swinging gas gauge, capacitor to slow response?

Discussion in 'Automotive Electronics' started by freicus, Jul 28, 2015.

  1. freicus

    Thread Starter New Member

    Jul 28, 2015
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    Hi,
    I have a 91 Jeep and the gas gauge swings like crazy when ever I hit a bump or turn a corner. Obvoiusly this is the float in the tank responding to the fluid level changing as it sloshes around.
    I saw on a Jeep forum that someone added a .001mF capacitor in parallel with the gauge and that slowed down the response to a more civilized movement.
    This did not seem to work for me.
    By the way, new gauge, new sending unit, new wire from gauge to sending unit and then to ground. 12 volt DC neg ground.
    Any ideas?
    thanks freicus
     
  2. crutschow

    Expert

    Mar 14, 2008
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    (By .001mF I assume you mean 1μF).:confused:
    I would try a larger capacitor, say 10μF on up (be sure and observe the proper polarity for the polarized capacitor).
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    It is going to take like a 1000uF to 10,000uF to do anything... Observe polarity...
     
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  4. #12

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    Nov 30, 2010
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    I'm used to seeing numbers like 20 ohms in a fuel sensor, so I'm with MikeML. About 5,000 uf will give a tenth of a second for a time constant. Not good and solid, but slow enough that the needle doesn't look like it is in fear of getting bent as it slams against the stops.
     
  5. ian field

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    Oct 27, 2012
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    Just to add a word of caution - if the TS is unlucky enough to hit an LC resonance, the gauge could swing even more wildly at one particular frequency.

    A Q-spoiler resistor in parallel with the gauge would act like a shunt and ruin the calibration - that only leaves series with the capacitor.
     
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  6. #12

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    If the sensor is less than 100 ohms, then the capacitor will already have less than 100 ohms in parallel with it.
    Not expecting a resonance problem.
     
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  7. ian field

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    Its so obvious you just can't see it.
     
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  8. crutschow

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    What's obvious? :confused:

    Certainly the small stray wire inductance would have no significant resonance anywhere near the meter's response time.
     
  9. ian field

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    Its highly unlikely to be a thermal (bi-metal strip) thermal fuel gauge - they take a few seconds to find their way to FSD if you suddenly apply the full rated voltage. They don't move much in a side impact! That's why such old technology persisted so long - the tank float can bob up and down like crazy and you still get an average reading.

    The vehicle instrument firm I worked for didn't make any bi-metal gauges (at least not in the factory zone I was in). They still did some legacy Eddy-current speedos, but were mostly converting to moving coil movements. Some were damped by a fair sized electrolytic and some were wound on an aluminium former that provided damping by being a shorted turn.

    My main job was on tachogenerators, but I filled in for anyone absent from the clean room.
     
  10. #12

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    Thanks for the job history, but please answer the question. What gauge might resonate with 100 ohms and 5,000 uf in parallel with it?
     
  11. ian field

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    A moving coil type.

    If the 100R is the sender rheostat - that is usually solenoid wound resistance wire, and it will also have significant inductance.
     
  12. #12

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    I don't think so.
    When did you ever see a wire wound potentiometer compensated for its inductance, or even measured for its inductance?

    Better yet, just show the resonance graph for 1 uH in parallel with 25 ohms, in parallel with 5,000 uf, in parallel with some microhenries in a moving coil.
     
    Last edited: Aug 3, 2015
  13. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I've seen wire wound resistors compensated for inductance... but now that you mention it, I've never seen a potentiometer with that sort of feature...
     
  14. ian field

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    You may not have noticed, but they tend not to use WW resistors in RF circuits because the inductance is *VERY* significant. In fact, at RF the spiral on a film resistor is too inductive.

    The tank sender has to run stone cold at all times, so it has a lot more turns than most WW resistors, and much larger former diameter. The inductance may not play a big part - but it *WILL* play a part.
     
  15. #12

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    Show your numbers.
     
  16. ian field

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    You mean you've been called on BS and you want me to do all your research for you.
    It doesn't work like that - years of experience have taught me that these apparently trivial things can and often do have unexpected and unwelcome effects.

    And I did say in the post that seems to have triggered your adversarial crusade; that the TS *COULD* be unlucky enough to hit an LC resonance.
     
  17. MikeML

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    Anybody who thinks that a Fuel Gauge or its Sender has enough inductance to effect its damping at fuel-slosh frequencies (a few Hz) is full of crap!
     
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  18. alfacliff

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    Dec 13, 2013
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    how aobut the resistance of the wire? the Q qould be spoiled by the resistance of the wire. also, when I checked the inductance of 25 ohm pots for a project (ever hear of the "miracle whip" antenna?) the inductance is very low and only significant at hf frequencies above 7 mhz. resonance with a large electrolytic? nonsense. if there were any, the meter would be incapable of moving fast enough to show it.
     
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  19. #12

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    No, it's the other way around. You've been called out. Extraordinary claims require extraordinary proof. Show any hint of a math formula or graph which shows the resonance response of this circuit.
     
  20. ian field

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    You'd be surprised how big an effect these small parasitic characteristics can have - I have been on numerous occasions.

    I pointed out something that *COULD* be a problem, and you launched into a tirade!!!
     
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