water goes to ground:). A broken wire stops electrical flow; electrons don't run out all over everything. A wire that touches ground (airframe) is a short, a low-resistance connection that results in a massive current flow that either trip a breaker or pops a fuse or causes an electrical fire. Fuses and breakers are there to protect the wire, not the component it's feeding. The component has resistance that limits the current flow through it; it's that wire that will light up if the current finds some alternate, low-resistance path to ground.

    The positive battery cable has no fuse or breaker in its circuit to the bus behind the instrument panel. That's why the master contactor is located close to the battery: so we can shut the master off if we smell smoke. A fuse or breaker would need to be close to the battery, where we couldn't reach it if it popped. And it would be a massive device, to handle the large current flow demanded by the starter. So mechanics should be paying close attention to the condition of those cables and connections so that shorts don't develop.
     
  32. flyingbrit

    flyingbrit Pre-takeoff checklist

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    Not quite all. I used to measure CRT anode and focus voltages with an analog electrostatic voltmeter. These are for all practical purposes an ideal DC voltmeter, with infinite resistance and zero current draw. Only useful in the kV range though.
     
  33. Dan Thomas

    Dan Thomas Final Approach

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    I ws aware of those but never used one. When I was young I did basic repairs to tv's and radios. Got few big shocks off the anodes, too.
     
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  34. weirdjim

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    I learned my first lessons about dielectric absorption on those suckers too. Nobody ever told me that if I discharged the anode the little electrons hiding in the anode filter capacitor dielectric can rear their ugly little heads for a few more minutes.

    Jim
     
  35. 3393RP

    3393RP Pattern Altitude

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    Good discussion. I guess I never bothered to think about the meter movement.

    Thanks