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Time of current rise in a coil

Discussion in 'General Electronics Discussion' started by seymourfroggs, Jun 2, 2016.

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  1. seymourfroggs

    seymourfroggs

    13
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    Sep 29, 2014
    Hello

    In a circuit where a short burst of current through a coil opens a valve, I need to estimate the time to reach a certain current level.

    The point is that there are two resistances, but I think only that of the coil matters. This is key to my question.

    We simply have 15 v applied across the circuit. There is 5.6Ω in series with the coil, whose resistance is 0.15Ω. The coil's inductance is 0.15 mH.

    So tau is L/R and my question is is tau 10^-3 from 0.15/0.0015 ? Or is it (0.15 + 5.6)/0.0015?

    I need to find the time to reach 5 A in the coil but I can do it if I know the value of tau.

    Thank you!

    Seymour
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    If I interpet what you have correctly, the current will only rise to 2.6A.
    Any resistance in series with the coil needs to be added to the coil resistance.
    To get 5A the total resistance will need to be less than 3Ω.
     
    seymourfroggs likes this.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    tau = L/R -> I(t) = I0*(1-exp(-t/tau))
    At t=tau the current has reached 63% of the max value I0.
    At t=3+tau the current has reached 95% of the max value I0.
    R=Rseries+Rcoil, of course.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Do you mean 3 * tau?
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Of course, thanks for spotting the typo.

    Harald
     
  6. seymourfroggs

    seymourfroggs

    13
    0
    Sep 29, 2014
    Thanks, all.
    I made a mistake - of course with that R the current would only be 2.6 A. The separate R should have been 2.8 Ω. (Two 5.6 Ω in parallel for a particular reason).
    But you have made it clear that the downstream R is included as I expected but wasn't sure.
     
  7. seymourfroggs

    seymourfroggs

    13
    0
    Sep 29, 2014
    Just for completeness, (I must have been nodding off!),
    The tau value (s) would be L/R which is 0.00015/2.95 which is
    0.05 ms,
    so we get to 5 A in ~150μs - 200μs.
    This fits with what I observe and is satisfactory.
    Thanks, guys.
     
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